Universal Serial Bus
Specification
Compaq
Digital Equipment Corporation
IBM PC Company
Intel
Microsoft
NEC
Northern Telecom
Revision 1.0
January 15, 1996
Universal Serial Bus Specification Revision 1.0
2
Scope of this Revision
The 1.0 revision of the specification is intended for product design. Every attempt has been made to ensure a
consistent and implementable specification. Implementations should ensure compliance with this revision.
Revision History
Revision Issue Date Comments
0.7 November 11, 1994 Supersedes 0.6e.
0.8 December 30, 1994 Revisions to Chapters 3-8, 10, and 11. Added
appendixes.
0.9 April 13, 1995 Revisions to all the chapters.
0.99 August 25, 1995 Revisions to all the chapters.
1.0 FDR November 13, 1995 Revisions to Chapters 1, 2, 5-11.
1.0 January 15, 1996 Edits to Chapters 5, 6, 7, 8, 9, 10, and 11 for
consistency.
Proposal for Universal Serial Bus Specification
Copyright © 1996, Compaq Computer Corporation, Digital Equipment Corporation,
IBM PC Company, Intel Corporation, Microsoft Corporation, NEC, Northern Telecom.
All rights reserved.
INTELLECTUAL PROPERTY DISCLAIMER
THIS SPECIFICATION IS PROVIDED “AS IS” WITH NO WARRANTIES WHATSOEVER INCLUDING ANY
WARRANTY OF MERCHANTABILITY, FITNESS FOR ANY PARTICULAR PURPOSE, OR ANY
WARRANTY OTHERWISE ARISING OUT OF ANY PROPOSAL, SPECIFICATION, OR SAMPLE.
A LICENSE IS HEREBY GRANTED TO REPRODUCE AND DISTRIBUTE THIS SPECIFICATION FOR
INTERNAL USE ONLY. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE,
TO ANY OTHER INTELLECTUAL PROPERTY RIGHTS IS GRANTED OR INTENDED HEREBY.
AUTHORS OF THIS SPECIFICATION DISCLAIM ALL LIABILITY, INCLUDING LIABILITY FOR
INFRINGEMENT OF PROPRIETARY RIGHTS, RELATING TO IMPLEMENTATION OF INFORMATION IN
THIS SPECIFICATION. AUTHORS OF THIS SPECIFICATION ALSO DO NOT WARRANT OR REPRESENT
THAT SUCH IMPLEMENTATION(S) WILL NOT INFRINGE SUCH RIGHTS.
GeoPort and Apple Desktop Bus are trademarks of Apple Computer, Inc.
Windows and Windows NT are trademarks and Microsoft and Win32 are registered trademarks of Microsoft
Corporation.
IBM, PS/2, and Micro Channel are registered trademarks of International Business Machines Corporation.
AT&T is a registered trademark of American Telephone and Telegraph Company.
Compaq is a registered trademark of Compaq Computer Corporation.
UNIX is a registered trademark of UNIX System Laboratories.
I2C is a trademark of Phillips Semiconductors.
DEC is a trademark of Digital Equipment Corporation.
All other product names are trademarks, registered trademarks, or servicemarks of their respective owners.
Please send comments via electronic mail to USB@fes.fm.intel.com
For industry information, refer to the USB Implementers Forum web page at http://www.teleport.com/~USB
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Contents
CHAPTER 1 INTRODUCTION
1.1 Motivation..........................................................................................................................................11
1.2 Objective of the Specification............................................................................................................11
1.3 Scope of the Document.......................................................................................................................12
1.4 Document Organization.....................................................................................................................12
CHAPTER 2 TERMS AND ABBREVIATIONS
CHAPTER 3 BACKGROUND
3.1 Goals for the Universal Serial Bus....................................................................................................23
3.2 Taxonomy of Application Space........................................................................................................23
3.3 Feature List........................................................................................................................................24
3.4 Some Existing Technologies...............................................................................................................26
CHAPTER 4 ARCHITECTURAL OVERVIEW
4.1 USB System Description....................................................................................................................27
4.1.1 Bus Topology................................................................................................................................28
4.2 Physical Interface...............................................................................................................................29
4.2.1 Electrical.......................................................................................................................................29
4.2.2 Mechanical ...................................................................................................................................30
4.3 Power..................................................................................................................................................30
4.3.1 Power Distribution........................................................................................................................30
4.3.2 Power Management.......................................................................................................................30
4.4 Bus Protocol.......................................................................................................................................30
4.5 Robustness..........................................................................................................................................31
4.5.1 Error Detection .............................................................................................................................31
4.5.2 Error Handling..............................................................................................................................31
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4.6 System Configuration.........................................................................................................................31
4.6.1 Attachment of USB Device...........................................................................................................31
4.6.2 Removal of USB Device...............................................................................................................32
4.6.3 Bus Enumeration...........................................................................................................................32
4.6.4 Inter-Layer Relationship................................................................................................................32
4.7 Data Flow Types.................................................................................................................................32
4.7.1 Control Transfers ..........................................................................................................................33
4.7.2 Bulk Transfers...............................................................................................................................33
4.7.3 Interrupt Transfers.........................................................................................................................33
4.7.4 Isochronous Transfers....................................................................................................................33
4.7.5 Allocating USB Bandwidth...........................................................................................................34
4.8 USB Devices........................................................................................................................................34
4.8.1 Device Characterizations...............................................................................................................34
4.8.2 Device Descriptions ......................................................................................................................35
4.9 USB Host: Hardware and Software .................................................................................................37
4.10 Architectural Extensions..................................................................................................................37
CHAPTER 5 USB DATA FLOW MODEL
5.1 Implementer Viewpoints....................................................................................................................39
5.2 Bus Topology......................................................................................................................................41
5.2.1 USB Host......................................................................................................................................41
5.2.2 USB Devices.................................................................................................................................42
5.2.3 Physical Bus Topology..................................................................................................................42
5.2.4 Logical Bus Topology...................................................................................................................43
5.2.5 Client Software to Function Relationship......................................................................................44
5.3 USB Communication Flow.................................................................................................................44
5.3.1 Device Endpoints..........................................................................................................................46
5.3.2 Pipes .............................................................................................................................................47
5.4 Transfer Types...................................................................................................................................49
5.5 Control Transfers...............................................................................................................................50
5.5.1 Data Format..................................................................................................................................50
5.5.2 Direction.......................................................................................................................................51
5.5.3 Packet Size Constraints.................................................................................................................51
5.5.4 Bus Access Constraints .................................................................................................................52
5.5.5 Data Sequences.............................................................................................................................53
5.6 Isochronous Transfers........................................................................................................................54
5.6.1 Data Format..................................................................................................................................54
5.6.2 Direction.......................................................................................................................................54
5.6.3 Packet Size Constraints.................................................................................................................54
5.6.4 Bus Access Constraints .................................................................................................................55
5.6.5 Data Sequences.............................................................................................................................56
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5.7 Interrupt Transfers............................................................................................................................56
5.7.1 Data Format..................................................................................................................................56
5.7.2 Direction.......................................................................................................................................56
5.7.3 Packet Size Constraints.................................................................................................................56
5.7.4 Bus Access Constraints.................................................................................................................57
5.7.5 Data Sequences.............................................................................................................................58
5.8 Bulk Transfers ...................................................................................................................................58
5.8.1 Data Format..................................................................................................................................59
5.8.2 Direction.......................................................................................................................................59
5.8.3 Packet Size Constraints.................................................................................................................59
5.8.4 Bus Access Constraints.................................................................................................................59
5.8.5 Data Sequences.............................................................................................................................60
5.9 Bus Access for Transfers ...................................................................................................................61
5.9.1 Transfer Management ...................................................................................................................61
5.9.2 Transaction Tracking ....................................................................................................................64
5.9.3 Calculating Bus Transaction Times...............................................................................................65
5.9.4 Calculating Buffer Sizes in Functions/Software.............................................................................67
5.9.5 Bus Bandwidth Reclamation.........................................................................................................67
5.10 Special Considerations for Isochronous Transfers.........................................................................67
5.10.1 Example Non-USB Isochronous Application...............................................................................68
5.10.2 USB Clock Model.......................................................................................................................71
5.10.3 Clock Synchronization................................................................................................................73
5.10.4 Isochronous Devices....................................................................................................................73
5.10.5 Data Prebuffering........................................................................................................................81
5.10.6 SOF Tracking..............................................................................................................................82
5.10.7 Error Handling............................................................................................................................82
5.10.8 Buffering for Rate Matching .......................................................................................................83
CHAPTER 6 MECHANICAL
6.1 Architectural Overview.....................................................................................................................85
6.2 Dimensioning Requirements..............................................................................................................85
6.3 Cable...................................................................................................................................................86
6.3.1 Cable Specification.......................................................................................................................86
6.3.2 Connector (Series A).....................................................................................................................90
6.3.3 Connector (Series B).....................................................................................................................96
6.3.4 Serial Bus Icon............................................................................................................................101
6.3.5 Plug/Receptacle Mechanical and Electrical Requirements...........................................................102
6.4 Cable Voltage Drop Requirements..................................................................................................107
6.5 Propagation Delay............................................................................................................................108
6.6 Grounding........................................................................................................................................108
6.7 Regulatory Information...................................................................................................................109
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CHAPTER 7 ELECTRICAL
7.1 Signaling...........................................................................................................................................111
7.1.1 USB Driver Characteristics .........................................................................................................111
7.1.2 Receiver Characteristics..............................................................................................................113
7.1.3 Signal Termination......................................................................................................................114
7.1.4 Signaling Levels..........................................................................................................................115
7.1.5 Data Encoding/Decoding.............................................................................................................121
7.1.6 Bit Stuffing .................................................................................................................................122
7.1.7 Sync Pattern................................................................................................................................123
7.1.8 Initial Frame Interval and Frame Adjustability............................................................................124
7.1.9 Data Signaling Rate.....................................................................................................................124
7.1.10 Data Signal Rise and Fall Time.................................................................................................124
7.1.11 Data Source Signaling...............................................................................................................125
7.1.12 Hub Signaling Timings..............................................................................................................126
7.1.13 Receiver Data Jitter...................................................................................................................127
7.1.14 Cable Delay...............................................................................................................................129
7.1.15 Bus Turnaround Time/Interpacket Delay...................................................................................129
7.1.16 Maximum End to End Signal Delay ..........................................................................................130
7.2 Power Distribution...........................................................................................................................131
7.2.1 Classes of Devices.......................................................................................................................131
7.2.2 Voltage Drop Budget...................................................................................................................135
7.2.3 Power Control During Suspend/Resume......................................................................................136
7.2.4 Dynamic Attach and Detach........................................................................................................136
7.3 Physical Layer..................................................................................................................................137
7.3.1 Environmental.............................................................................................................................137
7.3.2 Bus Timing/Electrical Characteristics .........................................................................................138
7.3.3 Timing Waveforms .....................................................................................................................142
CHAPTER 8 PROTOCOL LAYER
8.1 Bit Ordering.....................................................................................................................................145
8.2 SYNC Field.......................................................................................................................................145
8.3 Packet Field Formats.......................................................................................................................145
8.3.1 Packet Identifier Field.................................................................................................................145
8.3.2 Address Fields.............................................................................................................................146
8.3.3 Frame Number Field ...................................................................................................................147
8.3.4 Data Field ...................................................................................................................................147
8.3.5 Cyclic Redundancy Checks.........................................................................................................147
8.4 Packet Formats ................................................................................................................................148
8.4.1 Token Packets.............................................................................................................................148
8.4.2 Start of Frame Packets.................................................................................................................149
8.4.3 Data Packets................................................................................................................................149
8.4.4 Handshake Packets......................................................................................................................149
8.4.5 Handshake Responses..................................................................................................................150
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8.5 Transaction Formats........................................................................................................................152
8.5.1 Bulk Transactions .......................................................................................................................152
8.5.2 Control Transfers........................................................................................................................153
8.5.3 Interrupt Transactions .................................................................................................................155
8.5.4 Isochronous Transactions............................................................................................................156
8.6 Data Toggle Synchronization and Retry.........................................................................................157
8.6.1 Initialization via SETUP Token ..................................................................................................157
8.6.2 Successful Data Transactions......................................................................................................157
8.6.3 Data Corrupted or Not Accepted.................................................................................................158
8.6.4 Corrupted ACK Handshake.........................................................................................................158
8.6.5 Low Speed Transactions .............................................................................................................159
8.7 Error Detection and Recovery ........................................................................................................161
8.7.1 Packet Error Categories...............................................................................................................161
8.7.2 Bus Turnaround Timing..............................................................................................................161
8.7.3 False EOPs..................................................................................................................................162
8.7.4 Babble and Loss of Activity Recovery........................................................................................163
CHAPTER 9 USB DEVICE FRAMEWORK
9.1 USB Device States............................................................................................................................165
9.1.1 Visible Device States..................................................................................................................165
9.1.2 Bus Enumeration.........................................................................................................................169
9.2 Generic USB Device Operations .....................................................................................................170
9.2.1 Dynamic Attachment and Removal.............................................................................................170
9.2.2 Address Assignment....................................................................................................................170
9.2.3 Configuration..............................................................................................................................170
9.2.4 Data Transfer..............................................................................................................................171
9.2.5 Power Management.....................................................................................................................171
9.3 USB Device Requests .......................................................................................................................172
9.3.1 bmRequestType..........................................................................................................................172
9.3.2 bRequest.....................................................................................................................................173
9.3.3 wValue........................................................................................................................................173
9.3.4 wIndex........................................................................................................................................173
9.3.5 wLength......................................................................................................................................173
9.4 Standard Device Requests ...............................................................................................................173
9.4.1 Clear Feature...............................................................................................................................176
9.4.2 Get Configuration .......................................................................................................................176
9.4.3 Get Descriptor.............................................................................................................................176
9.4.4 Get Interface...............................................................................................................................177
9.4.5 Get Status....................................................................................................................................177
9.4.6 Set Address.................................................................................................................................179
9.4.7 Set Configuration........................................................................................................................179
9.4.8 Set Descriptor .............................................................................................................................179
9.4.9 Set Feature..................................................................................................................................180
9.4.10 Set Interface..............................................................................................................................180
9.4.11 Synch Frame.............................................................................................................................180
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9.5 Descriptors ................................................................................................................................ .......181
9.6 Standard USB Descriptor Definitions .............................................................................................182
9.6.1 Device.........................................................................................................................................182
9.6.2 Configuration..............................................................................................................................184
9.6.3 Interface......................................................................................................................................185
9.6.4 Endpoint......................................................................................................................................187
9.6.5 String..........................................................................................................................................188
9.7 Device Class Definitions...................................................................................................................189
9.7.1 Descriptors..................................................................................................................................189
9.7.2 Interface(s) and Endpoint Usage..................................................................................................189
9.7.3 Requests......................................................................................................................................189
9.8 Device Communications...................................................................................................................190
9.8.1 Basic Communication Mechanisms.............................................................................................192
CHAPTER 10 USB HOST: HARDWARE AND SOFTWARE
10.1 Overview of the USB Host.............................................................................................................195
10.1.2 Control Mechanisms..................................................................................................................198
10.1.3 Data Flow..................................................................................................................................198
10.1.4 Collecting Status and Activity Statistics....................................................................................199
10.1.5 Electrical Interface Considerations............................................................................................199
10.2 Host Controller Requirements.......................................................................................................199
10.2.1 State Handling...........................................................................................................................200
10.2.2 Serializer/Deserializer...............................................................................................................200
10.2.3 Frame Generation......................................................................................................................200
10.2.4 Data Processing.........................................................................................................................201
10.2.5 Protocol Engine.........................................................................................................................201
10.2.6 Transmission Error Handling.....................................................................................................201
10.3 Overview of Software Mechanisms ...............................................................................................202
10.3.1 Device Configuration................................................................................................................202
10.3.2 Resource Management ..............................................................................................................204
10.3.3 Data Transfers...........................................................................................................................205
10.3.4 Common Data Definitions.........................................................................................................205
10.4 Host Controller Driver...................................................................................................................206
10.5 Universal Serial Bus Driver...........................................................................................................207
10.5.1 Overview...................................................................................................................................207
10.5.2 USBD Command Mechanism Requirements .............................................................................209
10.5.3 USBD Pipe Mechanisms ...........................................................................................................211
10.5.4 Managing the USB via the USBD Mechanisms.........................................................................213
10.6 Operating System Environment Guides........................................................................................215
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CHAPTER 11 HUB SPECIFICATION
11.1 Overview ........................................................................................................................................217
11.2 Device Characteristics ...................................................................................................................217
11.2.1 Hub Architecture.......................................................................................................................217
11.2.2 Hub Connectivity......................................................................................................................218
11.2.3 Hub Port States .........................................................................................................................220
11.2.4 Bus State Evaluation.................................................................................................................224
11.2.5 Full vs. Low Speed Behavior.....................................................................................................224
11.2.6 Hub State Operation..................................................................................................................225
11.3 Hub I/O Buffer Requirements.......................................................................................................228
11.3.1 Pull-up and Pull-down Resistors................................................................................................229
11.3.2 Edge Rate Control.....................................................................................................................229
11.4 Hub Fault Recovery Mechanisms..................................................................................................230
11.4.1 Hub Controller Fault Recovery..................................................................................................230
11.4.2 False EOP.................................................................................................................................230
11.4.3 Repeater Fault Recovery...........................................................................................................230
11.4.4 Hub Frame Timer......................................................................................................................231
11.4.5 Hub Behavior Near EOF...........................................................................................................232
11.5 Suspend and Resume......................................................................................................................234
11.5.1 Global Suspend and Resume.....................................................................................................234
11.5.2 Selective Suspend and Resume..................................................................................................236
11.6 USB Hub Reset Behavior...............................................................................................................240
11.6.1 Hub Receiving Reset on Root Port...........................................................................................240
11.6.2 Per Port Reset...........................................................................................................................241
11.6.3 Power Bringup and Reset Delays..............................................................................................242
11.7 Hub Power Distribution Requirements.........................................................................................242
11.7.1 Overcurrent Indication ..............................................................................................................243
11.8 Hub Endpoint Organization..........................................................................................................243
11.8.1 Hub Information Architecture and Operation ............................................................................244
11.8.2 Port Change Information Processing..........................................................................................244
11.8.3 Hub and Port Status Change Bitmap..........................................................................................246
11.9 Hub Configuration.........................................................................................................................247
11.10 Hub Port Power Control..............................................................................................................247
11.11 Descriptors ...................................................................................................................................247
11.11.1 Standard Descriptors ...............................................................................................................248
11.11.2 Class-specific Descriptors .......................................................................................................249
11.12 Requests........................................................................................................................................252
11.12.1 Standard Requests...................................................................................................................252
11.12.2 Class-specific Requests...........................................................................................................253
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Chapter 1
Introduction
1.1 Motivation
The motivation for the Universal Serial Bus comes from three interrelated considerations:
• Connection of the PC to the telephone
It is well understood that the merge of computing and communication will be the basis for the next
generation of productivity applications. The movement of machine-oriented and human-oriented
data types from one location or environment to another depends on ubiquitous and cheap
connectivity. Unfortunately, the computing and communication industries have evolved
independently. The Universal Serial Bus provides a ubiquitous link that can be used across a wide
range of PC to telephone interconnects.
• Ease of use
The lack of flexibility in reconfiguring the PC has been acknowledged as the Achilles heel to its
further deployment. The combination of user friendly graphical interfaces and the hardware and
software mechanisms associated with new generation bus architectures like PCI, PnP ISA, and
PCMCIA has made computers less confrontational and easier to reconfigure. However, from the end
user point of view, the PC’s I/O interfaces such as serial/parallel ports, keyboard/mouse/joystick
interfaces, etc., do not have the attributes of plug and play.
• Port expansion
The addition of external peripherals continues to be constrained by port availability. The lack of a
bi-directional, low-cost, low-to-mid speed peripheral bus has held back the creative proliferation of
peripherals such as telephone/fax/modem adapters, answering machines, scanners, PDA’s,
keyboards, mice, etc. Existing interconnects are optimized for one or two point products. As each
new function or capability is added to the PC, a new interface has been defined to address this need.
The Universal Serial Bus is the answer to connectivity for the PC architecture. It is a fast, bi-
directional, isochronous, low-cost, dynamically attachable serial interface that is consistent with the
requirements of the PC platform of today and tomorrow.
1.2 Objective of the Specification
This document defines an industry standard Universal Serial Bus. The specification describes the bus
attributes, the protocol definition, types of transactions, bus management, and the programming interface
required to design and build systems and peripherals that are compliant with this standard.
The goal is to enable such devices from different vendors to inter-operate in an open architecture. The
specification is intended as an enhancement to the PC architecture spanning portable, business desktops,
and home environments. It is intended that the specification allow system OEMs and peripheral
developers adequate room for product versatility and market differentiation without the burden of
carrying obsolete interfaces or losing compatibility.
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1.3 Scope of the Document
• Target audience
The specification is primarily targeted to peripheral developers and system OEMs, but provides
valuable information for platform operating system/ BIOS/ device driver, adapter IHVs/ISVs, and
platform/adapter controller vendors.
• Benefit
This version of the Universal Serial Bus specification can be used for planning new products,
engineering an early prototype, and preliminary software development. All final products are
required to be compliant with the Universal Serial Bus Specification 1.0.
1.4 Document Organization
Chapters 1 through 5 provide an overview for all readers, while Chapters 6 through 11 contain detailed
technical information defining the Universal Serial Bus.
Peripheral implementers should particularly read Chapters 5 through 11.
Universal Serial Bus Host Controller implementers should particularly read Chapters 5, 6, 7, 8, 10, and
11.
Universal Serial Bus device driver implementers should particularly read Chapters 5, 9, and 10.
This document is complemented and referenced by the following related documents, which will be
released shortly:
• The Universal Serial Bus Device Class Specification
• The Universal Serial Bus Design Guide
Please contact the USB Implementers Forum for further details.
Readers are also requested to contact operating system vendors for operating system bindings specific to
the USB.
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Chapter 2
Terms and Abbreviations
This chapter lists and defines terms and abbreviations used throughout this specification.
Access.bus The Access.bus is developed by the Access.bus Industry Group, based on the
Phillips I2 C technology and a DEC software model. Revision 2.2 specifies
the bus for 100 kbs operation, but the technology has headroom to go up to
400 kbs.
ACK Acknowledgment. Handshake packet indicating a positive acknowledgment.
Active Device A device that is powered and not in the suspend state.
ADB See Apple Desktop Bus.
APM An acronym for Advanced Power Management. APM is a specification for
managing suspend and resume operations to conserve power on a host
system.
Apple Desktop Bus An expansion bus used by personal computers manufactured by Apple
Computer, Inc.
Asynchronous Data Data transferred at irregular intervals with relaxed latency requirements.
Asynchronous RA The incoming data rate, Fsi, and the outgoing data rate, Fso, of the RA
process are independent (i.e., no shared master clock).
Asynchronous SRC The incoming sample rate, Fsi, and outgoing sample rate, Fso, of the SRC
process are independent (i.e., no shared master clock).
Audio Device A device that sources or sinks sampled analog data.
AWG# The measurement of a wire’s cross section as defined by the American Wire
Gauge standard.
Babble Unexpected bus activity that persists beyond a specified point in a frame.
Bandwidth The amount of data transmitted per unit of time, typically bits per second
(bps) or bytes per second (Bps).
Big Endian A method of storing data that places the most significant byte of multiple
byte values at a lower storage addresses. For example, a word stored in big
endian format places the least significant byte at the higher address and the
most significant byte at the lower address. See Little Endian.
Bit A unit of information used by digital computers. Represents the smallest
piece of addressable memory within a computer. A bit expresses the choice
between two possibilities and is typically represented by a logical one (1) or
zero (0).
Bit Stuffing Insertion of a “0” bit into a data stream to cause an electrical transition on the
data wires allowing a PLL to remain locked.
bps Transmission rate expressed in bits per second.
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Bps Transmission rate expressed in bytes per second.
Buffer Storage used to compensate for a difference in data rates or time of
occurrence of events, when transmitting data from one device to another.
Bulk Transfer Nonperiodic, large bursty communication typically used for a transfer that
can use any available bandwidth and also be delayed until bandwidth is
available.
Bus Enumeration Detecting and identifying Universal Serial Bus devices.
Byte A data element that is eight bits in size.
Capabilities Those attributes of a Universal Serial Bus device that are administerable by
the host.
Characteristics Those qualities of a Universal Serial Bus device that are unchangeable; for
example, the device class is a device characteristic.
CHI An acronym for Concentration Highway Interface. CHI is a full duplex time
division multiplexed serial interface for digitized voice transfers in
communications systems. The current specification supports data transfer
rates up to 4.096 Mbs.
Client Software resident on the host that interacts with host software to arrange data
transfer between a function and the host. The client is often the data provider
and consumer for transferred data.
COM Port Communications port. On personal computers, an eight-bit asynchronous
serial port is typically used.
Configuring
Software The host software responsible for configuring a Universal Serial Bus device.
This may be a system configurator or software specific to the device.
Control Pipe Same as a message pipe.
Control Transfer One of four Universal Serial Bus Transfer Types. Control transfers support
configuration/command/status type communications between client and
function.
CRC See Cyclic Redundancy Check.
CTI Computer Telephony Integration.
Cyclic Redundancy
Check A check performed on data to see if an error has occurred in transmitting,
reading, or writing the data. The result of a CRC is typically stored or
transmitted with the checked data. The stored or transmitted result is
compared to a CRC calculated for the data to determine if an error has
occurred.
Default Address An address defined by the Universal Serial Bus Specification and used by a
Universal Serial Bus device when it is first powered or reset. The default
address is 00h.
Default Pipe The message pipe created by Universal Serial Bus system software to pass
control and status information between the host and a Universal Serial Bus
device’s Endpoint 0.
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Device A logical or physical entity that performs a function. The actual entity
described depends on the context of the reference. At the lowest level,
device may refer to a single hardware component, as in a memory device. At
a higher level, it may refer to a collection of hardware components that
perform a particular function, such as a Universal Serial Bus interface
device. At an even higher level, device may refer to the function performed
by an entity attached to the Universal Serial Bus; for example, a data/FAX
modem device. Devices may be physical, electrical, addressable, and
logical.
When used as a non-specific reference, a Universal Serial Bus device is
either a hub or a function.
Device Address The address of a device on the Universal Serial Bus. The Device Address is
the Default Address when the Universal Serial Bus device is first powered or
reset. Hubs and functions are assigned a unique Device Address by
Universal Serial Bus software.
Device Endpoint A uniquely identifiable portion of a Universal Serial Bus device that is the
source or sink of information in a communication flow between the host and
device.
Device Resources Resources provided by Universal Serial Bus devices, such as buffer space
and endpoints. See Host Resources and Universal Serial Bus Resources.
Device Software Software that is responsible for using a Universal Serial Bus device. This
software may or may not also be responsible for configuring the device for
use.
DMI An acronym for Desktop Management Interface. A method for managing
host system components developed by the Desktop Management Task Force.
Downstream The direction of data flow from the host or away from the host. A
downstream port is the port on a hub electrically farthest from the host that
generates downstream data traffic from the hub. Downstream ports receive
upstream data traffic.
Driver When referring to hardware, an I/O pad that drives an external load. When
referring to software, a program responsible for interfacing to a hardware
device; that is, a device driver.
DWORD Double word. A data element that is 2 words, 4 bytes, or 32 bits in size.
Dynamic Insertion
and Removal The ability to attach and remove devices while the host is in operation.
E2PROM See EEPROM.
EEPROM Electrically Erasable Programmable Read Only Memory. Non-volatile
rewritable memory storage technology.
End User The user of a host.
Endpoint See Device Endpoint.
Endpoint Address The combination of a Device Address and an Endpoint Number on a
Universal Serial Bus device.
Endpoint Number A unique pipe endpoint on a Universal Serial Bus device.
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EOF1 End of frame timing point #1. Used by the hub to monitor and disconnect
bus activity persisting near or past the end of a frame.
EOF2 End of frame timing point #2. Used by hubs to detect bus activity near the
end of frame.
EOP End of packet.
Fs See Sample Rate.
False EOP A spurious, usually noise induced, event that is interpreted by a packet
receiver as an end of packet.
FireWire Apple Computer’s implementation of the IEEE P1394 bus standard.
Frame The time from the start of one SOF token to the start of the subsequent SOF
token; consists of a series of transactions.
Frame Pattern A sequence of frames that exhibit a repeating pattern in the number of
samples transmitted per frame. For a 44.1 kHz audio transfer, the frame
pattern could be nine frames containing 44 samples followed by one frame
containing 45 samples.
Full-duplex Computer data transmission occurring in both directions simultaneously.
Function A Universal Serial Bus device that provides a capability to the host. For
example, an ISDN connection, a digital microphone, or speakers.
GeoPort A serial bus developed by Apple Computer, Inc. Current specification of the
GeoPort supports data transfer rates up to 2 Mbs and provides point to point
connectivity over a radius of 4 ft.
Handshake Packet A packet that acknowledges or rejects a specific condition. For examples,
see ACK and NACK.
Host The host computer system where the Universal Serial Bus host controller is
installed. This includes the host hardware platform (CPU, bus, etc.) and the
operating system in use.
Host Controller The host’s Universal Serial Bus interface.
Host Controller
Driver The Universal Serial Bus software layer that abstracts the host controller
hardware. Host Controller Driver provides an SPI for interaction with a host
controller. Host Controller Driver hides the specifics of the host controller
hardware implementation.
Host Resources Resources provided by the host, such as buffer space and interrupts. See
Device Resources and Universal Serial Bus Resources.
Hub A Universal Serial Bus device that provides additional connections to the
Universal Serial Bus.
Hub Tier The level of connect within a USB network topology given as the number of
hubs that that the data has to flow through.
I2CAcronym for the Inter-Integrated Circuits serial interface. The I2C interface
was invented by Philips Semiconductors.
IEEE P1394 A high performance serial bus. The P1394 is targeted at hard disk and video
peripherals, which may require bus bandwidth in excess of 100 Mb/s. The
bus protocol supports both isochronous and asynchronous transfers over the
same set of four signal wires.
Universal Serial Bus Specification Revision 1.0
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Industry Standard
Architecture The 8 and/or 16 bit expansion bus for IBM AT or XT compatible computers.
Integrated Services
Data Network An internationally accepted standard for voice, data, and signaling using
public, switched telephone networks. All transmissions are digital from end-
to-end. Includes a standard for out-of-band signaling and delivers
significantly higher bandwidth than POTS.
Interrupt Request A hardware signal that allows a device to request attention from a host. The
host typically invokes an interrupt service routine to handle the condition
which caused the request.
Interrupt Transfer One of four Universal Serial Bus Transfer Types. Interrupt transfers
characteristics are small data, non periodic, low frequency, bounded latency,
device initiated communication typically used to notify the host of device
service needs.
IRQ See Interrupt Request.
ISA See Industry Standard Architecture.
ISDN See Integrated Services Data Network.
Isochronous Data A stream of data whose timing is implied by its delivery rate.
Isochronous Device An entity with isochronous endpoints, as defined in the USB specification,
that sources or sinks sampled analog streams or synchronous data streams.
Isochronous Sink
Endpoint An endpoint that is capable of consuming an isochronous data stream.
Isochronous Source
Endpoint An endpoint that is capable of producing an isochronous data stream.
Isochronous
Transfer One of four Universal Serial Bus Transfer Types. Isochronous transfers are
used when working with isochronous data. Isochronous transfers provide
periodic, continuous communication between host and device.
Jitter A tendency toward lack of synchronization caused by mechanical or
electrical changes. More specifically, the phase shift of digital pulses over a
transmission medium.
kbs Transmission rate expressed in kilobits per second.
kBs Transmission rate expressed in kilobytes per second.
Line Printer Port A port used to access a printer. On most personal computers, an eight-bit
parallel interface is typically used.
Little Endian Method of storing data that places the least significant byte of multiple byte
values at lower storage addresses. For example, a word stored in little endian
format places the least significant byte at the lower address and the most
significant byte at the next address. See Big Endian.
LOA Loss of bus activity characterized by a start of packet without a
corresponding end of packet.
LPT Port See Line Printer Port.
LSB Least Significant Bit.
Mbs Transmission rate expressed in megabits per second.
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MBs Transmission rate expressed in megabytes per second.
Message Pipe A pipe that transfers data using a request/data/status paradigm. The data has
an imposed structure which allows requests to be reliably identified and
communicated.
Micro Channel
Architecture A 32 bit expansion bus used on some IBM PS/2 compatible computers.
Modem An acronym for Modulator/Demodulator. Component that converts signals
between analog and digital. Typically used to send digital information from
a computer over a telephone network which is usually analog.
MSB Most Significant Bit.
NACK Negative Acknowledgment. Handshake packet indicating a negative
acknowledgment.
Non Return to Zero
Invert A method of encoding serial data in which ones and zeroes are represented
by opposite and alternating high and low voltages where there is no return to
zero (reference) voltage between encoded bits. Eliminates the need for clock
pulses.
NRZI See Non Return to Zero Invert.
Object Host software or data structure representing a Universal Serial Bus entity.
Packet A bundle of data organized in a group for transmission. Packets typically
contain three elements: control information (e.g., source, destination, and
length), the data to be transferred, and error detection and correction bits.
Packet Buffer The logical buffer used by a Universal Serial Bus device for sending or
receiving a single packet. This determines the maximum packet size the
device can send or receive.
Packet ID A field in a Universal Serial Bus packet that indicates the type of packet, and
by inference the format of the packet and the type of error detection applied
to the packet.
PBX See Private Branch eXchange.
PCI See Peripheral Component Interconnect.
PCMCIA See Personal Computer Memory Card Industry Association.
Peripheral
Component
Interconnect
A 32- or 64-bit, processor independent, expansion bus used on personal
computers.
Personal Computer
Memory Card
International
Association
The organization that standardizes and promotes PC Card technology.
Phase A token, data, or handshake packet; a transaction has three phases.
Physical Device A device that has a physical implementation; e.g. speakers, microphones, and
CD players.
PID See Packet ID.
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Pipe A logical abstraction representing the association between an endpoint on a
device and software on the host. A pipe has several attributes; for example, a
pipe may transfer data as streams (Stream Pipe) or messages (Message Pipe).
Plain Old
Telephone Service Basic service supplying standard single line telephones, telephone lines, and
access to public switched networks.
Plug and Play A technology for configuring I/O devices to use non-conflicting resources in
a host. Resources managed by Plug and Play include I/O address ranges,
memory address ranges, IRQs, and DMA channels.
PnP See Plug and Play.
Polling Asking multiple devices, one at a time, if they have any data to transmit.
POR See Power On Reset.
Port Point of access to or from a system or circuit. For Universal Serial Bus, the
point where a Universal Serial Bus device is attached.
POTS See Plain Old Telephone Service.
Power On Reset Restoring a storage device, register, or memory to a predetermined state
when power is applied.
PLL Phase Locked Loop. A circuit that acts as a phase detector to keep an
oscillator in phase with an incoming frequency.
Private Branch
eXchange A privately owned telephone switching system which is not regulated as part
of the public telephone network.
Programmable
Data Rate Either a fixed data rate (single frequency endpoints), a limited number of
data rates (32 kHz, 44.1 kHz, 48 kHz, …), or a continuously programmable
data rate. The exact programming capabilities of an endpoint must be
reported in the appropriate class-specific endpoint descriptors.
Protocol A specific set of rules, procedures, or conventions relating to format and
timing of data transmission between two devices.
RA See Rate Adaptation.
Rate Adaptation The process by which an incoming data stream, sampled at Fsi is converted
to an outgoing data stream, sampled at Fso with a certain loss of quality,
determined by the rate adaptation algorithm. Error control mechanisms are
required for the process. Fsi and Fso can be different and asynchronous. Fsi
is the input data rate of the RA; Fso is the output data rate of the RA.
Request A request made to a Universal Serial Bus device contained within the data
portion of a SETUP packet.
Retire The action of completing service for a transfer and notifying the appropriate
software client of the completion.
Root Hub A Universal Serial Bus hub directly attached to the host controller. This hub
is attached to the host; tier 0.
Root Port The upstream port on a hub.
Sample The smallest unit of data on which an endpoint operates; a property of an
endpoint.
Sample Rate (Fs) The number of samples per second, expressed in Hertz.
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Sample Rate
Conversion A dedicated implementation of the RA process for use on sampled analog
data streams. The error control mechanism is replaced by interpolating
techniques.
SCSI See Small Computer Systems Interface.
Service A procedure provided by an SPI.
Service Interval The period between consecutive requests to a Universal Serial Bus endpoint
to send or receive data.
Service Jitter The deviation of service delivery from its scheduled delivery time.
Service Rate The number of services to a given endpoint per unit time.
Small Computer
Systems Interface A local I/O bus that allows peripherals to be attached to a host using generic
system hardware and software.
SOF An acronym for Start of Frame. The SOF is the first transaction in each
frame. SOF allows endpoints to identify the start of frame and synchronize
internal endpoint clocks to the host.
SPI See System Programming Interface.
SRC See Sample Rate Conversion.
Stage One part of the sequence composing a control transfer; i.e., the setup stage,
the data stage, and the status stage.
Stream Pipe A pipe that transfers data as a stream of samples with no defined Universal
Serial Bus structure.
Synchronization
Type A classification that characterizes an isochronous endpoint’s capability to
connect to other isochronous endpoints.
Synchronous RA The incoming data rate, Fsi, and the outgoing data rate, Fso, of the RA process
are derived from the same master clock. There is a fixed relation between
Fsi and Fso.
Synchronous SRC The incoming sample rate, Fsi, and outgoing sample rate, Fso, of the SRC
process are derived from the same master clock. There is a fixed relation
between Fsi and Fso.
System
Programming
Interface
A defined interface to services provided by system software.
TDM See Time Division Multiplexing.
Termination Passive components attached at the end of cables to prevent signals from
being reflected or echoed.
Time Division
Multiplexing A method of transmitting multiple signals (data, voice, and/or video)
simultaneously over one communications medium by interleaving a piece of
each signal one after another.
Time-out The detection of a lack of bus activity for some predetermined interval.
Token Generator See Initiator.
Token Packet A type of packet that identifies what transaction is to be performed on the
bus.
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Transaction The delivery of service to an endpoint; consists of a token packet, optional
data packet, and optional handshake packet. Specific packets are
allowed/required based on the transaction type.
Transfer One or more bus transactions to move information between a software client
and its function.
Transfer Type Determines the characteristics of the data flow between a software client and
its function. Four Transfer types are defined: control, interrupt, bulk, and
isochronous.
Turnaround Time The time a device needs to wait to begin transmitting a packet after a packet
has been received to prevent collisions on Universal Serial Bus. This time is
based on the length and propagation delay characteristics of the cable and the
location of the transmitting device in relation to other devices on Universal
Serial Bus.
Universal Serial
Bus A collection of Universal Serial Bus devices and the software and hardware
that allow them to connect the capabilities provided by functions to the host.
Universal Serial
Bus Device Includes hubs and functions. See device.
Universal Serial
Bus Interface The hardware interface between the Universal Serial Bus cable and a
Universal Serial Bus device. This includes the protocol engine required for
all Universal Serial Bus devices to be able to receive and send packets.
Universal Serial
Bus Resources Resources provided by Universal Serial Bus, such as bandwidth and power.
See Device Resources and Host Resources.
Universal Serial
Bus Software The host-based software responsible for managing the interactions between
the host and the attached Universal Serial Bus devices.
USB See Universal Serial Bus.
USBD See Universal Serial Bus Driver.
Universal Serial
Bus Driver The host resident software entity responsible for providing common services
to clients that are manipulating one or more functions on one or more Host
Controllers.
Upstream The direction of data flow towards the host. An upstream port is the port on
a device electrically closest to the host that generates upstream data traffic
from the hub. Upstream ports receive downstream data traffic.
Virtual Device A device that is represented by a software interface layer; e.g., a hard disk
with its associated device driver and client software that makes it able to
reproduce an audio .WAV file.
WFEOF2 Wait for EOF2 point. One of the four hub repeater states.
WFEOP Wait for end of packet. One of the four hub repeater states.
WFSOF Wait for start of frame. One of the four hub repeater states.
WFSOP Wait for start of packet. One of the four possible hub repeater states.
Word A data element that is two bytes or 16 bits in size.
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Chapter 3
Background
This chapter presents a brief description of the background of the Universal Serial Bus including design
goals, features of the bus, and existing technologies.
3.1 Goals for the Universal Serial Bus
The Universal Serial Bus is specified to be an industry standard extension to the PC architecture with a
focus on Computer Telephony Integration (CTI), consumer, and productivity applications. The following
criteria were applied in defining the architecture for the Universal Serial Bus:
• Ease of use for PC peripheral expansion
• Low-cost solution that supports transfer rates up to 12 Mbs
• Full support for the real-time data for voice, audio, and compressed video
• Protocol flexibility for mixed-mode isochronous data transfers and asynchronous messaging
• Integration in commodity device technology
• Comprehend various PC configurations and form factors
• Provide a standard interface capable of quick diffusion into product
• Enable new classes of devices that augment the PC’s capability
3.2 Taxonomy of Application Space
Figure 3-1 describes a taxonomy for the range of data traffic workloads that can be serviced over a
Universal Serial Bus. As can be seen, a 12 Mbs bus comprehends the mid-speed and low-speed data
ranges. Typically, mid-speed data types are isochronous and low-speed data comes from interactive
devices. The Universal Serial Bus being proposed is primarily a desktop bus but can be readily applied
to the mobile environment. The software architecture allows for future extension of the Universal Serial
Bus by providing support for multiple Universal Serial Bus host controllers.
Universal Serial Bus Specification Revision 1.0
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LOW SPEED
•Interactive Devices
•10-100 Kb/s
MEDIUM SPEED
•Phone, Audio,
Compressed Video
500Kb/s - 10Mbp/s
HIGH SPEED
•Video, Disk
•25-500 Mb/s
PERFORMANCE APPLICATIONS ATTRIBUTES
Ke y bo ard , Mo us e
Stylus
Game peripherals
Virtual Reality peripherals
Monitor Configuration
ISDN
PBX
POTS
Audio
Video
Disk
Dynamic Attach- Detach
Lower cost
H ot plug -u nplug
Ease of use
Multiple peripherals
Low cost
Ease of use
Guaranteed late nc y
Guaranteed Bandwidth
Multiple devices
High Bandwidth
Guaranteed late nc y
Ease of use
Figure 3-1. Application Space Taxonomy
3.3 Feature List
The Universal Serial Bus specification provides a selection of attributes that can achieve multiple
price-performance integration points and can enable functions that allow differentiation at the system and
component level. Features are categorized by benefits below:
Easy to use for end user
• Single model for cabling and connectors
• Electrical details isolated from end user; e.g., bus terminations
• Self identifying peripherals, automatic mapping of function to driver, and configuration
• Dynamically attachable and reconfigurable peripherals
Universal Serial Bus Specification Revision 1.0
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Wide range of workloads and applications
• Suitable for device bandwidths ranging from a few kbs to several Mbs
• Supports isochronous as well as asynchronous transfer types over the same set of wires
• Multiple Connections: Support for concurrent operation of many devices
• Support for up to 127 physical devices
• Supports transfer of multiple data and message streams between the host and devices
• Allows compound devices; i.e., peripherals composed of many functions
• Lower protocol overhead resulting in high bus utilization
Isochronous bandwidth
• Guaranteed bandwidth and low latencies appropriate for telephony, audio, etc.
• Isochronous workload may use entire bus bandwidth
Flexibility
• Wide range of packet sizes, allowing a range of device buffering options
• Wide range of device data rates by accommodating packet buffer size and latencies
• Flow control for buffer handling built into protocol
Robustness
• Error handling/fault recovery mechanism built into protocol
• Dynamic insertion and removal of devices identified in user perceived real-time
• Support for identification of faulty devices
Synergy with PC industry
• Simple protocol to implement and integrate
• Consistent with the PC Plug and Play architecture
• Leverages existing operating system interfaces
Low-cost implementation
• Low cost sub-channel at 1.5 Mbs
• Optimized for integration in peripheral and host hardware
• Suitable for development of low cost peripherals
• Low cost cables and connectors
• Utilizes commodity technologies
Universal Serial Bus Specification Revision 1.0
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Upgrade path
• Architecture upgradeable to support multiple Universal Serial Bus host controllers in a system
3.4 Some Existing Technologies
There are several technologies that are commonly considered to be serial buses. Each of these buses
were defined for a specific range of application(s). A few of them are listed below:
• Apple desktop bus (ADB)
This is a proprietary minimalist serial interface that provides a simple read/write protocol to up to 16
devices. The cost of hardware interface is estimated to be very low. The ADB supports data rates up
to 90 kbs, just enough to communicate with keyboards, pointing devices, or other desktop I/O
devices.
• Access.bus (A.b)
The Access.bus is being developed by the Access.bus Industry Group, based on the Philips I2C
technology and a DEC software model. The application space for the Access.bus is primarily
keyboards and pointing devices; however, A.b is more versatile then the ADB. The protocol has
well defined specifications for the dynamic attach, arbitration, data packets, configuration, and
software interface. While addressing is provided for up to 127 devices, the practical loading is
limited by cable lengths and power distribution considerations. Revision 2.2 of the A.b specification
specifies the bus for 100 kbs operation, but the technology has headroom to go up to 400 kbs using
the same separate clock and data wires.
• IEEE P1394
The IEEE P1394 is a high performance serial bus. The application space for P1394 is primarily hard
disk and video peripherals, which may require bus bandwidth in excess of 100 Mbs. The protocol
supports both isochronous and asynchronous transfers over the same set of four signal wires, broken
up as differential pair of clock and data signals. The P1394 specification is very well defined and the
first generation devices, based on the IEEE specification, are just coming to market. Current pricing
of P1394 solutions is considered competitive relative to SCSI disk interfaces, but not for generic
desktop connectivity.
• CHI
The Concentration Highway Interface (CHI) was developed by AT&T for terminals and digital
switches. CHI is a full duplex time division multiplexed serial interface for digitized voice transfers
in communications systems. The protocol consists of a number of fixed time slots that can carry
voice data and control information. The current specification supports data transfer rates up to
4.096 Mbs. The CHI bus has four signal wires: Clock, Framing, Receive data, and Transmit data.
Both, the Framing and the Clock signals are generated centrally (i.e., PBX switch).
• GeoPort
The GeoPort was originally developed by Apple Computer, Inc. to primarily enable Macintosh
telephony applications. Current specification of the GeoPort supports data transfer rates up to 2 Mbs
and provides point to point connectivity over a radius of 4 ft. The standard GeoPort specifies a 9-pin
connector (8 pins and an optional 9th power pin) and uses RS-422 signaling. Additionally, Apple
has defined an alternate 14-pin connector for extended cable lengths. The GeoPort protocol provides
three different operating modes: Beaconing, TDM, and Packetized transfer modes. Apple is
currently licensing the GeoPort specification.
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Chapter 4
Architectural Overview
This chapter presents an overview of the Universal Serial Bus architecture and key concepts. USB is a
cable bus that supports data exchange between a host computer and a wide range of simultaneously
accessible peripherals. The attached peripherals share USB bandwidth through a host scheduled token
based protocol. The bus allows peripherals to be attached, configured, used, and detached while the host
and other peripherals are in operation. This is referred to as dynamic (or hot) attachment and removal.
Later chapters describe the various components of the USB in greater detail.
4.1 USB System Description
A USB system is described by three definitional areas:
• USB interconnect
• USB devices
• USB host
The USB interconnect is the manner in which USB devices are connected to and communicate with the
host. This includes:
• Bus Topology: Connection model between USB devices and the host.
• Inter-layer Relationships: In terms of a capability stack, the USB tasks that are performed at each
layer in the system.
• Data Flow Models: The manner in which data moves in the system over the USB between producers
and consumers.
• Scheduling the USB: USB provides a shared interconnect. Access to the interconnect is scheduled
in order to support isochronous data transfers.
USB devices and the USB host are described in detail in subsequent sections.
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4.1.1 Bus Topology
The Universal Serial Bus connects USB devices with the USB host. The USB physical interconnect is a
tiered star topology. A hub is at the center of each star. Each wire segment is a point-to-point
connection between the host and a hub or function, or a hub connected to another hub or function.
Figure 4-1 illustrates the topology of the USB.
Host (Root Tier)
Tier 1
Tier 2
Tier 3
Tier 4
Hub 1
Hub 2 Node
Host
RootHub
Node
Hub 3 Hub 4 Node Node
Node
Node
Node
Figure 4-1. Bus Topology
4.1.1.1 The USB Host
There is only one host on any USB system. The USB interface to the host computer system is referred to
as the host controller. The host controller may be implemented in a combination of hardware, firmware,
or software. A root hub is integrated within the host system to provide one or more attachment points.
Additional information concerning the host may be found in Section 4.9 and in Chapter 10, USB Host:
Hardware and Software.
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4.1.1.2 USB Devices
USB devices are:
• Hubs, which provide additional attachment points to the USB
• Functions, which provide capabilities to the system; for example, an ISDN connection, a digital
joystick, or speakers
USB devices present a standard USB interface in terms of their:
• Comprehension of the USB protocol
• Response to standard USB operations such as configuration and reset
• Standard capability descriptive information
Additional information concerning USB devices may be found in Section 4.8 and in Chapter 9, USB
Devices.
4.2 Physical Interface
The physical interface of the USB is described in the electrical (Chapter 7) and mechanical (Chapter 6)
specifications for the bus.
4.2.1 Electrical
USB transfers signal and power over a four wire cable, shown in Figure 4-2. The signaling occurs over
two wires and point-to-point segments. The signals on each segment are differentially driven into a cable
of 90 Ω intrinsic impedance. The differential receiver features input sensitivity of at least 200 mV and
sufficient common mode rejection.
There are two modes of signaling. The USB full speed signaling bit rate is 12 Mbs. A limited capability
low speed signaling mode is also defined at 1.5 Mbs. The low speed method relies on less EMI
protection. Both modes can be simultaneously supported in the same USB system by mode switching
between transfers in a device transparent manner. The low speed mode is defined to support a limited
number of low bandwidth devices such as mice, since more general use would degrade the bus
utilization.
The clock is transmitted encoded along with the differential data. The clock encoding scheme is NRZI
with bit stuffing to ensure adequate transitions. A SYNC field precedes each packet to allow the
receiver(s) to synchronize their bit recovery clocks.
...
...
5 meters max
VBus
GND
D+
D-
VBus
GND
D+
D-
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
Figure 4-2. USB Cable
The cable also carries VBus and GND wires on each segment to deliver power to devices. VBus is
nominally +5 V at the source. USB allows cable segments of variable lengths up to several meters by
choosing the appropriate conductor gauge to match the specified IR drop and other attributes such as
device power budget and cable flexibility. In order to provide guaranteed input voltage levels and proper
termination impedance, biased terminations are used at each end of the cable. The terminations also
Universal Serial USB Specification Revision 1.0
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permit the detection of attach and detach at each port and differentiate between full speed and low speed
devices.
4.2.2 Mechanical
The mechanical specifications for cables and connectors are provided in Chapter 6. All devices have an
upstream connection. Upstream and downstream connectors are not mechanically interchangeable, thus
eliminating illegal loopback connections at hubs. The cable has four conductors: a twisted signal pair of
standard gauge and a power pair in a range of permitted gauges. The connector is four position, with
shielded housing, specified robustness, and ease of attach-detach characteristics.
4.3 Power
The specification covers two aspects of power:
• Power distribution over the USB deals with the issues of how USB devices consume power provided
by the host over the USB.
• Power management deals with how USB software and devices fit into the host-based power
management system.
4.3.1 Power Distribution
Each USB segment provides a limited amount of power over the cable. The host supplies power for use
by USB devices that are directly connected. In addition, any USB device may have its own power
supply. USB devices that rely totally on power from the cable are called bus-powered devices. In
contrast, those that have an alternate source of power are called self-powered devices. A hub also
supplies power for its connected USB devices. The architecture permits bus-powered hubs within certain
constraints of topology that are discussed later in Chapter 11. Self-powered devices must implement
prescribed power decoupling safety mechanisms. In Figure 4-4, the keyboard, pen, and mouse can all be
bus-powered devices.
4.3.2 Power Management
A USB host has a power management system which is independent of the USB. USB system software
interacts with the host’s power management system to handle system power events such as SUSPEND or
RESUME. Additionally, USB devices can carry USB-defined power management information which
allow them to be power managed by system software or generic device drivers.
The power distribution and power management features of USB allow it to be designed into power
sensitive systems such as battery based notebook computers.
4.4 Bus Protocol
All bus transactions involve the transmission of up to three packets. Each transaction begins when the
host controller, on a scheduled basis, sends a USB packet describing the type and direction of transaction,
the USB device address, and endpoint number. This packet is referred to as the Token Packet. The USB
device that is addressed selects itself by decoding the appropriate address fields. In a given transaction,
data is transferred either from the host to a device or from a device to the host. The direction of data
transfer is specified in the token packet. The source of the transaction then sends a Data Packet or
indicates it has no data to transfer. The destination in general responds with a Handshake Packet
indicating whether the transfer was successful.
The USB data transfer model between a source or destination on the host and an endpoint on a device is
referred to as a pipe. There are two types of pipes: stream and message. Stream data has no USB
defined structure while message data does. Additionally, pipes have associations of data bandwidth,
Universal Serial Bus Specification Revision 1.0
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transfer service type, and endpoint characteristics like directionality and buffer sizes. Pipes come into
existence when a USB device is configured. One message pipe, Control Pipe 0, always exists once a
device is powered in order to provide access to the device’s configuration, status, and control
information.
The transaction schedule allows flow control for some stream mode pipes. At the hardware level, this
prevents buffers from underrun or overrun situations by using a NACK handshake to throttle the data
rate. The token for a NACK’ed transaction is reissued when bus time is available. The flow control
mechanism permits the construction of flexible schedules that accommodate concurrent servicing of a
heterogeneous mix of stream mode pipes. Thus, multiple stream mode pipes can be serviced at different
intervals and with packets of different sizes.
4.5 Robustness
There are several attributes of the USB that contribute to its robustness:
• Signal integrity using differential drivers, receivers, and shielding
• CRC protection over control and data fields
• Detection of attach and detach and system-level configuration of resources
• Self-recovery in protocol, using time-outs for lost or broken packets
• Flow control for streaming data to ensure isochrony and hardware buffer management
• Data and control pipe constructs for ensuring independence from adverse interactions between
functions
4.5.1 Error Detection
The core bit error rate of the USB medium is expected to be close to that of a backplane and any glitches
will very likely be transient in nature. To provide protection against such transients, each of these
packets includes error protection fields. When data integrity is required, such as with lossless data
devices, an error recovery procedure may be invoked in hardware or software.
The protocol includes separate CRCs for control and data fields of each packet. A failed CRC is
considered to indicate a corrupted packet. The CRC gives 100% coverage on single and double bit
errors.
4.5.2 Error Handling
The protocol optionally allows for error handling in hardware or software. Hardware handling includes
reporting and retry of failed transfers. The host controller will retry an error three times before informing
the client software of the error. The client software can recover in an implementation specific way.
4.6 System Configuration
The USB supports USB devices attaching to and detaching from the USB at any point in time.
Consequently, enumerating the USB is an on-going activity which must accommodate dynamic changes
in the physical bus topology.
4.6.1 Attachment of USB Device
All USB devices attach to the USB via a port on specialized USB devices known as hubs. Hubs indicate
the attachment or removal of a USB device in its per port status. The host queries the hub to determine
the reason for the notification. The hub responds by identifying the port used to attach the USB device.
The host enables the port and addresses the USB device with a control pipe using the USB Default
Universal Serial USB Specification Revision 1.0
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Address. All USB devices are addressed using the USB Default Address when initially connected or
after they have been reset.
The host determines if the newly attached USB device is a hub or a function and assigns a unique USB
address to the USB device. The host establishes a control pipe for the USB device using the assigned
USB address and endpoint number zero.
If the attached USB device is a hub and USB devices are attached to its ports, then the above procedure is
followed for each of the attached USB devices.
If the attached USB device is a function, then attachment notifications will be dispatched by USB
software to interested host software.
4.6.2 Removal of USB Device
When a USB device has been removed from one of its ports, the hub automatically disables the port and
provides an indication of device removal to the host. Then the host removes knowledge of the USB
device from any host data structures.
If the removed USB device is a hub, the removal process must be performed for all of the USB devices
that were previously attached to the hub.
If the removed USB device is a function, removal notifications are sent to interested host software.
4.6.3 Bus Enumeration
Bus enumeration is the activity that identifies and addresses devices attached to a bus. For many buses,
this is done at startup time and the information collected is static. Since the USB allows USB devices to
attach to or detach from the USB at any time, bus enumeration for this bus is an on-going activity.
Additionally, bus enumeration for the USB also includes detection and processing of removals.
4.6.4 Inter-Layer Relationship
USB devices are logically divided into a USB device interface portion, a device portion, and a functional
portion. The host is logically partitioned into the USB host interface portion, the aggregate system
software portion (USB system software and host system software), and the device software portion.
Each of these portions is defined such that a particular USB task is the responsibility of only one portion.
The USB host and USB device portions correspond as shown in Table 4-1.
Table 4-1. Correlation Between Host and Device Layers
USB Host Portion USB Device Portion
Device Software Function
System Software Device
USB Interface USB Interface
4.7 Data Flow Types
The USB supports functional data and control exchange between the USB host and a USB device as a set
of either uni- or bi-directional fashions. USB data transfers take place between host software and a
particular endpoint on a USB device. A given USB device may support multiple data transfer endpoints.
The USB host treats communications with any endpoint of a USB device independently from any other
endpoint. Such associations between the host software and a USB device endpoint are called pipes. As
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an example, a given USB device could have an endpoint which would support a pipe for transporting data
to the USB device and another endpoint which would support a pipe for transporting data from the USB
device.
The USB architecture comprehends four basic types of data transfers:
• Control transfers that are used to configure a device at attach time and can be used for other device
specific purposes
• Bulk data transfers which are generated or consumed in relatively large and bursty quantities and has
wide dynamic latitude in transmission constraints
• Interrupt data transfers such as characters or coordinates with human perceptible echo or feedback
response characteristics
• Isochronous or streaming real time data transfers which occupy a prenegotiated amount of USB
bandwidth with a prenegotiated delivery latency
Any given pipe supports exactly one of the types of transfers described above. The USB data flow model
is described in more detail in Chapter 5.
4.7.1 Control Transfers
Control data is used by USB software to configure devices when they are first attached. Other driver
software can choose to used control transfers in implementation specific ways. Data delivery is lossless.
4.7.2 Bulk Transfers
Bulk data typically consists of larger amounts of data such as that used for printers or scanners. Bulk
data is sequential. Reliable exchange of data is ensured at the hardware level by using error detection in
hardware and, optionally, invoking a limited hardware retry. Also, the bandwidth taken up by bulk data
can be whatever is available and not being used for other transfer types.
4.7.3 Interrupt Transfers
A small, spontaneous data transfer from a device is referred to as interrupt data. Such data may be
presented for transfer by a device at any time and is delivered by the USB at a rate no slower than as is
specified by the device.
Interrupt data typically consists of event notification, characters, or coordinates that are organized as one
or more bytes. An example of interrupt data is the coordinates from a pointing device. Although an
explicit timing rate is not required, interactive data may have response time bounds which the USB must
support.
4.7.4 Isochronous Transfers
Isochronous data is continuous and real-time in creation, delivery, and consumption. Timing related
information is implied by the steady rate at which isochronous data is received and transferred.
Isochronous data must be delivered at the rate received to maintain its timing. In addition to delivery
rate, isochronous data may also be sensitive to delivery delays. For isochronous pipes, the bandwidth
required is typically based upon the sampling characteristics of the associated function. The latency
required is related to the buffering available at each endpoint.
A typical example of isochronous data is voice. If the delivery rate of these data streams is not
maintained, glitches in the data stream will occur due to buffer or frame underruns or overruns. Even if
data is delivered at the appropriate rate, delivery delays may degrade applications requiring real-time turn
around, such as telephony-based audio conferencing.
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The timely delivery of isochronous data is ensured at the expense of potential transient losses in the data
stream. In other words, any error in electrical transmission is not corrected by hardware mechanisms
such as retries. In practice, the core bit error rate of the USB is expected to be small enough not to be an
issue. USB isochronous data streams are allocated a dedicated portion of USB bandwidth to ensure that
data can be delivered at the desired rate. The USB is also designed for minimal delay of isochronous
data transfers.
4.7.5 Allocating USB Bandwidth
USB bandwidth is allocated among pipes. The USB allocates bandwidth for some pipes when a pipe is
established. USB devices are required to provide some buffering of data. It is assumed that USB devices
requiring more bandwidth are capable of providing larger sized buffers. The goal for the USB
architecture is to ensure that buffering induced hardware delay is bounded to within a few milliseconds.
USB’s bandwidth capacity can be allocated among many different data streams. This allows a wide
range of devices to be attached to the USB. For example, telephony devices ranging from 1B+D all the
way up to T1 capacity can be accommodated. Further, different device bit rates, with a wide dynamic
range, can be concurrently supported.
USB bandwidth allocation is blocking; i.e., if allocating an additional pipe would disturb preexisting
bandwidth or latency allocations, further pipe allocations are denied or blocked. When a pipe is closed,
the allocated bandwidth is freed up and may be reallocated to another pipe.
The USB Specification defines the rules for how each transfer type is allowed access to the bus.
4.8 USB Devices
USB devices are divided into device classes such as hub, locator, or text device. The hub device class
indicates a specially designated USB device which provides additional USB attachment points (refer to
Chapter 11). USB devices are required to carry information for self-identification and generic
configuration. They are also required at all times to display behavior consistent with defined USB device
states.
4.8.1 Device Characterizations
All USB devices are accessed by a unique USB address. Each USB device additionally supports one or
more endpoints with which the host may communicate. All USB devices must support a specially
designated Endpoint 0 to which the USB device’s USB control pipe will be attached.
Associated with Endpoint 0 is the information required to completely describe the USB device. This
information falls into the following categories:
• Standard. This is information whose definition is common to all USB devices and includes items
such as vendor identification, device class, and power management. Device, configuration,
interface, and endpoint descriptions carry configuration related information about the device.
Detailed information about these descriptors can be found in Chapter 9.
• Class. The definition of this information varies depending on the device class of the USB device.
• USB Vendor. The vendor of the USB device is free to put any information desired here. The
format, however, is not determined by this specification.
Additionally, each USB device carries USB control and status information. All USB devices support a
common access method via their USB control pipe.
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4.8.2 Device Descriptions
Two major divisions of device classes exist: hubs and functions. Only hubs have the ability to provide
additional USB attachment points. Functions provide additional capabilities to the host.
4.8.2.1 Hubs
Hubs are a key element in the plug-and-play architecture of USB. Figure 4-3 shows a typical hub. Hubs
serve to simplify USB connectivity from the user’s perspective and provide robustness at low cost and
complexity.
Hubs are wiring concentrators and enable the multiple attachment characteristics of USB. Attachment
points are referred to as ports. Each hub converts a single attachment point into multiple attachment
points. The architecture supports concatenation of multiple hubs.
The upstream port of a hub connects the hub towards the host. Each of the other downstream ports of a
hub allows connection to another hub or function. Hubs can detect attach and detach at each downstream
port and enable the distribution of power to downstream devices. Each downstream port can be
individually enabled and configured as either full or low speed. The hub isolates low speed ports from
full speed signaling.
A hub consists of two portions: the Hub Controller and the Hub Repeater. The repeater is a protocol
controlled switch between the upstream port and downstream ports. It also has hardware support for reset
and suspend/resume signaling. The controller provides the interface registers to allow communication
to/from the host. Hub specific status and control commands permit the host to configure a hub and to
monitor and control its ports.
HUB
Upstream
Port
Port
#1
Port
#7 Port
#6 Port
#5
Port
#4
Port
#2 Port
#3
Figure 4-3. A Typical Hub
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Figure 4-4 illustrates how hubs provide connectivity in a desktop computer environment.
Keyboard Monitor PC
Pen Mouse Speaker Mic Phone Hub
Host/HubHub/FunctionHub/Function
Function Function Function Function Function Hub
Figure 4-4. Hubs in a Desktop Computer Environment
4.8.2.2 Functions
A function is a USB device that is able to transmit or receive data or control information over the bus. A
function is typically implemented as a separate peripheral device with a cable that plugs into a port on a
hub. However, a physical package may implement multiple functions and an embedded hub with a
single USB cable. This is known as a compound device. A compound device appears to the host as a
hub with one or more permanently attached USB devices.
Each function contains configuration information that describes its capabilities and resource
requirements. Before a function can be used, it must be configured by the host. This configuration
includes allocating USB bandwidth and selecting function specific configuration options.
Examples of functions are:
• A locator device such as a mouse, tablet, or light pen
• An input device such as a keyboard
• An output device such as a printer
• A telephony adapter such as ISDN
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4.9 USB Host: Hardware and Software
The USB Host interacts with USB devices through the host controller. The host is responsible for the
following:
• Detecting the attachment and removal of USB devices
• Managing control flow between the host and USB devices
• Managing data flow between the host and USB devices
• Collecting status and activity statistics
• Providing a limited amount of power to attached USB devices
USB system software on the host manages interactions between USB devices and host-based device
software. There are five areas of interactions between USB system software and device software, they
are:
• Device enumeration and configuration
• Isochronous data transfers
• Asynchronous data transfers
• Power management
• Device and bus management information
Whenever possible, USB software uses existing host system interfaces to manage the above interactions.
For example, if a host system uses Advanced Power Management (APM) for power management, USB
system software connects to the APM message broadcast facility to intercept suspend and resume
notifications.
4.10 Architectural Extensions
The USB architecture comprehends extensibility at the interface between the Host Controller Driver and
USB driver. Implementations with multiple host controllers, and associated Host Controller Drivers, are
possible.
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Chapter 5
USB Data Flow Model
This chapter presents information about how data is moved across the USB that affects all implementers.
The information presented is at a level above the signaling and protocol definitions of the system.
Chapter 7, Electrical and Chapter 8, Protocol Layer should be consulted for more details about their
respective parts of the USB system. This chapter provides framework information that is further
expanded in Chapter 9, USB Device Framework, Chapter 10, USB Host: Hardware and Software, and
Chapter 11, Hub Specification. This chapter should be read by all implementers to understand key
concepts of the USB.
5.1 Implementer Viewpoints
The USB provides communication services between a host and attached USB devices. However, the
simple view an end user sees of attaching one or more USB devices to a host, as in Figure 5-1, is in fact a
little more complicated to implement than as indicated by the figure. Different views of the system are
required to explain specific USB requirements from the perspective of different implementers. Several
important concepts and features must be supported to provide the end user with the reliable operation
demanded from today’s personal computers. USB is presented in a layered fashion to ease explanation
and allow implementers of particular USB products to focus on the details related to their product.
USB Hos t USB Devi ce
Figure 5-1. Simple USB Host/Device View
Figure 5-2 shows a deeper overview of USB identifying the different layers of the system that will be
described in more detail in the remainder of the specification. In particular, there are four focus
implementation areas:
• USB Physical Device - A piece of hardware on the end of a USB cable that performs some useful
end user function.
• Client Software - Software that executes on the host corresponding to a USB device. This client
software is typically supplied with the operating system or provided along with the USB device.
• USB System Software - Software that supports USB in a particular operating system. Typically
supplied with the operating system independently of particular USB devices or client software.
• USB Host Controller (Host Side Bus Interface) - The hardware and software that allows USB devices
to be attached to a host.
There are shared rights and responsibilities between the four USB system components. The remainder of
this specification describes the details required to support robust, reliable communication flows between
a function and its client.
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Client SW
USB Host
Controller
USB Logical
Device
Function
Host Physical DeviceInterconnect
USB Bus
Interface
USB System SW
Actual communications flow
Logical communications flow
Implementation Focus Area
Function Layer
USB Device
Layer
USB Bus
Interface Layer
Figure 5-2. USB Implementation Areas
As shown in Figure 5-2, the simple connection of a host to a device requires interaction between a
number of layers and entities. The USB Bus Interface layer provides physical/signaling/packet
connectivity between the host and a device. The USB Device Layer is the view the USB System
software has for performing generic USB operations with a device. The Function Layer provides
additional capabilities to the host via an appropriate matched client software layer. The USB Device and
Function layers each have a view of logical communication within their layer that actually uses the USB
Bus Interface Layer to accomplish data transfer.
The physical view of USB communication as described in Chapters 6, 7, and 8 is related to the logical
communication view presented in Chapters 9 and 10. This chapter describes those key concepts that
affect USB implementers and should be read by all before proceeding to the remainder of the
specification to find those details most relevant to their product.
To describe and manage USB communication, the following concepts are important:
• Bus Topology: Section 5.2 presents the primary physical and logical components of USB and how
they interrelate.
• Communication Flow Models: Sections 5.3 through 5.8 describe how communication flows
between the host and devices through the USB and defines the four USB transfer types.
• Bus Access Management: Section 5.9 describes how bus access is managed within the host to
support a broad range of communication flows by USB devices.
• Special Consideration for Isochronous Transfers: Section 5.10 presents features of USB specific to
devices requiring isochronous data transfers. Device implementers for non-isochronous devices will
not need to read Section 5.10.
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5.2 Bus Topology
There are four main parts to USB topology:
• Host and Devices: The primary components of a USB system.
• Physical Topology: How USB elements are connected.
• Logical Topology: The roles and responsibilities of the various USB elements and how the USB
appears from the perspective of the host and a device.
• Client software to function relationships: How client software and its related function interfaces on a
USB device view each other.
5.2.1 USB Host
The host’s logical composition as shown in Figure 5-3 is:
• The USB host controller
• The aggregate USB system software (USB driver, host controller driver, and host software)
• The client
Client SW
USB Host
Controller
Host
USB System SW
Actual communications flow
Logical communications flow
Figure 5-3. Host Composition
The USB host occupies a unique position as the coordinating entity for the USB. In addition to its special
physical position, the host has specific responsibilities with regard to the USB and its attached devices.
The host controls all access to the USB. A USB device only gains access to the bus by being granted
access by the host. The host is also responsible for monitoring the topology of the USB.
For a complete discussion of the host and its duties, refer to Chapter 10, USB Host: Software and
Hardware.
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5.2.2 USB Devices
A USB physical device’s logical composition as shown in Figure 5-4 is:
• USB bus interface
• USB logical device
• Function
USB physical devices provide additional functionality to the host. The types of functionality provided by
USB devices vary widely. However, all USB logical devices present the same basic interface to the host.
This allows the host to manage the USB-relevant aspects of different USB devices in the same manner.
To assist the host in identifying and configuring USB devices, each device carries and reports
configuration related information. Some of the information reported is common among all logical
devices. Other information is specific to the functionality provided by the device. The detailed format of
this information varies depending on the device class of the device.
For a complete discussion of USB devices, refer to Chapter 9, USB Device Framework.
USB Logical
Device
Function
Physical Device
USB Bus
Interface
Actual communications flow
Logical communications flow
Figure 5-4. Physical Device Composition
5.2.3 Physical Bus Topology
Devices on the USB are physically connected to the host via a tiered star topology, as illustrated in
Figure 5-5. USB attachment points are provided by a special class of USB device known as a hub. The
additional attachment points provided by a hub are called ports. A host includes an embedded hub called
the root hub. The host provides one or more attachment points via the root hub. USB devices which
provide additional functionality to the host are known as functions. To prevent circular attachments, a
tiered ordering is imposed on the star topology of the USB. This results in the tree-like configuration
illustrated in Figure 5-5.
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RootHUB
HUB
HUB
Device
Compound device
HOST
Device
Device
Device
Device
Device
Device
Figure 5-5. USB Physical Bus Topology
Multiple functions may be packaged together in what appears to be a single physical device. For
example, a keyboard and a trackball might be combined in a single package. Inside the package, the
individual functions are permanently attached to a hub and it is the internal hub that is connected to the
USB. When multiple functions are combined with a hub in a single package, they are referred to as a
compound device. From the host’s perspective, a compound device is the same as a separate hub with
multiple functions attached. Figure 5-5 also illustrates a compound device.
5.2.4 Logical Bus Topology
While devices physically attach to the USB in a tiered, star topology, the host communicates with each
logical device as if it were directly connected to the root port. This creates the logical view illustrated in
Figure 5-6 that corresponds to the physical topology shown in Figure 5-5. Hubs are logical devices also,
but are not shown in Figure 5-6 to simplify the picture. Even though most host/logical device activities
use this logical perspective, the host maintains an awareness of physical topology to support processing
the removal of hubs. When a hub is removed, all of the devices attached to the hub must be removed
from the host’s view of the logical topology. A more complete discussion of hubs can be found in
Chapter 11, Hub Specification.
Host
Logical
Device
Logical
Device
Logical
Device
Logical
Device Logical
Device
Logical
Device
Logical
Device
Figure 5-6. USB Logical Bus Topology
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5.2.5 Client Software to Function Relationship
Even though the physical and logical topology of the USB reflects the shared nature of the bus, client
software(CSw) manipulating a USB function interface is presented with the view that it deals only with
its interface(s) of interest. Client software for USB functions must use USB software programming
interfaces to manipulate their functions as opposed to directly manipulating their functions via memory
or I/O accesses as with other buses (e.g., PCI, EISA, PCMCIA, etc.). During operation, client software
should be independent of other devices that may be connected to USB. This allows the designer of the
device and client software to focus on the hardware/software interaction design details. Figure 5-7
illustrates a device designer’s perspective of the relationships of client software and USB functions with
respect to the USB logical topology of Figure 5-6.
Client
Software
Func Func Func
Func
Func
Func Func
CSw
CSw
CSw
CSw
CSw
CSw
Figure 5-7. Client Software to Function Relationships
5.3 USB Communication Flow
USB provides a communication service between software on the host and its USB function. Functions
can have different communication flow requirements for different client to function interactions. USB
provides better overall bus utilization by allowing the separation of the different communication flows to
a USB function. Each communication flow makes use of some bus access to accomplish communication
between client and function. Each communication flow is terminated at an endpoint on a device. Device
endpoints are used to identify aspects of each communication flow.
The diagram in Figure 5-8 shows a more detailed view of Figure 5-2. The complete definition of the
actual communication flows of Figure 5-2 supports the logical device and function layer communication
flows. These actual communication flows cross several interface boundaries. Chapters 6, 7, and 8
describe the mechanical, electrical, and protocol interface definitions of the USB “wire.” Chapter 9
describes the USB device programming interface that allows a USB device to be manipulated from the
host side of the wire. Chapter 10 describes two host side software interfaces:
• Host Controller Driver (HCD) - the software interface between the USB host controller and USB
system software. This interface allows a range of host controller implementations without requiring
all host software to be dependent on any particular implementation. One USB Driver can support
different host controllers without requiring specific knowledge of a host controller implementation.
A host controller implementer provides an HCD implementation that supports the host controller.
• USB Driver (USBD) - the interface between USB system software and the client software. This
interface provides clients with convenient functions for manipulating USB devices.
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SIESIE
Host
Controller
USB Bus
Interface USB Bus
Interface
USB System SW
manages devices
USB L o gical
Device
a collection of
endpoints
Interconnect Physical DeviceHost
USB Wire
Buffers
Transfers
Transactions
Data Per
Endpoint
Interface
Specific
Function
a collection of
interfaces
Default Pipe
to Endpoint Zero
Pipe Bundl e
to an interface
Pipe, represents connection abstraction
between two horizontal entities
Data transport mechanism
USB-relevant format of transported data
No USB
Format
USB
Framed
Data
USB Framed
Data
USB
Framed
Data
No USB
Format
Interf ace x
Endpoint
Zero
Client SW
manages an interface
Mechanical,
Electrical,
Protocol
(Chapter 6,7,8)
USB Device
(Cha pter 9)
USB Host
(Cha pter 10)
Figure 5-8. USB Host/Device Detailed View
A USB logical device appears to the USB system as a collection of endpoints. Endpoints are grouped
into endpoint sets which implement an Interface. Interfaces are views to the function. System software
manages the device using the Default Pipe (associated with Endpoint 0). Client software manages an
Interface using pipe bundles (associated with an Endpoint Set). Client software requests that data be
moved across the USB between a buffer on the host and an endpoint on the USB device. The host
controller (or USB device depending on transfer direction) packetizes the data to move it over the USB.
The host controller also coordinates when bus access is used to move the packet of data over the USB.
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Figure 5-9 illustrates how communication flows are carried over pipes between endpoints and host side
memory buffers. The following sections describe endpoints, pipes, and communication flows in more
detail.
Client
Software
Interface
Endpoints
Comm Flows
Buffers
USB Logical D evice
Host
Pipes
Figure 5-9. USB Communication Flow
Software on the host communicates with a logical device via a set of communication flows. The set of
communication flows are selected by the device software/hardware designer(s) to efficiently match the
communication requirements of the device to the transfer characteristics provided by USB.
5.3.1 Device Endpoints
An endpoint is a uniquely identifiable portion of a USB device that is the terminus of a communication
flow between the host and device. Each USB logical device is composed of a collection of independently
operating endpoints. Software may only communicate with a USB device via one or more endpoints.
Each logical device has a unique address assigned by the system at device attachment time. Each
endpoint on a device has a device (design time) determined unique identifier, the endpoint number. The
combination of the device address and the endpoint number allows each endpoint to be uniquely
referenced.
An endpoint has characteristics that determine the type of transfer service required between the endpoint
and the client software. Endpoints describe themselves by:
• Their bus access frequency/latency requirements
• Their bandwidth requirements
• Their endpoint number
• The error handling behavior requirements
• Maximum packet size that the endpoint is capable of sending or receiving
• The transfer type for the endpoint (refer to Section 5.4 for details)
• For bulk and isochronous transfer types, the direction data is transferred between the endpoint and
the host
Endpoints are in an unknown state before being configured. Endpoints must not be accessed by the host
before being configured.
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5.3.1.1 Endpoint 0 Requirements
All USB devices are required to have an endpoint with endpoint number 0 that is used to initialize and
generically manipulate the logical device (e.g., to configure the logical device). Endpoint 0 provides
access to the device’s configuration information and allows generic USB status and control access.
Endpoint 0 supports control transfers as defined in Section 5.5. Endpoint 0 is always configured once a
device is attached and powered.
5.3.1.2 Non-endpoint 0 Requirements
Functions can have additional endpoints as required for their implementation. Low speed functions are
limited to two optional endpoints beyond the required Endpoint 0. Full speed devices can have additional
endpoints only limited by the protocol definition; i.e., a maximum of 16 input endpoints and 16 output
endpoints.
An endpoint cannot be used until it is configured. Endpoints, besides Endpoint 0, are configured as a
normal part of the device configuration process (refer to Chapter 9).
5.3.2 Pipes
A USB pipe is an association between an endpoint on a device and software on the host. Pipes represent
the ability to move data between software on the host via a memory buffer and an endpoint on a device.
There are two different, mutually exclusive, pipe communication modes:
• Stream. Data moving through a pipe has no USB defined structure.
• Message. Data moving through a pipe has some USB defined structure.
USB does not interpret the content of data it delivers through a pipe. Even though a message pipe
requires that data be structured according to USB definitions, the content of the data is not interpreted by
USB.
Additionally, pipes have associated with them:
• A claim on USB bus access and bandwidth usage.
• A transfer type.
• The associated endpoint’s characteristics such as directionality and maximum data payload sizes.
The data payload is the data that is carried in the data field of a data packet within a bus transaction
(as defined in Chapter 8).
Pipes come into existence when a USB device is configured. Since Endpoint 0 is always configured once
a device is powered, there is always a pipe for Endpoint 0. This pipe is called the Default Pipe. This
pipe is used by system software to determine device identification and configuration requirements, and to
configure the device. The Default Pipe can also be used by device specific software after the device is
configured. USB system software retains “ownership” of the Default Pipe and mediates use of the pipe
by other client software.
A software client normally requests data transfers via I/O Request Packets (IRPs) to a pipe and then
either waits or is notified when they are completed. Details about IRPs are defined in an operating
system specific manner. This specification uses the term to simply refer to an identifiable request by a
software client to move data between itself (on the host) and an endpoint of a device in an appropriate
direction. A software client can cause a pipe to return all outstanding IRPs if it desires. The software
client is notified that an IRP has completed when the bus transactions associated with it have completed
either successfully or due to errors.
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If there are no IRPs pending or in progress for a pipe, the pipe is idle and the host controller will take no
action with regard to the pipe; i.e., the endpoint for such a pipe will not see any bus transactions directed
to it. The only time bus activity is present for a pipe is when IRPs are pending for that pipe.
If a non-isochronous pipe encounters a STALL condition (refer to Chapter 8) or three bus errors are
encountered on any packet of an IRP, the IRP is aborted/retired, all outstanding IRPs are also retired, and
no further IRPs are accepted until the software client recovers from the condition (in an implementation
dependent way) and acknowledges the STALL or error condition via a USBD call. An appropriate status
informs the software client of the specific IRP result for error versus STALL (refer to Chapter 10).
Isochronous pipe behavior is described in Section 5.6.
An IRP may require multiple data payloads to move the client data over the bus. The data payloads for
such a multiple data payload IRP are expected to be maximum packet sized until the last data payload
that contains the remainder of the overall IRP. See each transfer type specific description for more
details. For such an IRP, short packets (i.e., less than maximum sized data payloads) on input that do not
completely fill an IRP data buffer can have one of two possible meanings depending upon the
expectations of a client.
• A client can expect a variable sized amount of data in an IRP. In this case, a short packet that does
not fill an IRP data buffer can be used simply as an inband delimiter to indicate “end of unit of data.”
The IRP should be retired without error and the host controller should advance to the next IRP.
• A client can expect a specific sized amount of data. In this case, a short packet that does not fill an
IRP data buffer is an indication of an error. The IRP should be retired, the pipe should be stalled,
and any pending IRPs associated with the pipe should also be retired.
Since the host controller must behave differently in the two cases and cannot know on its own which way
to behave for a given IRP, it is possible to indicate per IRP which behavior the client desires.
An endpoint can inform the host that it is busy by responding with a NAK. NAKs are not used as a retire
condition for returning an IRP to a software client. Any number of NAKs can be encountered during the
processing of a given IRP. A NAK response to a transaction does not constitute an error and is not
counted as one of the three errors described above.
5.3.2.1 Stream Pipes
Stream pipes deliver data in the data packet portion of bus transactions with no USB required structure on
the data content. Data flows in at one end of a stream pipe and out the other end in the same order.
Stream pipes are always unidirectional in their communication flow.
Data flowing through a stream pipe is expected to interact from what USB believes is a single client.
USB System software is not required to provide synchronization between multiple clients that may be
using the same stream pipe. Data presented to a stream pipe is moved through the pipe in sequential
order: first-in, first-out.
A stream pipe to a device is bound to a single device endpoint number in the appropriate direction (i.e.,
corresponding to an IN or OUT token as defined by the protocol layer). The device endpoint number for
the opposite direction can be used for some other stream pipe to the device.
Stream pipes support bulk, isochronous, and interrupt transfer types explained below.
5.3.2.2 Message Pipes
Message pipes interact with the endpoint in a different manner than stream pipes. First, a request is sent
to the USB device from the host. This request is followed by data transfer(s) in the appropriate direction.
Finally, a status stage follows at some later time by a response from the endpoint. In order to
accommodate the request/data/status paradigm, message pipes impose a structure on the communication
flow which allows commands to be reliably identified and communicated. Message pipes allow
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communication flow in both directions although the communication flow may be predominately one way.
The pipe for Endpoint 0, the Default Pipe, is always a message pipe.
USB system software ensures that multiple requests are not sent to an endpoint concurrently. An
endpoint is only required to service a single message request at a time per endpoint. Multiple software
clients on the host can make requests via the Default Pipe, but they are sent to the endpoint in a first-in,
first-out order. An endpoint can control the flow of information during the data and status stages based
on its ability to respond to the host transactions (refer to Chapter 8 for more details).
An endpoint will not normally be sent the next message from the host until the current message’s
processing at the endpoint has been completed. However, there are error conditions whereby a message
transfer can be aborted by the host and the endpoint can be sent a new message transfer prematurely
(from its perspective). From the perspective of the software manipulating a message pipe, an error on
some part of an IRP retires the current IRP and all queued IRPs. The software client that requested the
IRP is notified of the IRP completion with an appropriate error indication.
A message pipe to a device requires a single device endpoint number in both directions (IN and OUT
tokens). USB does not allow a message pipe to be associated with different endpoint numbers for each
direction.
Message pipes support the control transfer type explained below.
5.4 Transfer Types
USB transports data through a pipe between a memory buffer associated with a software client on the
host and an endpoint on the USB device. Data transported by message pipes is carried in a USB defined
structure, but USB allows device specific structured data to be transported within the USB defined
message data payload. USB also defines that data moved over the bus is packetized for any pipe (stream
or message), but ultimately the formatting and interpretation of the data transported in the data payload of
a bus transaction is the responsibility of the client software and function using the pipe. However, USB
provides different transfer types that are optimized to more closely match the service requirements of the
client software and function using the pipe. An IRP uses one or more bus transactions to move
information between a software client and its function.
Each transfer type determines various characteristics of the communication flow including:
• Data format imposed by USB
• Direction of communication flow
• Packet size constraints
• Bus access constraints
• Required data sequences
The designers of a USB device choose the capabilities for the device’s endpoints. When a pipe is
established for an endpoint, most of the pipe’s transfer characteristics are determined and remain fixed
for the lifetime of the pipe. Transfer characteristics that can be modified are described for each transfer
type.
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USB defines four transfer types:
• Control Transfers - Bursty, non-periodic, host software initiated request/response communication
typically used for command/status operations.
• Isochronous Transfers - Periodic, continuous communication between host and device typically used
for time relevant information. This transfer type also preserves the concept of time encapsulated in
the data. This does not imply, however, that the delivery needs of such data is always time-critical.
• Interrupt Transfers - Small data, non-periodic, low frequency, bounded latency, device initiated
communication typically used to notify the host of device service needs.
• Bulk Transfers - Non-periodic, large bursty communication typically used for data that can use any
available bandwidth and also be delayed until bandwidth is available.
Each transfer type is described in detail in the following four major sections. The data for any IRP is
carried by the data field of the data packet as described in Section 8.4.3. Chapter 8 also describes details
of the protocol that are affected by use of each particular transfer type.
5.5 Control Transfers
Control transfers allow access to different parts of a device. Control transfers are intended to support
configuration/command/status type communication flows between client software and its function. A
control transfer is composed of a setup bus transaction moving request information from host to function,
zero or more data transactions sending data in the direction indicated by the setup transaction, and a
status transaction returning status information from function to host. The status transaction returns
“success” when the endpoint has successfully completed processing the requested operation.
Section 8.5.2 describes the details of what packets, bus transactions, and transaction sequences are used
to accomplish a control transfer. Chapter 9 describes the details of the defined USB command codes.
Each USB device is required to implement Endpoint 0 with a control transfer type. This endpoint is used
by the USB system software as a control pipe. Control pipes provide access to the USB device’s
configuration, status, and control information. A function can provide endpoints for additional control
pipes for its own implementation needs.
The USB device framework (refer to Chapter 9) defines standard, device class, or vendor specific
requests that can be used to manipulate a device’s state. Descriptors are also defined that can be used to
contain different information on the device. Control transfers provide the transport mechanism to access
device descriptors and make requests of a device to manipulate its behavior.
Control transfers are only carried through message pipes. Consequently, data flows using control
transfers must adhere to USB data structure definitions as described in Section 5.5.1.
USB subsystem will make a “best effort” to support delivery of control transfers between the host and
devices. A function and its client software cannot request specific bus access frequency or bandwidth for
control transfers. USB system software may restrict the bus access and bandwidth that a device may
desire for control transfers. These restrictions are defined in Section 5.5.3 and Section 5.5.4.
5.5.1 Data Format
The setup packet has a USB defined structure that accommodates the minimum set of commands
required to enable communication between the host and a device. The structure definition allows vendor
specific extensions for device specific commands. The data transactions following setup have no USB
defined structure. The status transaction also has a USB defined structure. Specific control transfer
setup/data definitions are described in Section 8.5.2 and Chapter 9.
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5.5.2 Direction
Control transfers are supported via bi-directional communication flow over message pipes.
5.5.3 Packet Size Constraints
An endpoint for control transfers specifies the maximum data payload size that the endpoint can accept
from or transmit to the bus. USB defines the allowable maximum control data payload sizes for full
speed devices to be only 8, 16, 32, or 64 bytes. Low speed devices are limited to only an 8 byte
maximum data payload size. This maximum applies to the data payloads of the data packets following a
setup; i.e., the size specified is for the data field of the packet as defined in Chapter 8, not including other
protocol required information. A setup packet is always 8 bytes. A control endpoint always uses its
MaxPacketSize for data payloads.
All control endpoints are required to support a control data payload maximum size of 8 bytes after reset.
An endpoint can be designed to support a larger maximum data payload size. Such an endpoint reports in
its configuration information the value for its maximum data payload size. USB does not require that
data payloads transmitted be exactly the maximum size; i.e., if a data payload is less than the maximum,
it does not need to be padded to the maximum size.
All host controllers are required to have support for 8, 16, 32, and 64 byte maximum data payload sizes
for full speed control endpoints and only 8 byte maximum data payload sizes for low speed control
endpoints. No host controller is required to support larger or smaller maximum data payload sizes.
During configuration, USB system software reads the endpoint’s maximum data payload size and ensures
that no data payload will be sent to the endpoint that is larger than the supported size. The host will
always use a maximum data payload size of at least 8 bytes.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
MaxPacketSize (refer to Chapter 9). When a control transfer involves more data than can fit in one data
payload of the currently established maximum size, all data payloads are required to be maximum sized
except for the last data payload which will contain the remaining data. If an endpoint wants to transmit
less data than expected by the client software, a premature, less than maximum sized data payload will be
received by the host controller. This premature, less than maximum sized data payload causes the host
controller to advance to the status transaction instead of continuing on with another data transaction or
else stall the pipe as was outlined in Section 5.3.2. If a data payload is received that is larger than that
expected, the IRP for the control transfer will be aborted/retired and the pipe will stall future IRPs until
the condition is corrected and acknowledged.
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5.5.4 Bus Access Constraints
Control transfers can be used by full speed and low speed USB devices.
An endpoint has no way to indicate a desired bus access frequency for a control pipe. USB balances the
bus access requirements of all control pipes and the specific IRPs that are pending to provide “best
effort” delivery of data between client software and functions.
USB requires that part of each frame be reserved to be available for use by control transfers as follows:
• If the control transfers that are attempted (in an implementation dependent fashion) consume less
than 10% of the frame time, the remaining time can be used to support bulk transfers (refer to
Section 5.8).
• A control transfer that has been attempted and needs to be retried can be retried in the current or a
future frame; i.e., it is not required to be retried in the same frame.
• If there are more control transfers than reserved time, but there is additional frame time that is not
being used for isochronous or interrupt transfers, a host controller may move additional control
transfers as they are available.
• If there are too many pending control transfers than available frame time, control transfers are
selected to be moved over the bus as appropriate.
• If there are control transfers pending for multiple endpoints, control transfers for the different
endpoints are selected according to a fair access policy that is host controller implementation
dependent.
• A transaction of a control transfer that is frequently being retried should not be expected to consume
an unfair share of the frame time.
These requirements allow control transfers between host and devices to be regularly moved over the bus
with “best effort.”
All control transfers pending in a system contend for the same available bus time. Because of this, the
bus time made available for control transfers to a particular endpoint can be varied by USB system
software at its discretion. An endpoint and its client software cannot assume a specific rate of service for
control transfers. Bus time made available to a software client and its endpoint can be changed as other
devices are inserted into and removed from the system or also as control transfers are requested for other
device endpoints.
The bus frequency and frame timing limit the maximum number of successful control transfers within a
frame for any USB system to less than 29 full speed 8 byte data payloads or less than four low speed
8 byte data payloads. Table 5-1 lists information about different sized full speed control transfers and the
maximum number of transfers possible in a frame. This table was generated assuming zero length status
data stage transaction and one data stage transaction. The table illustrates the possible power of two data
payloads less than or equal to the allowable maximum data payload sizes.
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Table 5-1. Full Speed Control Transfer Limits
protoco l overhead (b ytes )
45 (9-syncs , 9-pids, 6- EP+CRC, 6- CRC,8-Set up data
7-byte inter packet delay (EOP, etc .))
data max Bandwidth Frame BW m ax Bytes Bytes/frame
pa ylo ad bytes/ sec pe r tr an sfe
r
transfe rs Rem aining usef ul data
1 32000 3% 32 28 32
2 62000 3% 31 43 62
4 120000 3% 30 30 120
8 224000 4% 28 16 224
16 384000 4% 24 36 384
32 608000 5% 19 37 608
64 832000 7% 13 83 832
max 1500000 1500
The 10% frame reservation for control transfers means that in a system with bus time fully allocated, all
full speed control transfers in the system contend for a nominal three control transfers per frame. Since
the USB subsystem uses control transfers for configuration purposes in addition to whatever other control
transfers other client software may be requesting, a given software client and its function should not
expect to be able to make use of this full bandwidth for its own control purposes. Host controllers are
also free to determine how the individual bus transactions for specific control transfers are moved over
the bus within and across frames. An endpoint could see all bus transactions for a control transfer within
the same frame or spread across several discontiguous frames. Finally, a host controller, for various
implementation reasons, may not be able to provide the theoretical maximum number of control transfers
per frame.
Both full speed and low speed control transfers contend for the same available frame time. Low speed
control transfers simply take longer to transfer. Table 5-2 lists information about different sized low
speed packets and the maximum number of packets possible in a frame. Also for both speeds, since a
control transfer is composed of several packets, the packets can be spread over several frames to spread
the bus time required across several frames.
Table 5-2. Low Speed Control Transfer Limits
protocol ov erhead (bytes)
46
data m ax Bandwidth Frame BW max Bytes Bytes/frame
payload (approx) per t ransfer transfers Remaining useful data
1 3000 25% 3 46 3
2 6000 26% 3 43 6
4 12000 27% 3 37 12
8 24000 29% 3 25 24
max 187500 187
5.5.5 Data Sequences
Control transfers require that a setup bus transaction be sent from the host to a device to describe the type
of control access that the device should perform. The setup transaction is followed by zero or more
control data transactions that carry the specific information for the requested access. Finally, a status
transaction completes the control transfer and allows the endpoint to return the status of the control
transfer to the client software. After the status transaction for a control transfer is completed, the host can
advance to the next control transfer for the endpoint. As described in Section 5.5.4, this next control
transfer will be moved over the bus at some host controller implementation defined time in the future.
The endpoint can be busy for a device specific numbers of frames during the data and status transactions
of the control transfer. During these times when the endpoint indicates it is busy (refer to Chapter 8 and
Chapter 9 for details), the host will retry the transaction at a later time.
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If a setup transaction is received by an endpoint before a previously initiated control transfer is
completed, the device must abort the current transfer/operation and handle the new control setup
transaction. A setup transaction should not normally be sent before the completion of a previous control
transfer. However, if a transfer is aborted, for example, due to errors on the bus, the host can send the
next setup transaction prematurely from the endpoint’s perspective.
After a STALL condition is encountered or an error is detected by the host, a control endpoint is allowed
to recover by accepting the next setup PID; i.e., recovery actions via some other pipe are not required for
control endpoints, but may be required by implementation for some. For the Default Pipe (Endpoint 0), a
device reset (by USBD) will ultimately be required to clear the STALL or error condition if the next
setup PID is not accepted.
USB provides robust error detection, recovery/retransmission for errors that occur during control
transfers. Transmitters and receivers can remain synchronized with regard to where they are in a control
transfer and recover with minimum effort. Retransmission of data and status packets can be detected by
a receiver via data retry indicators in the packet. A transmitter can reliably determine that its
corresponding receiver has successfully accepted a transmitted packet by information returned in a
handshake to the packet. The protocol allows for distinguishing a retransmitted packet from its original
packet except for a control setup packet. Setup packets may be retransmitted due to a transmission error;
however, setup packets cannot indicate that a packet is an original or a retried transmission.
5.6 Isochronous Transfers
In non-USB environments, isochronous transfers have the general implication of constant-rate, error-
tolerant transfers. In the USB environment, requesting an isochronous transfer type provides the
requester with the following:
• Guaranteed access to USB bandwidth with bounded latency
• As long as data is provided to the pipe, a constant data rate through the pipe is guaranteed
• In the case of a delivery failure due to error, no retrying of the attempt to deliver the data
While the USB isochronous transfer type is designed to support isochronous sources and destinations, it is
not required that software using this transfer type actually be isochronous in order to use the transfer type.
Section 5.10 presents more detail on special considerations for handling isochronous data on USB.
5.6.1 Data Format
USB imposes no data content structure on communication flows for isochronous pipes.
5.6.2 Direction
An isochronous pipe is a stream pipe and is, therefore, always unidirectional. An endpoint description
identifies whether a given isochronous pipe’s communication flow is into or out of the host. If a device
requires bi-directional isochronous communication flow, two isochronous pipes must be used, one in each
direction.
5.6.3 Packet Size Constraints
An endpoint in a given configuration for an isochronous pipe specifies the maximum size data payload
that it can transmit/receive. USB system software uses this information during configuration to ensure
that there is sufficient bus time to accommodate this maximum data payload in each frame. If there is
sufficient bus time for the maximum data payload, the configuration is established; if not, the
configuration is not established. USB system software does not adjust the maximum data payload size
for an isochronous pipe as was the case for a control pipe. An isochronous pipe can simply either be
supported or not supported in a given USB subsystem configuration.
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USB limits the maximum data payload size to 1023 bytes for each isochronous pipe. Table 5-3 lists
information about different sized isochronous transactions and the maximum number of transactions
possible in a frame.
Table 5-3. Isochronous Transaction Limits
protocol overhead (byt es)
9 (2-syncs, 2-pids, 2-EP+CRC, 2-CRC,
1 by te interpack et delay)
data max Bandwidth Frame BW m ax Bytes Bytes/frame
payload per trans. t rans. Remaining usef ul data
1 150000 1% 150 0 150
2 272000 1% 136 4 272
4 460000 1% 115 5 460
8 704000 1% 88 4 704
16 960000 2% 60 0 960
32 1152000 3% 36 24 1152
64 1280000 5% 20 40 1280
128 1280000 9% 10 130 1280
256 1280000 18% 5 175 1280
512 1024000 35% 2 458 1024
1023 1023000 69% 1 468 1023
max 1500000 1500
Any given transaction for a isochronous pipe need not be exactly the maximum size specified for the
endpoint. The size of a data payload is determined by the transmitter (client software or function) and
can vary as required from transaction to transaction. An endpoint can use the optional USB standard
sample header to indicate where in the sample stream this packet starts. This allows the receiver to
recover from packets lost due to errors. USB ensures that whatever size is presented to the host controller
is delivered on the bus. The actual size of a data payload is determined by the data transmitter and may
be less than the prenegotiated maximum size. Bus errors can change the actual size seen by the receiver.
However, these errors can be detected by either CRC on the data or knowledge the receiver has about the
expected size for any transaction.
5.6.4 Bus Access Constraints
Isochronous transfers can only be used by full speed devices.
USB requires that no more than 90% of any frame be allocated for periodic (isochronous and interrupt)
transfers.
An endpoint for an isochronous pipe does not include information about bus access frequency. All
isochronous pipes normally move exactly one data packet each frame (i.e., every 1 ms). Errors on the
bus or delays in operating system scheduling of client software can result in no packet being transferred
for a frame. An error indication is returned as status to the client software in such a case. A device can
also detect this situation by tracking SOF tokens and noticing two SOF tokens without an intervening
data packet for an isochronous endpoint.
The bus frequency and frame timing limit the maximum number of successful isochronous transactions
within a frame for any USB system to less than 151 full speed 1 byte data payloads. Finally, a host
controller, for various implementation reasons, may not be able to provide the theoretical maximum
number of isochronous transactions per frame.
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5.6.5 Data Sequences
Isochronous transfers do not support data retransmission in response to errors on the bus. A receiver can
determine that a transmission error occurred. The low level USB protocol does not allow handshakes to
be returned to the transmitter of an isochronous pipe. Normally handshakes would be returned to tell the
transmitter whether a packet was successfully received or not. For isochronous transfers, timeliness is
more important than correctness/retransmission, and given the low error rates expected on the bus, the
protocol is optimized assuming transfers normally succeed. Isochronous receivers can determine whether
they missed data during a frame. Also, a receiver can determine how much data was lost. Section 5.10
describes these USB mechanisms in more detail.
An endpoint for isochronous transfers never stalls since there is no handshake to report a STALL
condition. The host and client software can never encounter this case. Errors are reported as status
associated with the IRP for an isochronous transfer, but the isochronous pipe is not stalled in an error
case. If an error is detected, the host continues to process the data associated with the next frame of the
transfer. Limited error detection is possible since the protocol for isochronous transactions does not
allow per transaction handshakes.
5.7 Interrupt Transfers
The interrupt transfer type is designed to support those devices that need to communicate small amounts
of data infrequently, but with bounded service periods. Requesting a pipe with an interrupt transfer type
provides the requester with the following:
• Guaranteed maximum service period for the pipe
• Retry of transfer attempts at the next period, in the case of occasional delivery failure due to error on
the bus
5.7.1 Data Format
USB imposes no data content structure on communication flows for interrupt pipes.
5.7.2 Direction
An interrupt pipe is a stream pipe and is therefore always unidirectional. Further, an interrupt pipe is
only input to the host. Output interrupt pipes are not supported by USB.
5.7.3 Packet Size Constraints
An endpoint for an interrupt pipe specifies the maximum size data payload that it will transmit. The
maximum allowable interrupt data payload size is 64 bytes or less for full speed. Low speed devices are
limited to 8 bytes or less maximum data payload size. This maximum applies to the data payloads of the
data packets; i.e., the size specified is for the data field of the packet as defined in Chapter 8, not
including other protocol required information. USB does not require that data packets be exactly the
maximum size; i.e., if a data packet is less than the maximum, it does not need to be padded to the
maximum size.
All host controllers are required to have support for up to 64 byte maximum data payload sizes for full
speed interrupt endpoints and 8 bytes or less maximum data payload sizes for low speed interrupt
endpoints. No host controller is required to support larger maximum data payload sizes.
USB system software determines the maximum data payload size that will be used for a interrupt pipe
during device configuration. This size remains constant for the lifetime of a device’s configuration.
USB software uses the maximum data payload size determined during configuration to ensure that there
is sufficient bus time to accommodate this maximum data payload in its assigned period. If there is
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sufficient bus time, the pipe is established; if not, the pipe is not established. USB software does not
adjust the bus time made available to an interrupt pipe as was the case for a control pipe. An interrupt
pipe can simply either be supported or not in a given USB subsystem configuration. However, the actual
size of a data payload is still determined by the data transmitter and may be less than the maximum size.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
MaxPacketSize. A device can move data via an interrupt pipe that is larger than MaxPacketSize. A
software client can accept this data via an IRP for the interrupt transfer that requires multiple bus
transactions without requiring an IRP complete notification per transaction. This can be achieved by
specifying a buffer that can hold the desired data size. The size of the buffer is a multiple of
MaxPacketSize with some remainder. The endpoint must transfer each transaction except the last as
MaxPacketSize and the last transaction is the remainder. The multiple data transactions are moved over
the bus at the period established for the pipe.
When an interrupt transfer involves more data than can fit in one data payload of the currently
established maximum size, all data payloads are required to be maximum sized except for the last data
payload which will contain the remaining data. If an endpoint wants to transmit less data than expected
by the client software, a premature, less than maximum sized data payload will be received by the host
controller. This premature, less than maximum sized data payload causes the host controller to retire the
current IRP and advance to the next IRP or else stall the pipe as outlined in Section 5.3.2. If a data
payload is received that is larger than that expected, the interrupt IRP will be aborted/retired and the pipe
will stall future IRPs until the condition is corrected and acknowledged.
5.7.4 Bus Access Constraints
Interrupt transfers can be used by full speed and low speed devices.
USB requires that no more than 90% of any frame be allocated for periodic (isochronous and interrupt)
transfers.
The bus frequency and frame timing limit the maximum number of successful interrupt transactions
within a frame for any USB system to less than 108 full speed 1 byte data payloads or 14 low speed
1 byte data payloads. Finally, a host controller, for various implementation reasons, may not be able to
provide the above maximum number of interrupt transactions per frame.
Table 5-4 lists information about different sized full speed interrupt transactions and the maximum
number of transactions possible in a frame. Table 5-5 lists similar information for low speed interrupt
transactions.
Table 5-4. Full Speed Interrupt Transaction Limits
protocol overhead (byt es)
13 (3-syncs, 3- pids, 2-EP+CRC, 2-CRC,
3 by te interpack et delay)
data max Bandwidth Fram e BW m ax Bytes Bytes/frame
payload per trans. trans. Remaining useful data
1 107000 1% 107 2 107
2 200000 1% 100 0 200
4 352000 1% 88 4 352
8 568000 1% 71 9 568
16 816000 2% 51 21 816
32 1056000 3% 33 15 1056
64 1216000 5% 19 37 1216
max 1500000 1500
An endpoint for an interrupt pipe specifies its desired bus access period. A full speed endpoint can
specify a desired period from 1 ms to 255 ms. Low speed endpoints are limited to only specifying
10 ms to 255 ms. USB software will use this information during configuration to determine a period that
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can be sustained. The period provided by the system may be shorter than that desired by the device up to
the shortest period defined by USB. The client software and device can only depend on the fact that the
host will ensure that the time duration between two error free transactions (or two transaction attempts)
with the endpoint will be no longer than the desired period. Note that errors on the bus can prevent an
interrupt transaction from being successfully delivered over the bus and consequently exceed the desired
period. The period between any two transaction attempts can also vary over time; although it will never
exceed the desired period in error free cases. Also, the endpoint is only polled when the software client
has an IRP for an interrupt transfer pending. If the bus time for performing an interrupt transfer arrives
and there is no IRP pending, the endpoint will not be given an opportunity to transfer data at that time.
Once an IRP is requested, its data will be transferred at the next allocated period.
Table 5-5. Low Speed Interrupt Transaction Limits
prot o col overhe ad (by te s)
13
data max B andwidth Fram e BW max B ytes Bytes/fram e
payload (approx) per trans. trans. Rem aining usef ul data
1 13000 7% 13 5 13
2 24000 8% 12 7 24
4 44000 9% 11 0 44
8 64000 11% 8 19 64
max 187500 187
Interrupt transfers are moved over the USB by accessing an interrupt endpoint every period. The host has
no way to determine whether an endpoint will source an interrupt without accessing the endpoint and
requesting an interrupt transfer. If the endpoint has no interrupt data to transmit when accessed by the
host, it responds with a NAK. An endpoint should only provide interrupt data when it has an interrupt
pending to avoid having a software client erroneously notified of IRP complete. A zero length data
payload is a valid transfer and may be useful for some implementations.
5.7.5 Data Sequences
Interrupt transactions may use either alternating data toggle bits such that the bits are toggled only upon
successful transfer completion or a continuously toggling of data toggle bits. The host in any case must
assume that the device is obeying full handshake/retry rules as defined in Chapter 8. A device may
choose to always toggle DATA0/DATA1 PIDs so that it can ignore handshakes from the host. However,
in this case, the client software can miss some data packets when an error occurs because the host
controller interprets the next packet as a retry of a missed packet.
If a stall condition is detected on an interrupt pipe due to transmission errors or a STALL handshake
being returned from the endpoint, all pending IRPs are retired. Removal of the STALL condition is
achieved via software intervention through a separate control pipe. This recovery must also reset the data
toggle bit to DATA0 for the endpoint. The software client must also call a USBD Function to reset the
host’s data toggle to DATA0, acknowledge, and clear the stall condition on the host.
Interrupt transactions are retried due to errors detected on the bus that affect a given transfer.
5.8 Bulk Transfers
The bulk transfer type is designed to support devices that need to communicate relatively large amounts
of data at highly variable times where the transfer can be deferred until bandwidth is available.
Requesting a pipe with a bulk transfer type provides the requester with the following:
• Access to the USB on a bandwidth available basis
• Retry of transfers, in the case of occasional delivery failure due to error on the bus
• Guaranteed delivery of data, but no guarantees of bandwidth or latency
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Bulk transfers occur only on a bandwidth available basis. For a USB with large amounts of free
bandwidth, bulk transfers may happen relatively quickly; while for a USB with little bandwidth available,
bulk transfers may trickle out over a relatively long period of time.
5.8.1 Data Format
USB imposes no data content structure on communication flows for bulk pipes.
5.8.2 Direction
A bulk pipe is a stream pipe and, therefore, always has communication flowing either into or out of the
host for a given pipe. If a device requires bi-directional bulk communication flow, two bulk pipes must
be used, one in each direction.
5.8.3 Packet Size Constraints
An endpoint for bulk transfers specifies the maximum data payload size that the endpoint can accept
from or transmit to the bus. USB defines the allowable maximum bulk data payload sizes to be only 8,
16, 32, or 64 bytes. This maximum applies to the data payloads of the data packets; i.e.; the size
specified is for the data field of the packet as defined in Chapter 8, not including other protocol required
information.
A bulk endpoint is designed to support a maximum data payload size. A bulk endpoint reports in its
configuration information the value for its maximum data payload size. USB does not require that data
payloads be transmitted exactly the maximum size; i.e., if a data payload is less than the maximum, it
does not need to be padded to the maximum size.
All host controllers are required to have support for 8, 16, 32, and 64 byte maximum packet sizes for bulk
endpoints. No host controller is required to support larger or smaller maximum packet sizes.
During configuration, USB system software reads the endpoint’s maximum data payload size and ensures
that no data payload will be sent to the endpoint that is larger than the supported size.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
reported MaxPacketSize. When a bulk IRP involves more data than can fit in one maximum sized data
payload, all data payloads are required to be maximum size except for the last data payload which will
contain the remaining data. If an endpoint transmits less data than expected by the client software, a
premature, less than maximum sized data payload will be received by the host controller. This
premature, less than maximum sized data payload causes the host controller to retire the current IRP and
advance to the next IRP or else stall the pipe as was outlined in Section 5.3.2. If a data payload is
received that is larger than that expected, the pipe will stall and all pending bulk IRPs for that endpoint
will be aborted/retired.
5.8.4 Bus Access Constraints
Bulk transfers can only be used by full speed devices.
An endpoint has no way to indicate a desired bus access frequency for a bulk pipe. USB balances the bus
access requirements of all bulk pipes and the specific IRPs that are pending to provide “good effort”
delivery of data between client software and functions. Moving control transfers over the bus has priority
over moving bulk transfers.
There is no frame time guaranteed to be available for bulk transfers as there is for control transfers. Bulk
transfers are only moved over the bus on a bandwidth available basis. If there is frame time that is not
being used for other purposes, bulk transfers will be moved over the bus. If there is no time in a frame
available for bulk transfers, no bulk transfers will be moved in that frame. If there are too many pending
bulk transfers for the available frame time, bulk transfers are selected to be moved over the bus as
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appropriate. If there are bulk transfers pending for multiple endpoints, bulk transfers for the different
endpoints are selected according to a fair access policy that is host controller implementation dependent.
All bulk transfers pending in a system contend for the same available bus time. Because of this, the bus
time made available for bulk transfers to a particular endpoint can be varied by USB system software at
its discretion. An endpoint and its client software cannot assume a specific rate of service for bulk
transfers. Bus time made available to a software client and its endpoint can be changed as other devices
are inserted into and removed from the system or also as bulk transfers are requested for other device
endpoints. Client software cannot assume ordering between bulk and control transfers; i.e., in some
situations bulk transfers can be delivered ahead of control transfers.
The bus frequency and frame timing limit the maximum number of successful bulk transactions within a
frame for any USB system to less than 72 8 byte data payloads. Table 5-6 lists information about
different sized bulk transactions and the maximum number of transactions possible in a frame. Host
controllers are free to determine how the individual bus transactions for specific bulk transfers are moved
over the bus within and across frames. An endpoint could see all bus transactions for a bulk transfer
within the same frame or spread across several discontiguous frames. Finally, a host controller, for
various implementation reasons, may not be able to provide the above maximum number of transactions
per frame.
Table 5-6. Bulk Transaction Limits
prot oc o l overhead (by t es)
13 (3-syncs, 3-pid s, 2-E P+CRC, 2- C RC
3 byte i nterpacket delay)
data max Bandwidth Ba ndwidth m ax By tes Bytes/fram e
payl oad byt es/sec per t rans. t r ans. Rem ain i n g usef u l dat a
1 107000 1% 107 2 107
2 200000 1% 100 0 200
4 352000 1% 88 4 352
8 568000 1% 71 9 568
16 816000 2% 51 21 816
32 1056000 3% 33 15 1056
64 1216000 5% 19 37 1216
max 1500000 1500
5.8.5 Data Sequences
Bulk transactions use data toggle bits that are toggled only upon successful transaction completion to
preserve synchronization between transmitter and receiver when transactions are retried due to errors.
Bulk transactions are initialized to DATA0 when the endpoint is configured by an appropriate control
transfer. The host will also start the first bulk transaction with DATA0. If a bulk pipe is stalled, the data
toggle on the host is reset to DATA0 when the stall is acknowledged by the software client via a USBD
function. The endpoint has its stall condition cleared via an appropriate control transfer. That action also
resets the endpoint’s data toggle to DATA0.
Bulk transactions are retried due to errors detected on the bus that affect a given transaction.
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5.9 Bus Access for Transfers
Accomplishing any data transfer between the host and a USB device requires some use of the USB
bandwidth. Supporting a wide variety of isochronous and asynchronous devices requires that each
device’s transfer requirements are accommodated. The process of assigning bus bandwidth to devices is
called Transfer Management. There are several entities on the host that coordinate the information
flowing over USB: Client software, the USB Driver (USBD), and the Host Controller Driver (HCD).
Implementers of these entities need to know the key concepts related to bus access:
• Transfer Management - The entities and the objects that support communication flow over USB.
• Transaction Tracking - The USB mechanisms that are used to track transactions as they move
through the USB system.
• Bus Time - The time it takes to move a packet of information over the bus.
• Device/Software Buffer Size - The space required to support a bus transaction.
• Bus Bandwidth Reclamation - Conditions where bandwidth that was allocated to other transfers but
was not used and can now be possibly reused by control and bulk transfers.
The previous sections focused on how client software relates to a function and what the logical flows are
over a pipe between the two entities. This section focuses on the different parts of the host and how they
must interact to support moving data over the USB. This information may also be of interest to device
implementers to understand aspects of what the host is doing when a client requests a transfer and how
that transfer is presented to the device.
5.9.1 Transfer Management
Transfer Management involves several entities that operate on different objects in order to move
transactions over the bus:
• Client Software - Consumes/Generates function specific data to/from a function endpoint via calls
and callbacks requesting IRPs with USBD interface.
• USB Driver (USBD) - Converts data in client IRPs to/from device endpoint via calls/callbacks with
appropriate HCD. A single client IRP may involve one or more transfers.
• Host Controller Driver (HCD) - Converts IRPs to/from transactions (as required by a host controller
implementation) and organizes them for manipulation by the host controller. Interactions between
the host controller driver and its hardware is implementation dependent and outside the scope of the
USB specification.
• Host Controller - Takes transactions and generates bus activity via packets to move function specific
data across the bus for each transaction.
Figure 5-10 shows how the entities are organized as information flows between client software and the
USB. The objects of primary interest to each entity are shown at the interfaces between entities.
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Transfers
USBD
Host Controller
Transaction List
Client Software Data
Transactions
Packets
USB
HCD
Interface
Transaction
Transaction
USBD
Interface
HCD
HW/SW
Interface
IRPs
Figure 5-10. USB Information Conversion From Client Software to Bus
5.9.1.1 Client Software
Client software determines what transfers need to be made with a function. It uses appropriate operating
system specific interfaces to request IRPs. Client software is only aware of the set of pipes (i.e., the
interface) it needs to manipulate its function. The client is aware and adheres to all bus access and
bandwidth constraints as described previously for each transfer type. The requests made by the client
software are presented via the USBD interface.
Some clients may manipulate USB functions via other device class interfaces defined by the operating
system and may themselves not make direct USBD calls. However, there is always some lowest level
client that makes USBD calls to pass IRPs to USBD. All IRPs presented are required to adhere to the
prenegotiated bandwidth constraints set when the device was attached to the bus and configured. If a
function is moved from a non-USB environment to USB, the driver that would have directly manipulated
the function hardware via memory or I/O accesses is the lowest client software in the USB environment
that now interacts with USBD to manipulate its USB function.
After client software has requested a transfer of its function and the request has been serviced, the client
software gets notified of the completion status of the IRP. If the transfer involved function to host data
transfer, the client software can access the data in the data buffer associated with the completed IRP.
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The USBD interface is defined in Chapter 10.
5.9.1.2 USB Driver
USBD is involved in mediating bus access at two general times while a device is attached to the bus
during configuration and normal transfers. When a device is attached and configured, USBD is involved
to ensure that the desired device configuration can be accommodated on the bus. The USBD receives
configuration requests from the configuring software which describe the desired device configuration:
endpoint(s), transfer type(s), transfer period(s), data size(s), etc. USBD either accepts or rejects a
configuration request based on bandwidth availability and the ability to accommodate that request type
on the bus. If it accepts the request, USBD creates a pipe for the requester of the desired type and with
appropriate constraints as defined for the transfer type.
The configuration aspects of USBD are typically operating system environment specific and heavily
leverage the configuration features of the operating system to avoid defining additional (redundant)
interfaces.
Once a device is configured, the software client can request IRPs to move data between it and its function
endpoints.
5.9.1.3 Host Controller Driver
HCD is responsible for tracking the IRPs in progress and ensuring that USB bandwidth and frame time
maximums are never exceeded. When IRPs are made for a pipe, HCD adds them to the transaction list.
When an IRP is complete, HCD notifies the requesting software client of the completion status for the
IRP. If the IRP involved data transfer from the function to the software client, the data was placed in the
client-indicated data buffer.
IRPs are defined in an operating system dependent manner.
5.9.1.4 Transaction List
The transaction list is a host controller implementation dependent description of the current outstanding
set of bus transactions that need to be run on the bus. A typical transaction list consists of a series of
frame descriptions in some host controller implementation dependent representation. Only HCD and its
host controller have access to the specific representation. Each frame description contains transaction
descriptions in which parameters such as data size in bytes, the device address and endpoint number, and
the memory area to which data is to be sent or received are identified.
A transaction list and the interface between HCD and its host controller is typically represented in an
implementation dependent fashion and is not defined explicitly as part of the USB specification.
5.9.1.5 Host Controller
The host controller has access to the transaction list and translates it into bus activity. In addition, the
host controller provides a reporting mechanism whereby the status of a transaction (done, pending,
stalled, etc.) can be obtained. The host controller converts transactions into appropriate implementation
dependent activities that result in USB packets moving over the bus topology rooted in the root hub.
The host controller ensures that the bus access rules defined by the protocol are obeyed; e.g., inter-packet
timings, time-outs, babble, etc. The HCD interface provides a way for the host controller to participate in
whether a new pipe is allowed access to the bus. This is done because host controller implementations
can have restrictions/constraints on the minimum inter-transaction times they may support for
combinations of bus transactions.
The interface between the transaction list and the host controller is hidden within an HCD and host
controller implementation. The host controller is typically implemented in hardware.
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5.9.2 Transaction Tracking
A USB function sees data flowing across the bus in packets as described in Chapter 8. The host
controller uses some implementation dependent representation to track what packets to transfer to/from
what endpoints at what time or in what order. Most client software does not want to deal with packetized
communication flows since this involves a degree of complexity and interconnect dependency that limits
the implementation. USB system software (USBD and HCD) provides support for matching data
movement requirements of a client to packets on the bus. The host controller hardware and software uses
IRPs to track information about one or more transactions that combine to deliver a transfer of information
between the client software and the function. Figure 5-11 summarizes how transactions are organized
into IRPs for the four transfer types. Detailed protocol information for each transfer type can be found in
Chapter 8. More information about client software views of IRPs can be found in Chapter 10 and in the
operating system specific information for a particular operating system.
Data Flo w Ty pes
Control Transf er
Int errupt Transfer
Isochronous Transfer
Bulk Transfer
A control tr ansfer is anOUT
Setup transaction followed
b y multip le IN o r O U T Da ta
transactions followed by
one “opposite of data
direction” Status
Transaction.
On e o r more IN
Data Transactions.
On e o r more IN / OU T
Data Transactions.
On e o r more IN / OU T
Data Transactions.
IRP
Transaction Transaction
Transaction
All transfers are
composed of one or more
tra n saction s. A nd an I RP
corresponds to one or
more tran sfe r s.
IRP
Setup
Transaction Data
Transaction
IRP IN Transact io n IN Transact io n
IRP Transaction Transaction
Transaction
IRP Transaction Transaction
Transaction
Status
Transaction
Additional
Control Transfers
Figure 5-11. Transfers for Communication Flows
Even though IRPs track the bus transactions that need to occur to move a specific data flow over USB,
host controllers are free to choose how the particular bus transactions are moved over the bus subject to
the USB defined constraints; e.g., exactly one transaction per frame for isochronous transfers. In any
case, an endpoint will see transactions in the order they appear within an IRP unless errors occur. For
example, Figure 5-12 shows two IRPs, one each for two pipes where each IRP contains three
transactions. For any transfer type, a host controller is free to move the first transaction of the first IRP
followed by the first transaction of the second IRP somewhere in Frame 1, while moving the second
transactions of each IRP in opposite order somewhere in Frame 2. If these are isochronous transfer types,
that is the only degree of freedom a host controller has. If these are control or bulk transfers, a host
Universal Serial Bus Specification Revision 1.0
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controller could further move more or less transactions from either IRP within either frame. Functions
cannot depend on seeing transactions within an IRP back to back within a frame.
Pipe Pipe
Frame 1 Token, Data,
Handshake
(2-0)
Token Data,
Handshake
(1-0)
Frame 2
Token, Data,
Handshake
(2-1)
Token, Data,
Handshake
(1-1)
IRP 1 Transaction
1-0 Transaction
1-1 Transaction
1-2
IRP 2 Transaction
2-0 Transaction
2-1 Transaction
2-2
Figure 5-12. Arrangement of IRPs to Transactions/Frames
5.9.3 Calculating Bus Transaction Times
When the USB system software allows a new pipe to be created for the bus, it must calculate how much
bus time is required for a given transaction. That bus time is based on the maximum packet size
information reported for an endpoint, the protocol overhead for the specific transaction type request, the
overhead due to signaling imposed bit-stuffing, inter-packet timings required by the protocol,
inter-transaction timings, etc. These calculations are required to ensure that the time available in a frame
is not exceeded. The equations used to determine transaction bus time are:
KEY
Data_bc Byte count of data payload
Host_Delay Time required for the host to prepare for or recover
from the transmission; host controller implementation
specific
Floor() Integer portion of argument
Hub_LS_Setup The time provided by the host controller for hubs to
enable low speed ports; Measured as the delay from end
of PRE PID to start of low speed SYNC; minimum of 4
full speed bit times.
BitStuffTime Function that calculates theoretical additional time
required due to bit stuffing in signaling; worst case
is (1.1667*8*Data_bc)
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Full Speed (Input)
Non-Isochronous Transfer (Handshake Included)
= 9107 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Isochronous Transfer (No Handshake)
= 7268 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Full Speed (Output)
Non-Isochronous Transfer (Handshake Included)
= 9107 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Isochronous Transfer (No Handshake)
= 6265 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Low Speed (Input)
= 64060 + (2 * Hub_LS_Setup) +
(676.67 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Low Speed (Output)
= 64107 + (2 * Hub_LS_Setup) +
(667.0 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
The Bus Times in the above equations are in nanoseconds and take into account propagation delays due
to distance the device is from the host. These are typical equations that can be used to calculate Bus
Time; however, different implementations may choose to use coarser approximations of these times.
The actual bus time taken for a given transaction will almost always be less than that calculated since bit
stuffing overhead is data dependent. Worst case bit stuffing is calculated as 1.1667 times the raw time
(i.e., the BitStuffTime function multiplies the Data_bc by 8*1.1667 in the equations). This means that
there will almost always be time unused on the bus (subject to data pattern specifics) after all regularly
scheduled transactions have completed. By placing all bulk/control transactions at the end of a frame, bit
stuffing time can be calculated as less than worst case. This more aggressive calculation comes at the
cost of having some bulk/control transfer’s transaction fail in a given frame every once in a while due to
exceeding the frame time when enough of the previous transactions require worst case bit stuffing. The
failed transaction can be retried, will seldom happen given random data patterns, and can result in a
better allocation estimate for isochronous and interrupt transfer times. In any case, the bus time made
available due to less bit stuffing can be reused as discussed in Section 5.9.5.
The Host_Delay term in the equations is host controller and system dependent and allows for additional
time a host controller may require due to delays in gaining access to memory or other implementation
dependencies. This term is incorporated into an implementation of these equations by using the Transfer
Constraint Management functions provided by the HCD interface. These equations are typically
implemented by a combination of USBD and HCD software working in cooperation. The results of these
calculations are used to determine whether a transfer or pipe creation can be supported in a given USB
configuration.
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5.9.4 Calculating Buffer Sizes in Functions/Software
Client software and functions both need to provide buffer space for pending data transactions awaiting
their turn on the bus. For non-isochronous pipes, this buffer space only needs to be large enough to hold
the next data packet. If more than one transaction request is pending for a given endpoint, the buffering
for each transaction must be supplied. Methods to calculate precise absolute minimum buffering a
function may require because of specific interactions defined between its client software and the function
are outside the scope of the USB specification.
The host controller is expected to be able to support an unlimited number of transactions pending for the
bus subject to available system memory for buffer and descriptor space, etc. Host controllers are allowed
to limit how many frames into the future they allow a transaction to be requested.
For isochronous pipes, Section 5.10.4 describes details affecting host side and device side buffering
requirements. In general, buffers need to be provided to hold approximately twice the amount of data
that can be transferred in 1 ms.
5.9.5 Bus Bandwidth Reclamation
USB bandwidth and bus access are granted based on a calculation of worst case bus transmission time
and required latencies. However, due to the constraints placed on different transfer types and the fact
that the bit stuffing bus time contribution is calculated as a constant but is data dependent, there will
frequently be bus time remaining in each frame time versus what the frame transmission time was
calculated to be. In order to support the most efficient use of the bus bandwidth, control and bulk
transfers are candidates to be moved over the bus as bus time becomes available. Exactly how a host
controller supports this is implementation dependent. A host controller can take into account the transfer
types of pending IRPs and implementation specific knowledge of remaining frame time to reuse
reclaimed bandwidth.
5.10 Special Considerations for Isochronous Transfers
Support for isochronous data movement between the host and a device is one of the system capabilities
supported by USB. Delivering isochronous data reliably over USB requires careful attention to detail.
The responsibility for reliable delivery is shared by several USB entities: the device/function, the bus,
the host controller, and one or more software agents. Since time is a key part of an isochronous transfer,
it is important for USB designers to understand how time is dealt within USB by these different entities.
All isochronous devices must report their capabilities in the form of device specific descriptors. The
capabilities should also be provided in a form that the potential customer can use to decide whether the
device offers a solution to his problem(s). The specific capabilities of a device can justify price
differences.
In any communication system, the transmitter and receiver must be synchronized enough to deliver data
robustly. In an asynchronous communication system, data can be delivered robustly by allowing the
transmitter to detect that the receiver has not received a data item correctly and simply retrying
transmission of the data.
In an isochronous communication system, the transmitter and receiver remain time and data synchronized
to deliver data robustly. USB does not support transmission retry of isochronous data so that minimal
bandwidth can be allocated to isochronous transfers and time synchronization is not lost due to a retry
delay. However, it is critical that a USB isochronous transmitter/receiver pair still remain synchronized
both in normal data transmission cases and in cases where errors occur on the bus.
In many systems that deal with isochronous data, a single global clock is used to which all entities in the
system synchronize; e.g., the PSTN - Public Switched Telephone Network. Given that a broad variety of
devices with different natural frequencies may be attached to USB, no single clock can provide all the
features required to satisfy the synchronization requirements of all devices and software while still
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supporting the cost targets of mass market PC products. USB defines a clock model that allows a broad
range of devices to coexist on the bus and have reasonable cost implementations.
This section presents options or features that can be used by isochronous endpoints to minimize behavior
differences between a non-USB implemented function and a USB version of the function. An example is
included to illustrate the similarities and differences of non-USB and USB versions of a function.
The remainder of the section presents key concepts of:
• USB Clock Model - What clocks are present in a USB subsystem that have impact on isochronous
data transfers.
• USB Frame Clock to Function Clock Synchronization Options - How the USB Frame clock can
relate to a function clock.
• Start of Frame Tracking - Responsibilities/Opportunities of isochronous endpoints with respect to the
Start of Frame (SOF) token and USB Frames.
• Data Prebuffering - Requirements on accumulating data before generation/transmission/consumption.
• Error Handling - Isochronous specific details for error handling.
• Buffering for Rate Matching - Equations that can be used to calculate buffer space required for
isochronous endpoints.
5.10.1 Example Non-USB Isochronous Application
The example used is a reasonably general case example. Other simpler or more complex cases are
possible and the relevant USB features identified can be used or not as appropriate.
The example consists of an 8 kHz mono microphone connected through a mixer driver that sends the
input data stream to 44 kHz stereo speakers. The mixer expects the data to be received and transmitted at
some sample rate and encoding. A rate matcher driver on input and output converts the sample rate and
encoding from the natural rate and encoding of the device to the rate and encoding expected by the
mixer. Figure 5-13 illustrates this example.
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Mono
Microphone
8kHz Sample Clock
(1 byte/sample)
Each DD has
independent
service rate
44.1KHz Sample Clock
(4 bytes/sample)
2x1 Byte Buffer
(2 Samples)
CD Stereo
Speakers
2x4 Byte Buffer
(2 Samples)
Microphone
Device Driver Speaker
Device Driver
Software
Hardware
2x160 Byte Buffer
(2 Services,
160 samples per service)
2x3528 Byte Buffer
(2 Services,
882 samples per service)
Master Clock
Transfer
Complete
Interrupt
Transfer
Complete
Interrupt
Traditional Bus
(e.g. PCI, ISA, ...)
Single sample
transfers
Rate
Matcher Rate
Matcher
Mixer Device
Driver
DMA
controller
1 sample at a time 1 sample at a time
1 speaker DD
service period
(n sample)
slop buffer
20ms service
period
20ms service
period
8MHz Bus Clock
Figure 5-13. Non-USB Isochronous Example
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A master clock (can be provided by software driven from the real time clock) in the PC is used to awaken
the mixer to ask the input source for input data and to provide output data to the output sink. In this
example, assume it awakens every 20 ms. The microphone and speakers each have their own sample
clocks that are unsynchronized with respect to each other or the master mixer clock. The microphone
produces data at its natural rate (1 byte samples, 8,000 times a second) and the speakers consume data at
their natural rate (4 byte samples, 44,100 times a second). The three clocks in the system can drift and
jitter with respect to each other. Each rate matcher may also be running at a different natural rate than
either the mixer driver, the input source/driver, or output sink/driver.
The rate matchers also monitor the long term data rate of their device compared to the master mixer
clock and interpolate an additional sample or merge two samples to adjust the data rate of their device to
the data rate of the mixer. This adjustment may be required every couple of seconds, but typically occurs
infrequently. The rate matchers provide some additional buffering to carry through a rate match.
Note that some other application might not be able to tolerate sample adjustment and would need some
other means of accommodating master clock to device clock drift or else would require some means of
synchronizing the clocks to ensure that no drift could occur.
The mixer always expects to receive exactly a service period of data (20 ms service period) from its input
device and produce exactly a service period of data for its output device. The mixer can be delayed up to
less than a service period if data or space is not available from its input/output device. The mixer
assumes that such delays don’t accumulate.
The input and output devices and their drivers expect to be able to put/get data in response to a hardware
interrupt from the DMA controller when their transducer has processed one service period of data. They
expect to get/put exactly one service period of data. The input device produces 160 bytes (10 samples)
every service period of 20 ms. The output device consumes 3528 bytes (882 samples) every 20 ms
service period. The DMA controller can move a single sample between the device and the host buffer at
a rate much faster than the sample rate of either device.
The input and output device drivers provide two service periods of system buffering. One buffer is
always being processed by the DMA controller. The other buffer is guaranteed to be ready before the
current buffer is exhausted. When the current buffer is emptied, the hardware interrupt awakens the
device driver and it calls the rate matcher to give it the buffer. The device driver requests a new IRP with
the buffer before the current buffer is exhausted.
The devices can provide two samples of data buffering to ensure that they always have a sample to
process for the next sample period while the system is reacting to the previous/next sample.
The service periods of the drivers are chosen to survive interrupt latency variabilities that may be present
in the operating system environment. Different operating system environments will require different
service periods for reliable operation. The service periods are also selected to place a minimum interrupt
load on the system since there may be other software in the system that requires processing time.
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5.10.2 USB Clock Model
Time is present in the USB system via clocks. In fact, there are multiple clocks in a USB system that
must be understood:
• Sample clock - This clock determines the natural data rate of samples moving between client
software on the host and the function. This clock does not need to be different between non-USB
and USB implementations.
• Bus clock - This clock runs at a 1.000 ms period (1 kHz frequency) and is indicated by the rate of
Start of Frame(SOF) packets on the bus. This clock is somewhat equivalent to the 8 MHz clock in
the non-USB example. In the USB case, the bus clock is often a lower frequency clock than the
sample clock, whereas the bus clock is almost always a higher frequency clock than the sample clock
in a non-USB case.
• Service clock - This clock is determined by the rate at which client software runs to service IRPs that
may have accumulated between executions. This clock also can be the same in the USB and
non-USB cases.
In most operating system environments that exist today, it is not possible to support a broad range of
isochronous communication flows if each device driver must be interrupted for each sample for fast
sample rates. Therefore, multiple samples, if not multiple packets, will be processed by client software
and then given to the host controller to sequence over the bus according to the prenegotiated bus access
requirements. Figure 5-14 presents an example for a reasonable USB clock environment equivalent to
the non-USB example in Figure 5-13.
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3 byte packets
Feedback Info
Host
Controller
Mono
Microphone
Hub
1KHz Bus Clock
8kHz Sample Clock
(1 byte/sample) 44.1KHz Sample Clock
(4 bytes/sample)
7-9 Byte Packets
7-9 samples per packet
8+9 Byte Buffer
(2 Packets)
172-184 Byte Packets
43-46 samples per packet
CD Stereo
Speakers
(44+45+1+1)x4
Byte Buffer
(2 Packets)
Software
Hardware
2x161 Byte Buffer
(2 Services,
159-161 samples per
service,
20 packets/service)
2x3532 Byte Buffer
(2 Services,
881-883 samples per
service
20 packets/service)
USB SW
Transfer
Complete
Interrupt
Microphone
Device Driver Speaker
Device Driver
Master Clock
Rate
Matcher Rate
Matcher
Mixer Device
Driver
QueueBuffer QueueBuffer
Transfer
Complete
Interrupt
1x3 Byte Buffer
(1 Services,
1 feedback per service
1 packets/service)
Each DD has
independent
service rate
1 sample
slop buffer
1x3 Byte
Buffer
(1 Packets)
1 speaker DD
service period
(n sample)
slop buffer
20ms service
period
20ms service
period
Figure 5-14. USB Isochronous Application
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Figure 5-14 shows a typical round trip path of information from a microphone as an input device to a
speaker as an output device. The clocks, packets, and buffering involved are also shown. Figure 5-14
will be explored in more detail in the following sections.
The focus of this example is to identify the differences introduced by USB compared to the previous
non-USB example. The differences are in the areas of buffering, synchronization given the existence of a
USB bus clock, and delay. The client software above the device drivers can be unaffected in most cases.
5.10.3 Clock Synchronization
In order for isochronous data to be manipulated reliably, the three clocks identified above must be
synchronized in some fashion. If the clocks are not synchronized, several clock to clock attributes can be
present that can be undesirable:
• Clock drift - Two clocks that are nominally running at the same rate can, in fact, have
implementation differences that result in one clock running faster or slower than the other over long
periods of time. If uncorrected, this variation of one clock compared to the other can lead to having
too much or too little data when data is expected to always be present at the time required.
• Clock jitter - A clock may vary its frequency over time due to changes in temperature, etc. This may
also alter when data is actually delivered compared to when it is expected to be delivered.
• Clock to clock phase differences - If two clocks are not phase locked, different amounts of data may
be available at different points in time as the beat frequency of the clocks cycle out over time. This
can lead to quantization/sampling related artifacts.
The bus clock provides a central clock with which USB hardware devices and software can synchronize
to one degree or another. However, the software will, in general, not be able to phase or frequency lock
precisely to the bus clock given the current support for “real time-like” operating system scheduling
support in most PC operating systems. Software running in the host can, however, know that data moved
over USB is packetized. For isochronous transfer types, a single packet of data is moved exactly once
per frame and the frame clock is reasonably precise. Providing the software with this information allows
it to adjust the amount of data it processes to the actual frame time that has passed.
5.10.4 Isochronous Devices
USB includes a framework for isochronous devices which defines Synchronization Types, how
isochronous endpoints provide data rate feedback, and how they can be connected together. Isochronous
devices include sampled analog devices; for example, audio and telephony devices, and synchronous data
devices. Synchronization Type classifies an endpoint according to its capability to synchronize its data
rate to the data rate of the endpoint that it is connected to. Feedback is provided by indicating accurately
what the required data rate is, relative to the SOF frequency. The ability to make connections depends on
the quality of connection that is required, the endpoint synchronization type, and the capabilities of the
host application which is making the connection. Additional Device Class-specific information may be
required, depending on the application.
Note that the term “data” is used very generally, and may refer to data which represents sampled analog
information (like audio), or it may be more abstract information. “Data rate” refers to the rate at which
analog information is sampled, or the rate at which data is clocked.
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The following information is required in order to determine how to connect isochronous endpoints:
• Synchronization Type
− Asynchronous - unsynchronized, although sinks provide data rate feedback
− Synchronous - synchronized to USB’s SOF
− Adaptive - synchronized using feedback or feedforward data rate information
• Available data rates
• Available data formats
Synchronization Type and data rate information are needed to determine if an exact data rate match
exists between source and sink, or if an acceptable conversion process exists which would allow the
source to be connected to the sink. It is the responsibility of the application to determine whether the
connection can be supported within available processing resources and other constraints (like delay).
Specific USB device classes define how to describe Synchronization Type and data rate information.
Data format matching and conversion is also required for a connection, but it is not a unique requirement
for isochronous connections. Details about format conversion can be found in other documents related to
specific formats.
5.10.4.1 Synchronization Type
Three distinct synchronization types are defined. Table 5-7 presents an overview of endpoint
synchronization characteristics for both source and sink endpoints. The types are presented in order of
increasing capability.
Table 5-7. Synchronization Characteristics
Source Sink
Asynchronous Free running Fs
Provides implicit feedforward (data stream)
Free running Fs
Provides explicit feedback (interrupt pipe)
Synchronous Fs locked to SOF
Uses implicit feedback (SOF)
Fs locked to SOF
Uses implicit feedback (SOF)
Adaptive Fs locked to sink
Uses explicit feedback (control pipe)
Fs locked to data flow
Uses implicit feedforward (data stream)
5.10.4.1.1 Asynchronous
Asynchronous endpoints cannot synchronize to SOF or any other clock in the USB domain. They source
or sink an isochronous data stream at either a fixed data rate (single frequency endpoints), a limited
number of data rates (32 kHz, 44.1 kHz, 48 kHz, …), or a continuously programmable data rate. If the
data rate is programmable, it is set during initialization of the isochronous endpoint. Asynchronous
devices must report their programming capabilities in the class specific endpoint descriptor as described
in their Device Class specification. The data rate is locked to a clock external to USB or to a free
running internal clock. These devices place the burden of data rate matching elsewhere in the USB
environment. Asynchronous source endpoints carry their data rate information implicitly in the number
of samples they produce per frame. Asynchronous sink endpoints must provide explicit feedback
information to an adaptive driver (refer to Section 5.10.4.2).
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An example of an asynchronous source is a CD-audio player that provides its data based on an internal
clock or resonator. Another example is a Digital Audio Broadcast (DAB) receiver or a Digital Satellite
Receiver (DSR). Here too, the sample rate is fixed at the broadcasting side and is beyond USB control.
Asynchronous sink endpoints could be low cost speakers, running off of their internal sample clock.
Another case arises when there are two or more devices present on USB that need to have mastership
control over SOF generation in order to operate as synchronous devices. This could happen if there were
two telephony devices, each locked to a different external clock. One telephony device could be digitally
connected to a PBX which is not synchronized to the ISDN. The other device could be connected
directly to the ISDN. Each device will source or sink data to/from the network side at an externally
driven rate. Since only one of the devices can take mastership over SOF, the other will sink or source
data at a rate which is asynchronous to SOF. This example indicates that every device capable of SOF
mastership may be forced to operate as an asynchronous device.
5.10.4.1.2 Synchronous
Synchronous endpoints can have their clock system (their notion of time) controlled externally through
SOF synchronization. These endpoints must be doing one of the following:
• Slaving their sample clock to the 1 ms SOF tick (by means of a programmable PLL).
• Controlling the rate of USB SOF generation so that their data rate becomes automatically locked to
SOF. In case these endpoints are not granted SOF mastership, they must degenerate to the
asynchronous mode of operation (refer to the asynchronous example).
Synchronous endpoints may source or sink isochronous data streams at either a fixed data rate (single
frequency endpoints), a limited number of data rates (32 kHz, 44.1 kHz, 48 kHz, …), or a continuously
programmable data rate. If programmable, the operating data rate is set during initialization of the
isochronous endpoint. The number of samples or data units generated in a series of USB frames is
deterministic and periodic. Synchronous devices must report their programming capabilities in the class
specific endpoint descriptor as described in their Device Class specification.
An example of a synchronous source is a digital microphone that synthesizes its sample clock from SOF
and produces a fixed number of audio samples every USB frame. Another possibility is a 64 kb/s bit-
stream from an ISDN “modem.” If the USB SOF generation is locked to the PSTN clock (perhaps
through the same ISDN device), the data generation will also be locked to SOF and the endpoint will
produce a stable 64 kb/s data stream, referenced to the SOF time notion.
5.10.4.1.3 Adaptive
Adaptive endpoints are the most capable endpoints possible. They are able to source or sink data at any
rate within their operating range. Adaptive source endpoints produce data at a rate which is controlled by
the data sink. The sink provides feedback (refer to Section 5.10.4.2) to the source which allows the
source to know the desired data rate of the sink. Adaptive endpoints can communicate with all types of
sink endpoints. For adaptive sink endpoints, the data rate information is embedded in the data stream.
The average number of samples received during a certain averaging time determines the instantaneous
data rate. If this number changes during operation, the data rate is adjusted accordingly.
The data rate operating range may center around one rate (e.g., 8 kHz), select between several
programmable or auto detecting data rates (32 kHz, 44.1 kHz, 48 kHz, …), or may be within one or more
ranges (e.g., 5 kHz to 12 kHz, 44 kHz to 49 kHz). Adaptive devices must report their programming
capabilities in the class specific endpoint descriptor as described in their Device Class specification
An example of an adaptive source is a CD player that contains a fully adaptive sample rate converter
(SRC) so that the output sample frequency no longer needs to be 44.1 kHz but can be anything within the
operating range of the SRC. Adaptive sinks include such endpoints as high-end digital speakers,
headsets, etc.
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5.10.4.2 Feedback
An asynchronous sink provides feedback to an adaptive source by indicating accurately what its desired
data rate (Ff) is, relative to the USB SOF frequency. The required data rate is accurate to better than one
sample per second (1 Hz) in order to allow a high quality source rate to be created and to tolerate delays
and errors in the feedback loop.
The Ff value consists of a fractional part, in order to get the required resolution with 1 kHz frames, and
an integer part, which gives the minimum number of samples per frame. Ten bits are required to resolve
one sample within a 1 kHz frame frequency (1000 / 2^10 = 0.98). This is a 10 bit fraction, represented in
unsigned fixed binary point 0.10 format. The integer part needs 10 bits (2^10 = 1024) to encode up to
1023 1-byte samples per frame. The 10 bit integer is represented in unsigned fixed binary point 10.0
format. The combined Ff value can be coded in unsigned fixed binary point 10.10 format, which fits into
three bytes (24 bits). Since the maximum integer value is fixed to 1023, the 10.10 number will be left-
justified in the 24 bits, so that it has a 10.14 format. Only the first 10 bits behind the binary point are
required. The lower 4 bits may be optionally used to extend the precision of Ff; otherwise, they shall be
reported as 0. The bit and byte ordering follows the definitions of other multi-byte fields contained in
Chapter 8.
Each frame, the adaptive source adds Ff to any remaining fractional sample count from the previous
frame, sources the number of samples in the integer part of the sum, and retains the fractional sample
count for the next frame. The source can look at the behavior of Ff over many frames to determine an
even more accurate rate, if it needs to.
The sink can determine Ff by counting cycles of a clock with a frequency of Fs * 2^P for a period of
2^(10-P) frames, where P is an integer. P is practically bound to be in the range [0,10] because there is
no point in using a clock slower than Fs, and no point in trying to update more than once a frame. The
counter is read into Ff and reset every 2^(10-P) frames. As long as no clock cycles are skipped, the count
will be accurate over the long term. An endpoint only needs to implement the number of counter bits
that it requires for its maximum Ff.
A digital telephony endpoint, for example, will usually derive its 8 kHz Fs by dividing down the 64 kHz
clock (P=3) which it uses to serialize the data stream. The 64 kHz clock phase can also give an
additional 1 bit of accuracy, effectively giving P=4. This would give Ff updates every 2^(10-4) = 64
frames. A 13-bit counter would be required to obtain Ff, with 3 bits for 8 samples per frame, and 10 bits
for the fractional part. The 13 bits would provide a 3.10 field within the 10.14 Ff value, with the
remaining bits set to 0.
The choice of P is endpoint specific, and should be between 1 and 9, inclusive. Larger values of P are
preferred, since they reduce the size of the frame counter and increase the rate at which Ff is updated.
More frequent updates result in a tighter control of the source data rate, which reduces the buffer space
required to handle Ff changes. P should be less than 10 so that Ff is averaged across at least two frames
in order to reduce SOF jitter effects. P should not be 0 in order to keep the deviation in the number of
samples sourced to less than 1 in the event of a lost Ff value.
Interrupt transfers are used to read Ff from the feedback register at periodic intervals. The desired
reporting rate for the feedback should be 2^(10-P) ms (frames). Ff will be reported at most once per
update period. There is nothing to be gained by reporting the same Ff value more than once per update
period. The endpoint may choose to only report F f if the updated value has changed from the previous Ff
value.
It is possible that the source will deliver one too many or one too few samples over a long period, due to
errors or accumulated inaccuracies in measuring Ff. The sink must have sufficient buffer capability to
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accommodate this. When the sink recognizes this condition, it should adjust the reported Ff value to
correct it. This may also be necessary to compensate for relative clock drifts. The implementation of
this correction process is endpoint specific and is not specified.
An adaptive source may obtain the sink data rate information from an adaptive sink which is locked to
the same clock as the sink, as would be the case for a two-way speech connection. In this case, the
feedback pipe is not needed.
5.10.4.3 Connectivity
In order to fully describe the source-to-sink connectivity process, an interconnect model is presented.
The model indicates the different components involved and how they interact to establish the connection.
The model provides for multi-source/multi-sink situations. Figure 5-15 illustrates a typical situation
(highly condensed and incomplete). A physical device is connected to the host application software
through different hardware and software layers as described in the USB specification. At the client
interface level, a “virtualized” device is presented to the application. From the application standpoint,
only virtual devices exist. It is up to the device driver and client software to decide what the exact
relation is between physical and virtual device.
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Source
device drive rSink
device drive r
device drive rSink
Source devic e drive r Client
isoc . pipe
isoc . pipe
isoc . pipe
isoc . pipe
Client
Client
Client
Clientdevice drive r
ha rd di sk
Application
Ho s t env iron men t
USB environm ent
physica l sourc es
vir tua l sou rces
vir tua l sinks
physica l s inks de vice drive r Client
CD-ROM
Figure 5-15. Example Source/Sink Connectivity
Device manufacturers (or operating system vendors) must provide the necessary device driver software
and client interface software to convert their device from the physical implementation to a USB
compliant software implementation, the virtual device. As stated before, depending on the capabilities
built into this software, the virtual device can exhibit different synchronization behavior from the
physical device. However, the synchronization classification equally applies to both physical and virtual
devices. All physical devices belong to one of the three possible synchronization types. Therefore, the
capabilities that have to be built into the device driver and/or client software are the same as the
capabilities of a physical device. The word “application” must be replaced by “device driver/client
software.” In the case of a physical source to virtual source connection, “virtual source device” must be
replaced by “physical source device” and “virtual sink device” must be replaced by “virtual source
device.” In the case of a virtual sink to physical sink connection, “virtual source device” must be
replaced by “virtual sink device” and “virtual sink device” must be replaced by “physical sink device.”
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Placing the rate adaptation functionality into the device driver/client software layer has the distinct
advantage of isolating all applications, using the device from the specifics and problems associated with
rate adaptation. Applications that would otherwise be multi-rate degenerate to simpler mono-rate
systems.
Note that the model is not limited to only USB devices. A CD-ROM drive, for example, containing
44.1 kHz audio can appear as either an asynchronous, synchronous, or adaptive source. Asynchronous
operation means that the CD-ROM fills its buffer at the rate that it reads data from the disk, and the
driver empties the buffer according to its USB service interval. Synchronous operation means that the
driver uses the USB service interval (e.g., 10 ms) and nominal sample rate of the data (44.1 kHz) to
determine to put out 441 samples every USB service interval. Adaptive operation would build in a
sample rate converter to match the CD-ROM output rate to different sink sampling rates.
Using this reference model, it is possible to define what operations are necessary to establish connections
between various sources and sinks. Furthermore, the model indicates at what level these operations must
or can take place. First there is the stage where physical devices are mapped onto virtual devices and
vice versa. This is accomplished by the driver and/or client software. Depending on the capabilities
included in this software, a physical device can be transformed into a virtual device of an entirely
different synchronization type. The second stage is the application that uses the virtual devices. Placing
rate matching capabilities at the driver/client level of the software stack relieves applications
communicating with virtual devices from the burden of performing rate matching for every device that is
attached to them. Once the virtual device characteristics are decided, the actual device characteristics are
not any more interesting than the actual physical device characteristics of another driver.
As an example, consider a mixer application that connects at the source side to different sources, each
running at their own frequencies and clocks. Before mixing can take place, all streams must be
converted to a common frequency and locked to a common clock reference. This action can be
performed in the physical to virtual mapping layer or it can be handled by the application itself for each
source device independently. Similar actions must be performed at the sink side. If the application sends
the mixed data stream out to different sink devices, it can either do the rate matching for each device
itself or it can rely on the driver/client software to do that if possible.
Table 5-8 indicates at the intersections what actions the application must perform to connect a source
endpoint to a sink endpoint.
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Table 5-8 Connection Requirements
Source Endpoint
Sink Endpoint Asynchronous Synchronous Adaptive
Asynchronous Async Source/Sink RA
See Note 1. Async SOF/Sink RA
See Note 2. Data + Feedback
Feedthrough
See Note 3.
Synchronous Async Source/SOF RA
See Note 4. Sync RA
See Note 5. Data Feedthrough +
Application Feedback
See Note 6.
Adaptive Data Feedthrough
See Note 7. Data Feedthrough
See Note 8. Data Feedthrough
See Note 9.
Notes:
1. Asynchronous RA in the application. Fsi is determined by the source, using the feedforward information
embedded in the data stream. Fso is determined by the sink, based on feedback information from the sink.
If nominally Fsi = Fso, the process degenerates to a feedthrough connection if slips/stuffs due to lack of
synchronization are tolerable. Such slips/stuffs will cause audible degradation in audio applications.
2. Asynchronous RA in the application. Fsi is determined by the source but locked to SOF. Fso is
determined by the sink, based on feedback information from the sink. If nominally Fsi = Fso, the process
degenerates to a feedthrough connection if slips/stuffs due to lack of synchronization are tolerable. Such
slips/stuffs will cause audible degradation in audio applications.
3. If Fso falls within the locking range of the adaptive source, a feedthrough connection can be established.
Fsi = Fso, and both are determined by the asynchronous sink, based on feedback information from the sink.
If Fso falls outside the locking range of the adaptive source, the adaptive source is switched to synchronous
mode and Note 2 applies.
4. Asynchronous RA in the application. Fsi is determined by the source. Fso is determined by the sink and
locked to SOF. If nominally Fsi = Fso, the process degenerates to a feedthrough connection if slips/stuffs
due to lack of synchronization are tolerable. Such slips/stuffs will cause audible degradation in audio
applications.
5. Synchronous RA in the application. Fsi is determined by the source and locked to SOF. Fso is determined
by the sink and locked to SOF. If Fsi = Fso, the process degenerates to a loss-free feedthrough
connection.
6. The application will provide feedback to synchronize the source to SOF. Then the adaptive source appears
to be a synchronous endpoint and Note 5 applies.
7. If Fsi falls within the locking range of the adaptive sink, a feedthrough connection can be established.
Fsi = Fso and are determined by and locked to the source.
If Fsi falls outside the locking range of the adaptive sink, synchronous RA is done in the host to provide an
Fso that is within the locking range of the adaptive sink.
8. If Fsi falls within the locking range of the adaptive sink, a feedthrough connection can be established.
Fso = Fsi and are determined by the source and locked to SOF.
If Fsi falls outside the locking range of the adaptive sink, synchronous RA is done in the host to provide an
Fso that is within the locking range of the adaptive sink.
9. The application will use feedback control to set Fso of the adaptive source when the connection is set up.
The adaptive source operates as an asynchronous source in the absence of ongoing feedback information
and Note 7 applies.
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In cases where RA is needed but not available, the rate adaptation process could be mimicked by sample
dropping/stuffing. The connection could then still be made, possibly with a warning about poor quality;
otherwise, the connection cannot be made.
5.10.4.3.1 Audio Connectivity
When the above is applied to audio data streams, the rate adaptation process is replaced by sample rate
conversion, which is a specialized form of rate adaptation. Instead of error control, some form of sample
interpolation is used to match incoming and outgoing sample rates. Depending on the interpolation
techniques used, the audio quality (distortion, signal to noise ratio, etc.) of the conversion can vary
significantly. In general, higher quality requires more processing power.
5.10.4.3.2 Synchronous Data Connectivity
For the synchronous data case, rate adaptation is used. Occasional slips/stuffs may be acceptable to many
applications which implement some form of error control. Error control includes error detection and
discard, error detection and retransmit, or forward error correction. The rate of slips/stuffs will depend
on the clock mismatch between the source and sink, and may be the dominant error source of the
channel. If the error control is sufficient, then the connection can still be made.
5.10.5 Data Prebuffering
USB requires that devices prebuffer data before processing/transmission to allow the host more flexibility
in managing when each pipe’s transaction is moved over the bus from frame to frame.
For transfers from function to host, the endpoint must accumulate samples during frame X until it
receives the Start of Frame (SOF) token packet for frame X+1. It “latches” the data from frame X into its
packet buffer and is now ready to send the packet containing those samples during frame X+1. When it
will send that data during the frame is determined solely by the host controller and can vary from frame
to frame.
For transfers from host to function, the endpoint will accept a packet from the host sometime during
frame Y. When it receives the SOF for frame Y+1, it can then start processing the data received in frame
Y.
This approach allows an endpoint to use the SOF token as a stable clock with very little jitter/drift when
the host controller moves the packet over the bus while also allowing the host controller to vary within a
frame precisely when the packet is actually moved over the bus. This prebuffering introduces some
additional delay between when a sample is available at an endpoint and when it moves over the bus
compared to an environment where the bus access is at exactly the same time offset from SOF from
frame to frame.
Figure 5-16 shows the time sequence where for a function to host transfer (IN process), data D0 is
accumulated during frame Fi at time Ti, and transmitted to the host during frame FI+1. Similarly, for a host
to function transfer (OUT process), data D0 is received by the endpoint during frame Fi+1 and processed
during frame Fi+2.
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Time:
Frame:
Data on Bus:
OUT Process:
IN Process
T
i
T
i+1
T
i+2
T
i+3
... T
m
T
m+1
...
F
i
F
i+1
F
i+2
F
i+3
... F
m
F
m+1
...
D
0
D
1
D
2
... D
0
D
1
...
D
0
D
1
... D
0
...
D
0
D
1
... D
0
...
Figure 5-16. Data Prebuffering
5.10.6 SOF Tracking
Functions supporting isochronous pipes must receive and comprehend the SOF token to support
prebuffering as previously described. Given that SOFs can be corrupted, a device must be prepared to
recover from a corrupted SOF. These requirements limit isochronous transfers to full speed devices only,
since low speed devices do not see SOFs on the bus. Also, since SOF packets can be damaged in
transmission, devices that support isochronous transfers need to be able to synthesize the existence of an
SOF that they may not see due to a bus error.
Isochronous transfers require the appropriate data to be transmitted in the corresponding frame. USB
requires that when an isochronous transfer is presented to the host controller, it identifies the frame
number for the first frame. The host controller must not transmit the first transaction before the indicated
frame number. Each subsequent transaction in the IRP must be transmitted in succeeding frames. If
there are no transactions pending for the current frame, then the host controller must not transmit
anything for an isochronous pipe. If the indicated frame number is passed, the host controller must skip
(i.e., not transmit) all transactions until the one corresponding to the current frame is reached.
5.10.7 Error Handling
Isochronous transfers provide no data packet retries (i.e., no handshakes are returned to a transmitter by a
receiver) so that timeliness of data delivery is not perturbed. However, it is still important for the agents
responsible for data transport to know when an error occurs and how the error affects the communication
flow. In particular, for a sequence of data packets (A,B,C,D), USB allows sufficient information such
that a missing packet (A,_,C,D) can be detected and will not unknowingly be turned into an incorrect
data or time sequence (A,C,D or A,_,B,C,D). The protocol provides four mechanisms that support this:
exactly one packet per frame, SOF, CRC, and bus transaction timeout.
Isochronous transfers require exactly 1 data transaction every frame for normal operation. USB does not
dictate what data is transmitted in each frame. The data transmitter/source determines specifically what
data to provide. This regular data per frame provides a framework that is fundamental to detecting
missing data errors. Any phase of a transaction can be damaged during transmission on the bus.
Chapter 8 describes how each error case affects the protocol.
Since every frame is preceded by an SOF packet and a receiver can see SOFs on the bus, a receiver can
determine that its expected transaction did not occur between two SOFs. Additionally, since even an
SOF packet can be damaged, a device must be able to reconstruct the existence of a missed SOF as
described in Section 5.10.6.
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A data packet may be corrupted on the bus; therefore, CRC protection allows a receiver to determine that
the data packet it received was corrupted.
Finally, the protocol defines the details that allow a receiver to determine via bus transaction timeout that
it is not going to receive its data packet after it has successfully seen its token packet.
Once a receiver has determined that a data packet was not received, it may need to know the size of the
data that was missed in order to recover from the error with regard to its functional behavior. If the
communication flow is always the same data size per frame, then the size is always a known constant.
However, in some cases the data size can vary from frame to frame. In this case, the receiver and
transmitter have an implementation dependent mechanism to determine the size of the lost packet.
In summary, whether a transaction is actually moved successfully over the bus or not, the transmitter and
receiver always advance their data/buffer streams one transaction per frame to keep data per time
synchronization. The detailed mechanisms described above allow detection, tracking, and reporting of
damaged transactions so that a function or its client software can react to the damage in a function
appropriate fashion. The details of that function/application specific reaction are outside the scope of the
USB specification.
5.10.8 Buffering for Rate Matching
Given that there are multiple clocks that affect isochronous communication flows in USB, buffering is
required to rate match the communication flow across USB. There must be buffer space available both in
the device per endpoint and on the host side on behalf of the client software. These buffers provide space
for data to accumulate until it is time for a transfer to move over USB. Given the natural data rates of the
device, the maximum size of the data packets that move over the bus can also be calculated. Figure 5-17
shows the equations used to determine buffer size on the device and host and maximum packet size that
must be requested to support a desired data rate. These equations allow a device and client software
design time determined service clock rate (variable X), sample clock rate (variable C), and sample size
(variable S). USB only allows one transaction per bus clock. These equations should provide design
information for selecting the appropriate packet size that an endpoint will report in its characteristic
information and the appropriate buffer requirements for the device/endpoint and its client software.
Figure 5-14 shows actual buffer, packet, and clock values for a typical isochronous example.
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X Service Clock
1KHz Bus Clock
C Sample Clock
(S byte/sample)
M = (2 * N * P) Byte Buffer
for 2 Services,
N = (CEIL(1KHz / X)) packets
per service
B = 2 * P
Byte Buffer
(2 Packets)
P = (CEIL(C / 1KHz) * S)
Byte Packets
Isochronous Rate (Clock) Matching
By Buffering
Figure 5-17. Packet and Buffer Size Formulas for Rate Matched Isochronous Transfers
The USB data model assumes that devices have some natural sample size and rate. USB supports the
transmission of packets that are multiples of sample size to make error recovery handling easier when
isochronous transactions are damaged on the bus. If a device has no natural sample size or if its samples
are larger than a packet, it should describe its sample size as being one byte. If a sample is split across a
data packet, the error recovery can be harder when an arbitrary transaction is lost. In some cases, data
synchronization can be lost unless the receiver knows in what frame number each partial sample is
transmitted. Furthermore, if the number of samples can vary due to clock correction (e.g., for a
non-derived device clock), it may be difficult or inefficient to know when a partial sample is transmitted.
Therefore, USB does not split samples across packets.
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Chapter 6
Mechanical
This chapter provides the mechanical specification for the cables and connectors for USB hubs,
functions, and hosts. The specification includes the dimensions, materials, electrical, and reliability
requirements.
6.1 Architectural Overview
The physical topology of a USB channel consists of connecting a hub or function to another hub,
function, or host. There are two speeds at which the channel can operate. The fully rated speed of
12 Mbs requires the use of a shielded cable with two internal power conductors and two internal signal
conductors. For lower cost and lower speed, a sub-channel at 1.5 Mbs is allowed by the specification
with the use of unshielded cabling.
Two types of plugs and receptacles are specified for USB: series A and series B. Series A plugs and
receptacles are to be used for those devices on which the external cable is permanently attached to
devices such as keyboards, mice, and hubs. Series B plugs and receptacles are to be used for those
devices that require an external connector so that the USB cabling is detachable, such as printers,
scanners, and modems. There may be internal connectors that will need to meet the electrical
requirements of the USB specification, but the mechanical aspects of the internal connector are not part
of the USB specification. All cables that have a series A or series B connector should meet the
construction requirements of the fully rated channel and have maximum gauge power conductors.
Series A and B connectors cannot be interchanged; therefore, there is no possibility that the integrity of
the bus will be compromised.
6.2 Dimensioning Requirements
Default tolerances are listed in Table 6-1, unless otherwise specified. The dimensions are in millimeters.
Table 6-1. Default Tolerances
Over 1 to
5Over 5 to
30 Over 30
to 100 Over 100
to 300 Over 300
to 1000 Over 1000
to 3000 Over 3000
to 5000
±0.3 ±0.4 ±0.6 ±0.8 ±1.6 ±2.5 ±10
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6.3 Cable
All hubs and functions as defined in this specification will have one permanently attached cable or be
terminated with a series B connector.
The standard USB cable will consist of one pair of 20-28 AWG wire for power distribution with another
28 AWG pair twisted, with a shield and overall jacket. This will be used for typical peripherals operating
at the rated 12 Mbs signaling.
An alternative cable of identical gauge but without the twisted conductors and shield can be used for
1.5 Mbs signaling. This will be used in a sub-channel application where the wider bandwidth is not
needed.
In all other respects, the mechanical specifications for the sub-channel will be identical to the fully rated
specification.
6.3.1 Cable Specification
This specification defines the detailed requirements of a twisted pair, 28 AWG, PVC, round cable with two
power leads (non twisted) for fully rated devices as well as a four-conductor cable with an overall jacket for
the sub-channel devices.
6.3.1.1 Applicable Documents
Underwriters’ Laboratory, Inc.
UL-STD-94 Tests for Flammability of Plastic Materials
for Parts in Devices and Appliances
UL-Subject-444 Communication Cables
American Standard Test Materials
ASTM-D-4565 Physical and Environmental Performance
Properties of Insulation and Jacket for
Telecommunication Wire and Cable, Test
Standard Method
ASTM-D-4566 Electrical Performance Properties of
Insulation and Jacket for Telecommunication
Wire and Cable, Test Standard Method
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6.3.1.2 Requirements
Mechanical
Material/Finish:
Outer Jacket: Polyvinyl chloride (PVC)
Color: Recommended; frost white
Conductor Insulation: Semi-rigid PVC for power conductors and polyethylene (optionally
foamed) or equivalent meeting the requirements of Table 6-4 for the signal pair (fully rated
12 Mbs only).
Conductors: Refer to Table 6-2 for power distribution conductors. The signaling conductor
pair is 28 AWG.
Table 6-2. Conductors - Pair for Power Distribution
Gauge and Conductor Outer Diameter
28 AWG - .84 ±.05 mm
26 AWG - 1.00 ±.05 mm
24 AWG - 1.10 ±.07 mm
22 AWG - 1.30 ±.07 mm
20 AWG - 1.50 ±.08 mm
Cable Construction:
Fully Rated: Cable shall consist of four conductors; one twisted pair with 28 AWG
conductors (data pair), one non-twisted pair (power distribution pair) with an overall jacket.
The twisted pair shall have one twist per 6-8 cm.
Sub-Channel: Cable shall consist of four conductors: one pair with 28 AWG conductors
(data pair) and one pair for power distribution with an overall jacket.
Outer Jacket:
A. Outside Diameter:
Fully rated and sub-channel: 3.4 to 5.3 mm.
B. Color: Frost white recommended.
Conductor Insulation:
A. Outside Diameter: Refer to Table 6-2.
B. Color: Refer to Table 6-5.
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Conductors:
Fully Rated:
A. 28 AWG stranded - twisted pair
B. Non twist - One pair per Table 6-2 stranded selected as needed for proper DC
power distribution.
C. Shield: Required for EMI compliance. Suggest aluminized mylar wrap with a
28 AWG drain wire and 65% minimum. coverage tinned copper mesh over the
foil.
Sub-Channel:
A. 28 AWG stranded - One pair.
B. Non Twist - One pair per Table 6-2 stranded selected as needed for proper DC
power distribution.
Break Strength: 45 Newtons minimum when tested in accordance with ASTM-D-4565.
Electrical:
Voltage Rating: 30 V (rms) maximum.
Conductor Resistance: As shown in Table 6-3, when tested in accordance with ASTM-D-4565.
Refer to Section 6.4 for limitations on DC voltage drop.
Table 6-3. Conductor Resistance
Gauge DC Resistance (max.)
28 0.232 Ω/m
26 0.145 Ω/m
24 0.0909 Ω/m
22 0.0574 Ω/m
20 0.0358 Ω/m
Resistance Unbalance: The resistance unbalance between the two conductors shall not exceed 5%
when tested in accordance with ASTM-D-4566.
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Length: Maximum cable length shall not exceed 3 meters for the sub-channel and 5 meters for the
fully rated channel.
Fully rated only:
Attenuation: The attenuation of the signal pair measured in accordance with ASTM-D-4566
shall not exceed the values in Table 6-4.
Characteristic Impedance: The characteristic impedance of the signal pair shall be
90 Ω ±15%, when measured in accordance with ASTM-D-4566 over the frequency
range of 1-16 MHz.
Propagation delay of the fully rated twisted signal pair must be equal to or less than 30 ns
over the length of cable used in the frequency range of 1-16 MHz. Refer to Section 6.5 if
the cabling cannot meet this requirement. Note that the signal pair might require a foamed
polyethylene insulation on the signal pair to meet the propagation delay requirements for a
full length cable.
Skew: The maximum skew between the two signal pair must be less than 65.6 ps/m at 10 kHz.
Table 6-4. Signal Attenuation
Frequency (MHz) Attenuation (maximum) dB/305 m
0.064 4.80
0.256 6.70
0.512 8.20
0.772 9.40
1.000 12.0
4.000 24.0
8.000 35.0
10.000 38.0
16.000 48.0
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Environmental:
Temperature Rating: -40 °C to 60 °C storage; 0 °C to 40 °C operating.
Laboratory Approvals: Item shall be UL listed per UL Subject 444. Class 2, Type CM for
Communication Cable Requirements.
Flammability: Plastic material used in the construction of this item shall meet the
flammability requirements of NEC Article 800.
Marking: Item shall be legibly and permanently marked with the vendor name or symbol,
UL File Number, Type CM (UL).
Qualification: All suppliers, when requested, must be able to supply appropriate
documentation to show conformance to the requirements of this chapter.
All electrical measurements should be made with a sample cable removed from the reel or container.
The cable must rest on a non-conductive surface or be on aerial supports.
Table 6-5. Cable Color Code
Wire Color
+ Data Green
- Data White
VCC Red
Ground Black
6.3.2 Connector (Series A)
Figures 6-1 through 6-7 describe the Series A connector.
6.3.2.1 Plug (Series A)
The USB (Series A) plug is a four-position plug with a shielded housing compatible with the cabling as
described in Section 6.3. The following guidelines ensure intermateability. The recommended color is
frost white for the overmold. Internal plastic features can be frost white or equivalent.
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Figure 6-1. Plug Connector (Series A)
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Figure 6-2. Plug Contact Detail (Series A)
The termination of the conductors to the plug contacts may be done as deemed appropriate by the
connector’s manufacturing process.
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6.3.2.2 Receptacle (Series A)
Figure 6-3. Receptacle (Series A)
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There are four variants of the receptacle available for general use. They are vertical, right angled, panel
mount, and stacked right angled with SMT as well as through hole variants. However, as long as the
interface requirements of the specification are met, it is up to the implementer as to what form the
receptacle will take. Internal plastic features should be frost white or equivalent.
6.3.2.3 Connector Mating Features (Series A)
Figure 6-4. Connector Mating Features (Series A)
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6.3.2.4 Receptacle PWB Foot Print (Series A)
Figure 6-5. PWB Footprint for Receptacle, SMT (Series A)
Figure 6-6. PWB Footprint for Receptacle, Throughhole (Series A)
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Figure 6-7. PWB Footprint for Receptacle, Stacked Right Angle (Series A)
6.3.3 Connector (Series B)
Figures 6-8 through 6-12 describe the Series B connector.
6.3.3.1 Plug (Series B)
The USB (Series B) plug is a four position plug with shielded housing compatible with the cabling as
described in Section 6.3. The following guidelines ensure intermateability. The recommended color is
frost white for the overmold. The internal features can be frost white or equivalent.
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Figure 6-8. Plug Connector (Series B)
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Figure 6-9. Plug Contact Detail (Series B)
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6.3.3.2 Receptacle (Series B)
Figure 6-10. Receptacle (Series B)
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6.3.3.3 Connector Mating Features (Series B)
Figure 6-11. Connector Mating Features (Series B)
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6.3.3.4 Receptacle PWB Foot Print (Series B)
Figure 6-12. PWB Footprint for Receptacle, Throughhole (Series B)
6.3.4 Serial Bus Icon
The USB icon, shown in Figure 6-13, should be molded into the connector and also placed on the product
for ease of identifying the USB port. It is recommended that the icon on the product and the one on the
plug be adjacent to each other when the plug and receptacle are mated. This icon can be used for both
series A and B connector schemes. On the plug, there should be a 0.635 mm rectangular recessed area
around the icon such that there is a perceptible feel of the icon.
Figure 6-13. USB Icon Artwork
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6.3.5 Plug/Receptacle Mechanical and Electrical Requirements
6.3.5.1 Contact Numbering (Series A and B)
Table 6-6. Contact Numbering
Contact Number Signal Name Comment
1 VC C Cable power
2 - Data
3 + Data
4 Ground Cable ground
6.3.5.2 Ratings
Voltage: 30 Vac (rms).
Current: 1 A maximum per contact not to exceed 30 °C temperature rise.
Temperature: -40 °C to 60 °C storage; 0 °C to 40 °C operating.
6.3.5.3 Performance and Test Description
Product is designed to meet electrical, mechanical, and environmental performance requirements
specified in Table 6-7. Unless otherwise specified, all tests shall be performed at ambient environmental
conditions. Cable construction and/or part number used for testing must be included with test report.
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Table 6-7. Test Requirements and Procedures Summary
Test Description Requirement Procedure
Examination of product Meets requirements
of Section 6.3 Visual, dimensional, and functional compliance
ELECTRICAL
Termination resistance 30 mΩ maximum EIA 364-23
Subject mated contacts assembled in housing to
20 mV maximum open circuit at 100 mA
maximum. See Figure 6-14.
Insulation resistance 1000 MΩ minimum EIA 364-21
Test between adjacent contacts of mated and
unmated connector assemblies
Dielectric withstanding
voltage 750 Vac at sea level EIA 364-20
Test between adjacent contacts of mated and
unmated connector assemblies
Capacitance 2 pF maximum EIA 364-30
Test between adjacent circuits of unmated
connectors at 1 kHz
MECHANICAL
Vibration, random No discontinuities of
1 µs or longer
duration.
See Note.
EIA 364-28 Condition V Test letter A. Subject
mated connectors to 5.35 G's rms. Fifteen
minutes in each of three mutually perpendicular
planes. See Figure 6-15.
Physical shock No discontinuities of
1 µs or longer
duration.
See Note.
EIA 364-27 Condition H. Subject mated
connectors to 30 G's half-sine shock pulses of
11 ms duration. Three shocks in each direction
applied along three mutually perpendicular
planes, 18 total shocks. See Figure 6-15 for the
test setup.
Durability See Note. EIA 364-09
Mate and unmate connector assemblies for 1500
cycles at maximum rate of 200 cycles per hour
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Table 6-7. Test Requirements and Procedures Summary (Continued)
Test Description Requirement Procedure
Mating force 35 Newtons
maximum EIA 364-13
Measure force necessary to mate connector
assemblies at maximum rate of 12.5 mm per
minute.
Unmating force 10 Newtons
minimum EIA 364-13
Measure force necessary to unmate connector
assemblies at maximum rate of 12.5 mm per
minute.
Cable Retention Cable shall not
dislodge from
cable crimp.
Apply axial load of 25 Newtons to the cable.
ENVIRONMENTAL
Thermal shock See Note. EIA 364-32 Test Condition I.
Subject mated connectors to five cycles between
-55 °C and 85 °C.
Humidity See Note. EIA-364-31 Method II Test Condition A. Subject
mated connectors to 96 hours at 40 °C with 90 to
95% RH.
Temperature life See Note. EIA-364-17 Test Condition 3 Method A. Subject
mated connectors to temperature life at 85 °C for
250 hours.
Note:
Shall meet visual requirements, show no physical damage, and shall meet requirements of additional tests
as specified in the test sequence listed in Table 6-8.
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Table 6-8. Product Qualification Test Sequence
Test or Examination Test Group (a)
12 3
Test Sequence (b)
Examination of product 1,10 1,5 1,9
Termination resistance 3,7 2,4
Insulation resistance 3,7
Dielectric withstanding
voltage 4,8
Capacitance 2
Vibration 5
Physical shock 6
Durability 4
Mating force 2
Unmating force 8
Thermal shock 5
Humidity 6
Cable Retention 9
Temperature life 3(c)
Notes:
(a) Refer to Section 6.3.5.4.
(b) Numbers indicate sequence in which tests are performed.
(c) Precondition samples with 10 cycles durability.
6.3.5.4 Sample Selection
Samples shall be prepared in accordance with applicable manufacturers’ instructions and shall be selected
at random from current production. Test groups 1, 2, and 3 shall consist of a minimum of eight
connectors. A minimum of 30 contacts shall be selected and identified. Unless otherwise specified,
these contacts shall be used for all measurements.
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Figure 6-14. Termination Resistance Measurement Points
Figure 6-15. Vibration and Physical Shock Mounting Fixture
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6.3.5.5 Additional Requirements
Flammability: Plastic material used in the construction of this item, shall be rated 94V-0, per
UL-STD-94.
Marking: USB icon per Figure 6-13 on plug. Recommended that OEM’s add an icon near the receptacle
on end product where possible or practical.
Qualification: All suppliers when requested must be able to supply appropriate documentation to show
conformance to the requirements of this chapter.
6.4 Cable Voltage Drop Requirements
The USB physical layer specification requires that the maximum power distribution voltage drop between two
hubs or between hubs and functions should be 350 mV max. The table below lists the nominal lengths of
power distribution cabling for each gauge of conductor. The following is a formula for the voltage drop to an
unpowered hub of 350 mV.
Vunpowered _hub = Vswitch + 4 * Vconnector + 2 * Vcable
Where: Vswitch
= Imax * (board resistance and FET resistance) = 100 mV (max. by
definition)
Vconnector = Imax * 30 mΩ (connector resistance) = 15 mV
Vcable = Imax * cable resistance
Imax = 500 mA
With the above information, Vcable = 95 mV, assuming two connectors are used in the cable assembly,
For a 95 mV drop using copper wire at 20 °C, Table 6-9 lists cable lengths with a current of 500 mA.
Figure 6-16 shows the voltage drop distribution.
Table 6-9. Cable Lengths vs. Gauge
Gauge Resistance Length (Max.)
28 0.232 Ω/m .81 m
26 0.145 Ω/m 1.31 m
24 0.091 Ω/m 2.08 m
22 0.057 Ω/m 3.33 m
20 0.036 Ω/m 5.00 m
Note: This table does not include additional temperature effects (approximately 10%).
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Figure 6-16. Cable and Connector Voltage Drop Distribution
It is recommended that each individual implementer verify proper DC voltage drop. If the implementer
uses different materials than above, then it is responsible for proper DC voltage at the unpowered hub.
To meet the 5 meter maximum length requirement of this specification, a wire range of 20 AWG to
28 AWG is needed for the DC power distribution conductors.
Note: For typical functions that do not require 500 mA, smaller wire gauges can be used as appropriate
per the voltage drop requirements.
6.5 Propagation Delay
If the cabling you have selected cannot meet the requirements of Section 6.3.1.2, then use Table 6-10 to
limit the cable length for fully rated channels.
Table 6-10. Propagation Delay vs. Cable Length
Cable Propagation Delay Specification Maximum Cable Length
9.0 ns/m 3.3 m
8.0 ns/m 3.7 m
7.0 ns/m 4.3 m
6.5 ns/m 4.6 m
Note: The implementation must use the shortest cable that meets the requirements of
Section 6.3.1.2, Section 6.4, and Section 6.5.
6.6 Grounding
The shield must be terminated to the connector plug for completed assemblies. At the host end, the
shield, DC power, and chassis ground should be bonded together. The complete bus should have only
one DC ground point at the host end. All other devices should not connect the shield or DC return to
chassis ground. This prevents circulating low frequency currents. However, AC coupling is permitted
for EMI compliance. The coupling impedance must be less than 250 kΩ at 60 Hz and not greater than
15 Ω between 3 and 30 MHz. The dielectric voltage rating of the capacitor must be 250 Vac (rms).
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6.7 Regulatory Information
Recommendation and guidelines for the installation of this cabling per applicable local regulations are
the responsibility of the OEM. It is recommended that guidelines such as EIA CB8-1981[4] and
ANSI/NFPA 70-1984 as well as local codes and regulations be followed.
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Chapter 7
Electrical
This chapter describes the electrical specification for the USB. It contains signaling, power distribution,
and physical layer specifications.
7.1 Signaling
The signaling specification for the USB is described in the following subsections.
7.1.1 USB Driver Characteristics
The USB uses a differential output driver to drive the USB data signal onto the USB cable. The static
output swing of the driver in its low state must be below the VOL of 0.3 V with a 1.5 kΩ load to 3.6 V and
in its high state must be above the VOH of 2.8 V with a 15 kΩ load to ground as listed in Table 7-4. The
output swings between the differential high and low state must be well balanced to minimize signal skew.
Slew rate control on the driver is required to minimize the radiated noise and cross talk. The driver’s
outputs must support three-state operation to achieve bi-directional half duplex operation. High
impedance is also required to isolate the port from downstream devices that are being hot inserted or
which are connected but powered down. The driver must tolerate a voltage on the signal pins of -0.5 V
to 3.8 V with respect to local ground reference without damage. It must tolerate this voltage for 10.0 µs
while the driver is active and driving, and tolerate the condition indefinitely when the driver is in its high
impedance state.
7.1.1.1 Full Speed (12 Mbs) Driver Characteristics
A full speed USB connection is made through a shielded, twisted pair cable with a characteristic
impedance (Z0) of 90 Ω ±15% and a maximum length of 5 meters. The impedance of each of the drivers
must be between 29 Ω and 44 Ω. The data line rise and fall times must be between 4 ns and 20 ns,
smoothly rising or falling (monotonic), and be well matched to minimize RFI emissions and signal skew.
For a CMOS implementation, the driver impedance will typically be realized by a CMOS driver with an
impedance significantly less than this resistance with a discrete series resistor making up the balance.
Figure 7-1 shows an example of how a full speed driver might be done using two identical CMOS buffers
which have an output impedance between 3 Ω and 15 Ω and two series resistors of 27 Ω. Figure 7-2
shows the full speed driver signal waveforms.
27ΩΩ
27ΩΩ
Outp ut Imped. = 3ΩΩ(min.) to 15ΩΩ(max.)
D- (30ΩΩ to 42ΩΩ Equiv. Imped.)
D+ (30ΩΩ to 42ΩΩ Equiv. Imped.)
OEData Identical
CMOS
Buffers
Figure 7-1. Example Full Speed CMOS Driver Circuit
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VSS
One Bit
Time
(12Mb/s)
Driver
Signal Pins
One-Way
Trip Cable
Delay
VSS
Signal pins pass
input spec levels
after one cable
delay
Receiver
Signal Pins
VSE (min)
VSE (max)
Figure 7-2. Full Speed Driver Signal Waveforms
7.1.1.2 Low Speed (1.5 Mbs) Driver Characteristics
A low speed USB connection is made through an unshielded, untwisted wire cable a maximum of
3 meters in length. The rise and fall time of the signals on this cable must be greater than 75 ns to keep
RFI emissions under FCC class B limits, and less than 300 ns to limit timing delays and signaling skews
and distortions. The driver must reach the specified static signal levels with smooth rise and fall times,
and minimal reflections and ringing when driving the cable (see Figure 7-3). This cable and driver are
used only on network segments between low speed devices and the ports to which they are connected.
VSS
Signal pins
pass output
spec levels
with minimal
reflections and
ringing
One Bit
Time
(1.5Mb/s)
Driver
Signal Pins
VSE (max)
VSE (min)
Figure 7-3. Low Speed Driver Signal Waveforms
7.1.1.3 Driver Usage
Full speed buffers are used on the upstream ports (towards the host) of all hubs and full speed functions.
All devices with hubs must be full speed devices. A full speed driver will be used to send data at both
and low speed data rates. However, the signaling always uses full speed signaling conventions (refer to
Table 7-1) and edge rates. Running at low speed data rates does not change the driver’s characteristics.
Low speed buffers are used on the upstream ports of low speed functions. The downstream ports of all
hubs (including the host) are required to be capable of both driver characteristics, such that any type of
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device can be plugged in to these ports (see Figure 7-5 and Figure 7-6). Low speed drivers only send at
the low speed data rate using low speed signaling conventions (refer to Table 7-1) and edge rates.
7.1.2 Receiver Characteristics
A differential input receiver must be used to accept the USB data signal. The receiver must feature an
input sensitivity of at least 200 mV when both differential data inputs are in the range of at least 0.8 V to
2.5 V with respect to its local ground reference. This is called the common mode input voltage range.
Proper data reception is also required when the differential data lines are outside the common mode
range, as shown in Figure 7-4. The receiver must tolerate static input voltages between -0.5 V to 3.8 V
with respect to its local ground reference without damage. In addition to the differential receiver, there
must be a single-ended receiver for each of the two data lines. The receivers must have a switching
threshold between 0.8 V and 2.0 V (TTL inputs). It is recommended that the single-ended receiver have
some hysteresis to reduce its sensitivity to noise.
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
Co mm on M od e Inpu t Volta ge (vo lts)
Minimum Differential Sensitivity (volts)
Figure 7-4. Differential Input Sensitivity Over Entire Common Mode Range
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7.1.3 Signal Termination
The USB is terminated at the hub and function ends as shown in Figure 7-5 and Figure 7-6. Full speed
and low speed devices are differentiated by the position of the pull-up resistor on the downstream end of
the cable. Full speed devices are terminated as shown in Figure 7-5 with the pull-up on the D+ line.
Low speed devices are terminated as shown in Figure 7-6 with the pull-up on the D- line.
The pull-up terminator is a 1.5 kΩ ±5% resistor tied to a voltage source between 3.0 V and 3.6 V
referenced to the local ground. The pulldown terminators are resistors of 15 kΩ ±5% connected to their
local ground.
F.S./L.S. USB
Transceiver
Host or
Hub Port
Twisted Pair Shielded
R1
D+
D-D-
D+
R1F.S. USB
Transceiver
Hub Port 0
or
Full Speed Function
5 Meters max.
Z0=90ΩΩ±±15%
R1=15KΩΩ
R2=1.5KΩΩ
R2
Figure 7-5. Full Speed Device Cable and Resistor Connections
F.S./L.S. USB
Transceiver
Host or
Hub Port
Untwisted, Unshielded
R1
D+
D-D-
D+
R1
Slow Slew Rate
Buffers
L.S. USB
Transceiver
Low Speed Function
3 Meters max.
R1=15KΩΩ
R2=1.5KΩΩ
R2
Figure 7-6. Low Speed Device Cable and Resistor Connections
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7.1.4 Signaling Levels
Table 7-1 summarizes the USB signaling levels. The signaling levels are described below and in the
following sections.
Table 7-1. Signaling Levels
Bus State Signaling Levels
From Originating Driver At Receiver
Differential “1” (D+) - (D-) > 200 mV and D+ or D- > VSE (min.)
Differential “0” (D+) - (D-) < -200 mV and D+ or D- > VSE (min.)
Data J State:
Low Speed
Full Speed
Differential “0”
Differential “1”
Data K State:
Low Speed
Full Speed
Differential “1”
Differential “0”
Idle State:
Low Speed
Full Speed
Differential “0” and D- > VSE (max.) and D+ < VSE (min.)
Differential “1” and D+ > VSE (max.) and D- < VSE (min.)
Resume State:
Low Speed
Full Speed
Differential “1” and D+ > VSE (max.) and D- < VSE (min.)
Differential “0” and D- > VSE (max.) and D+ < VSE (min.)
Start of Packet
(SOP) Data lines switch from Idle to K State
End of Packet
(EOP) D+ and D- < VSE (min) for 2 bit
times1 followed by an Idle for 1 bit
time
D+ and D- < VSE(min) for ≥ 1 bit
time2 followed by a J State
Disconnect
(Upstream only) (n.a.) D+ and D- < VSE(max) for ≥ 2.5 µs
Connect
(Upstream only) (n.a.) D+ or D- > VSE(max) for ≥ 2.5 µs
Reset
(Downstream only) D+ and D- < VSE for ≥10 ms D+ and D- < VSE (min)for ≥ 2.5 µs
(must be recognized within 5.5 µs)3
Note 1: The width of EOP is defined in bit times relative to the speed of transmission.
Note 2: The width of EOP is defined in bit times relative to the device type receiving the EOP.
Note 3: These times apply to an active device that is not in the suspend state.
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The J and K data states are the two logical levels used to communicate differential data in the system.
Differential signaling is measured from the point where the data line signals cross over. Differential data
signaling is not concerned with the level at which the signals cross, as long as it is within the common
mode range. When the bus is not in the differential signaling mode, the data lines have to be outside the
VSE switching threshold range to be valid, as in the idle and resume states. Note that the idle and resume
states are logically equivalent to the J and K states respectively.
Table 7-1 shows the J and K states for a full speed to be inverted from those of low speed. The sense of
data, idle, and resume signaling is set by the type of device that is being attached to a port. If a full speed
device is attached to a port, that segment of the USB network uses full speed signaling conventions (and
fast rise and fall times) even if the data being sent across the data lines is at the low speed data rate. The
low speed signaling conventions shown in Table 7-1 (plus slow rise and fall times) are only used between
a low speed device and the port to which it is attached. The method of determining the device type, and,
therefore, the signaling convention used, is described in the next section.
7.1.4.1 Connect and Disconnect Signaling
All ports on the downstream side of the host or a hub have pull-down resistors on both the D+ and D-
lines. All devices have a pull-up resistor on one of the data lines on their upstream port. The type of
device determines which data line has the pull-up resistor. Full speed devices have the pull-up on the D+
line (see Figure 7-5) and low speed devices have the pull-up on the D- line (see Figure 7-6). When a
device is attached to hub or host but the data lines are not being driven, these resistors create a quiescent
bias condition on the lines such that the data line with the pull-up is above 2.8 V and the other data line is
near ground. This is called the idle state.
When no function is attached to the downstream port of the host or hub or the pull-up resistor on an
attached device is not powered, the pull-down resistors will cause both D+ and D- to be pulled below the
single-ended low threshold of the host or hub port. This creates a state called a single-ended zero (SE0)
on the downstream port. A disconnect condition is indicated if an SE0 persists on a downstream port for
more than 2.5 µs (30 full speed bit times). Note that disconnect signaling applies only in an upstream
direction (see Figure 7-7).
A connect condition will be detected when a device is connected to the host or hub’s port, and one of the
data lines is pulled above the single-ended high threshold level for more than 2.5 µs (30 full speed data
bit times). The data line that is high when the port state changes from disconnected to connected sets the
idle state for this bus segment and determines whether the connected device is a full speed device or a
low speed device. All signaling levels given in Table 7-1 are set for this network segment (and this
segment alone) once the idle state is determined. Figure 7-8 shows a full speed device connection
sequence and Figure 7-9 shows a low speed device connection sequence.
All hub ports start out in an implied disconnected state after power is applied to that port. If a device is
connected to the port, the port goes through the connect sequence described above to detect the device
type and set the port signaling characteristics (refer to Section 11.2.3).
Universal Serial Bus Specification Revision 1.0
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VOH (min)
VSE (max)
VSE (min)
VOL (max)
VSS
Device
Disconnected Disconnect
Detected
≥≥ 2.5 µµs
Figure 7-7. Disconnect Detection
VOH (min)
VSE (max)
VSE (min)
VOL (max)
VSS
Device
Connected
D-
D+
Connect
Detected
≥≥ 2.5µµs
Figure 7-8. Full Speed Device Connect Detection
VOH (min)
VSE (max)
VSE (min)
VOL (max)
VSS
Device
Connected
D+
D-
Connect
Detected
≥≥ 2.5 µµs
Figure 7-9. Low Speed Device Connect Detection
Universal Serial Bus Specification Revision 1.0
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7.1.4.2 Data Signaling
Data transmission within a packet is done with differential signals. A differential 1 on the bus is
represented by D+ being at least 200 mV more positive than D- as seen at the receiver, and a differential
0 is represented by D- being at least 200 mV more positive than D+ as seen at the receiver. The signal
cross over point must be between 1.3 V and 2.0 V.
The start of a packet (SOP) is signaled by the originating port by driving the D+ and D- lines from the
idle state to the opposite logic level (K state). This switch in levels represents the first bit of the Sync
field. Hubs must limit the distortion of the width of the first bit after SOP when it is retransmitted to less
than 5 ns. Distortion can be minimized by matching the nominal data delay through the hub with the
output enable delay of the hub.
The single-ended 0 state is used to signal an end of packet (EOP). The single-ended 0 state is indicated
by both D+ and D- being below 0.8 V. EOP will be signaled by driving D+ and D- to the single-ended 0
state for two bit times followed by driving the lines to the idle state for one bit time. The transition from
the single-ended 0 to the idle state defines the end of the packet. The idle state is asserted for 1 bit time
and then both the D+ and D- output drivers are placed in their high-impedance state. The bus termination
resistors hold the bus in the idle state. Figure 7-10 shows the signaling for start and end of a packet.
VOH (min)
VSE (max)
VSE (min)
VOL (max)
VSS Bus Idle First Bit
of Packet
SOP
VOH (min)
VSE (max)
VSE (min)
VOL (max)
VSS
Bus Driven
to Id le State
Last Bit
of Packet
Bus Idle
Bus
Floats
EOP
Strobe
Figure 7-10. Packet Transaction Voltage Levels
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7.1.4.3 Reset Signaling
A reset is signaled downstream from a hub port on the bus by the presence of an extended SE0 at the
upstream port of a device. After the reset is removed, the device will be in the attached, but not yet
addressed or configured state (refer to Section 9.1). Note that reset signaling applies only in the
downstream direction.
The reset signal can be generated by host command on any hub or host controller port. The reset signal
must be generated for a minimum of 10 ms. The port that generated the reset will be sent to the logically
disconnected state at the end of the reset. If a device is connected to the port, the bus pull-up resistor will
determine the device type (low or full speed) and the port will end up in the disabled state (refer to
Section 11.2.3).
An active device (powered and not in the suspend state) seeing a single-ended zero on its upstream port
for more than 2.5 µs may treat that signal as a reset, but must have interpreted the signaling as a reset
within 5.5 µs. A device that recognizes a reset from a SE0 between 32 and 64 full speed bit times or
between 4 and 8 low speed bit times satisfies these requirements. The reset signal propagates through all
enabled ports of any hubs downstream of the signaling port, but does not propagate through any ports that
are disabled. A bus-powered hub that receives a reset on its root port removes power from all its
downstream ports. After the reset is removed, all devices that received the reset are set to their default
USB address and are in the unconfigured state. All ports on a hub that received a reset are disabled.
Hubs must be able to establish connectivity, and all devices must be able to accept device address via a
SET_ADDRESS command (refer to Section 9.4) no later than 10 ms after the reset is removed. Failure
to establish connectivity or accept an address may cause the device not to be recognized by the USB
enumerator. In the case of a hub, it may also cause any devices connected to that hub not to be
recognized. All other requests for data or service, except SETUP packets (refer to Section 8.4.5.4), can
be NAKed for a period up to 5.0 seconds after which the device is declared defective and is not
recognized.
Reset can wake a device from the suspended mode. It is recommended that the device wait for its clocks
to stabilize before accepting the reset to avoid spurious single-ended zero events from causing the device
to reset. A device may take up to 10 ms to wake up from the suspended state.
7.1.4.4 Suspending
All devices must support the suspend mode. Devices can go into the suspended mode from any powered
state. They go into the suspend state when they see a constant idle state on their bus lines for more than
3.0 ms. Any bus activity will keep a device out of the suspend state. The SOF packet (refer to
Section 8.4.2) is guaranteed to occur once a frame to keep full speed devices awake during normal bus
operation. Hubs that are not in the suspend state keep low speed devices awake by generating a low
speed EOP on enabled ports that are attached to low speed devices (refer to Section 11.2.5.1). When a
device is in the suspend state, it draws less than 500 µA from the bus. Even though suspended devices
draw at most 500 µA from the bus, hub ports must be able to supply their maximum rated current to the
downstream devices when the hub is in the suspended state. This is necessary to support remote wake-up
as described in Section 7.2.3.
All devices can be awakened from the suspend state by switching the bus state to the resume state, by
normal bus activity, or by signaling a reset. Some devices have the ability to be awakened by actions
associated with their internal functions and then cause signaling on their upstream connection to wake or
alert the rest of the system. This feature is called remote wake-up and is described in Section 7.1.4.5.
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7.1.4.4.1 Global Suspend
Global suspend is used when no communication is desired anywhere on the bus and the whole network is
placed in the suspend state. The host signals the start of suspend by ceasing all its transmissions
(including the SOF token). As each device on the bus recognizes the lack of activity, and that the bus is
in the idle state for the appropriate length of time, it goes into its suspend state. As hubs go into the
suspended state, they cease to send the low speed EOP on any downstream low-speed configured ports.
7.1.4.4.2 Selective Suspend
The system software may want to conserve power by suspending only certain segments of the topology,
while continuing regular operation on the remaining segments. Segments of the network can be
selectively put into the suspend state by suspending the hub port to which that segment is attached. The
suspended port will block activity to this segment of the bus and the attached devices will go into suspend
after the appropriate delay as described above.
Any non-hub device can be suspended in this way. Any hub not involved in connecting the remaining
devices to the host may also be suspended by disabling the port to which it is attached. Devices that are
selectively suspended can still alert the system with a remote wake-up signaling, although the process is
slightly different. A description of the suspend port state can be found in Section 11.2.3 and selective
suspend is further described in Section 11.5.2.
7.1.4.5 Resume
Once a device is in the suspend state, its operation can be resumed by receiving non-idle signaling on the
bus, or it can signal the system to resume operation if it has the remote wake-up capability. The resume
signaling state is used by the host or a device with remote wake-up to awaken the system. Hubs play an
important role in the propagation and generation of resume signaling. The following description is an
outline of a general global resume sequence. A complete description of the resume sequence, the special
cases caused by selective suspend, and the role of the hub is given in Section 11.5.
The host may signal resume anytime after it places the bus into the suspend mode. It must send the
resume signaling for at least 20 ms and then end the resume signaling with a standard low speed EOP
(two low speed bit times of SE0 followed by a transition to the idle state). The 20 ms of resume
signaling insures that all devices in the network that are enabled to see the resume are awakened. The
EOP tears down the connectivity established by the resume signaling and prepares the hubs for normal
operation. After resuming the bus, the host must begin sending bus traffic (at least the SOF token) within
3 ms to keep the system from going back into the suspend state.
A device with remote wake-up capability must wait for at least 5 ms after the bus is in the idle state
before sending the remote wake-up resume signaling. This allows the hubs to get into their suspend state
and prepare for propagating resume signaling. The remote wake-up device must hold the resume
signaling for at least 10 ms and no more than 15 ms. At the end of the resume signaling, the device puts
its data lines into the high impedance state.
The hub upstream of the remote wake-up device will propagate the resume signaling to its root port and
all of its enabled downstream ports, including the port which originally signaled the resume. The hub
must begin this rebroadcast of the resume signaling within 50 µs of receiving the original resume. The
resume signal will propagate in this manner upstream until it reaches the host (or a suspended hub port -
refer to Section 11.5.2.1) which will reflect the resume downstream on its enabled ports, as in any other
hub. Meanwhile, the hubs that transported the resume to the host wake up within 10 ms. After a 1 ms
delay, they reverse the direction of connectivity in their root port from upstream to downstream and
reflect the signal state on the root port onto their enabled downstream ports. This puts the host in control
of the resume signaling which then proceeds as described above.
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Port connects and disconnects can also cause the affected hub to send a resume signal and awaken the
system. Refer to Section 11.5 for more details.
(Note that the host can wake up the entire bus by resetting it. This requires that the entire bus must be re-
enumerated and reconfigured.)
Refer to Section 7.2.3 for a description of power control during suspend and resume.
7.1.5 Data Encoding/Decoding
The USB employs NRZI data encoding when transmitting packets. In NRZI encoding, a 1 is represented
by no change in level and a 0 is represented by a change in level. Figure 7-11 shows a data stream and
the NRZI equivalent and Figure 7-12 is a flow diagram for NRZI. The high level represents the J state on
the data lines in this and subsequent figures showing NRZI encoding. A string of zeros causes the NRZI
data to toggle each bit time. A string of ones causes long periods with no transitions in the data.
00
11
11
00
11
00
11
00
00
00
11
00
00
11
11
00
DD
aa
tt
aa
NN
RR
ZZ
II
II
dd
ll
ee
II
dd
ll
ee
Figure 7-11. NRZI Data Encoding
Power Up
Begi n Packet Transmission
No Packet
Transmission
No
No Yes
Yes
Idle
Is Packet
Transfer
Done?
Is Data
Bit = 0?
No Data
Transition
Fetch the
Data Bit
Transition
Data
Figure 7-12. Flow Diagram for NRZI
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7.1.6 Bit Stuffing
In order to ensure adequate signal transitions, bit stuffing is employed by the transmitting device when
sending a packet on the USB (see Figure 7-13 and Figure 7-14). A 0 is inserted after every six
consecutive 1’s in the data stream before the data is NRZI encoded to force a transition in the NRZI data
stream. This gives the receiver logic a data transition at least once every seven bit times to guarantee the
data and clock lock. The receiver must decode the NRZI data, recognize the stuffed bits, and discard
them. Bit stuffing is enabled beginning with the Sync Pattern and throughout the entire transmission.
The data “one” that ends the Sync Pattern is counted as the first one in a sequence. Bit stuffing is always
enforced, without exception. If required by the bit stuffing rules, a zero bit will be inserted even if it is
the last bit before the end-of-packet (EOP) signal.
Data Encoding Sequence:
Bit Stuffed Data
Raw Data
NRZI
Encoded Data
Idle
Sync Pattern
Sync Pattern
Sync Pattern
Packet Data
Packet Data
Stuffed Bit
Six Ones
Packet Data
Figure 7-13. Bit Stuffing
Universal Serial Bus Specification Revision 1.0
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Power Up
Be gin Packet Transmission
No Packet
Transmission Idle
Reset Bit
Counter to 0
No
Yes
Is Packet
Transfer
Done?
Get Next
Bit
=0
=1
Bit
Value?
No
Yes
Counter
= 6?
Increment
the Co unter
Rese t the Bit
Counter to 0
Insert a
Zero Bit
Figure 7-14. Flow Diagram for Bit Stuffing
7.1.7 Sync Pattern
The NRZI bit pattern shown in Figure 7-15 is used as a synchronization pattern and is prefixed to each
packet. This pattern is equivalent to a data pattern of seven 0’s followed by a 1 (0x80).
I
d
le
SYNC P ATTERN
NRZI Data
Encoding
P
ID
0
P
ID
1
Figure 7-15. Sync Pattern
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7.1.8 Initial Frame Interval and Frame Adjustability
The USB defines a frame interval to be 1.000 ms long. The frame interval is measured from the start of
the Start of Frame (SOF) PID in one frame to the same point in the SOF token of the next frame. The
permitted tolerance on the initial frame interval is ±0.05% (500 ppm). This tolerance includes
inaccuracies from all sources: initial frequency accuracy, crystal capacitive loading, supply voltage on
the oscillator, temperature, and aging.
The host controller must be able to adjust the frame interval. There are two possible components to the
frame interval adjustability. If the host’s data rate clock is not exactly 12.00 Mbs, then the initial ±0.05%
frame interval accuracy can be met by changing the default number of bits per frame from the nominal of
12,000. (A host controller component, that has a range of possible clock source values, may have to
make this initial frame count a programmable value. The range of these values is given in Section 7.1.9.)
An additional adjustability of ±15 full speed bit times is required to allow the host to synchronize to an
external time reference. The frame interval can be reprogrammed by no more than one full speed bit
time each adjustment during normal bus operation.
Hubs and certain full speed functions need to track the frame interval. They also are required to have
sufficient frame timing adjustability to compensate for its own frequency tolerance and track the host’s
±15 full speed bit time adjustability range.
7.1.9 Data Signaling Rate
The full speed data rate is nominally 12 Mbs. The data rate tolerance for host, hub, and full speed
functions is ±0.25% (2500 ppm). The accuracy of the host controller’s data rate must be known to better
than ±0.05% (500 ppm) in order to meet the frame interval accuracy. This tolerance includes
inaccuracies from all sources: initial frequency accuracy, crystal capacitive loading, supply voltage on
the oscillator, temperature, and aging.
The low speed data rate is nominally 1.5 Mbs. The permitted frequency tolerance for low speed
functions is ±1.5% (15000 ppm). This tolerance includes inaccuracies from all sources: initial frequency
accuracy, crystal capacitive loading, supply voltage on the oscillator, temperature, and aging. The jitter
in the low speed data rate must be less than 10 ns. This tolerance allows the use of resonators in low
cost, low speed devices
7.1.10 Data Signal Rise and Fall Time
The output rise time and fall time are measured between 10% and 90% of the signal (see Figure 7-16).
Edge transition time for the rising and falling edges of full speed data signals is 4 ns (minimum) and 20
ns (maximum) measured with a capacitive load (CL) of 50 pF. The rise and fall times must be well
matched. The rise and fall time of low speed signals is 75 ns (minimum) into a load of 50 pF and 300 ns
(maximum) into a capacitive load of 350 pF. In both cases, the rising and falling edges should be
smoothly transitioning (monotonic) when driving their respective cables to avoid excessive EMI. (In full
speed signaling, the driver and cable impedances are not matched, so reflections from the receiver end of
the cable are expected.)
Low Speed: 75ns at CL = 50pF, 300ns at CL = 350pFFull Speed: 4 to 20ns at CL = 50pF
Differential
Data Lines 10%
Rise Time
90%
Fall Tim e
tF
tR
10%
90%
CL
CL
Figure 7-16. Data Signal Rise and Fall Time
Universal Serial Bus Specification Revision 1.0
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7.1.11 Data Source Signaling
This section covers the timing characteristics of data produced and sent from a device (the data source).
Section 7.1.12 covers the timing characteristics of data that is transmitted through the repeater section of
a hub. In this section, TPERIOD is defined as the actual period of the data rate which can have a range as
defined in Section 7.1.9.
7.1.11.1 Data Source Jitter
The source of data can have some variation (jitter) in the timing of edges of the data transmitted. The
time between any set of data transitions is N * TPERIOD ± jitter time, where ‘N’ is the number of bits
between the transitions. The data jitter is measured with the same capacitive load used for maximum rise
and fall times and is measured at the crossover points of the data lines as shown in Figure 7-17.
TPERIOD
Differential
Data Lines
Crossover
Points
Paired
Transitions
Consecutive
Transitions
Figure 7-17. Data Jitter
For full speed transmissions, the jitter time for any consecutive differential data transitions must be
within ±2.0 ns and within ±1.0 ns for any set of paired differential data transitions. For low speed
transmissions, the jitter time for any consecutive differential data transitions must be within ±25 ns and
within ±10 ns for any set of paired differential data transitions. These jitter numbers include timing
variations due to differential buffer delay and rise/fall time mismatches, internal clock source jitter and to
noise and other random effects.
7.1.11.2 EOP Width
The width of the SE0 in the EOP is about 2 * TPERIOD. The EOP width is measured with the same
capacitive load used for maximum rise and fall times and is measured at the same level as the differential
signal crossover points of the data lines (see Figure 7-18).
TPERIOD
Differential
Data Lines
EOP
Width
Data
Crossover
Level
Figure 7-18. EOP Width Timing
For full speed transmissions, the EOP width from the transmitter must be between 160 ns and 175 ns. For
low speed transmissions, the transmitter’s EOP width must be between 1.25 µs and 1.50 µs. These
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ranges include timing variations due to differential buffer delay and rise/fall time mismatches and to
noise and other random effects.
The full speed receiver must accept a 82 ns wide SE0 followed by a J transition as a valid EOP. An SE0
narrower than 40 ns or any SE0 not followed by a J transition must be rejected as an EOP. The receiver
may reject or accept an EOP between 40 ns and 82 ns as dictated by implementation depending on
sampling and synchronization. A low speed receiver must accept a 670 ns wide SE0 followed by a J
transition as a valid EOP. An SE0 narrower than 330 ns or an SE0 not followed by a J transition must be
rejected as an EOP. An EOP between 330 ns and 670 ns may be rejected or accepted as above. The host
is an exception to this rule and always uses full speed EOP timing rules while receiving and either data
rate. (This is necessary to support end-of-frame error recovery. Refer to Section 11.4.5.) Any SE0 that
is 2.5 µs or wider is automatically a reset or a disconnect (depending on direction).
7.1.12 Hub Signaling Timings
The propagation of a full speed, differential data signal through a hub is shown in Figure 7-19. The
downstream signaling is measured without a cable connected to the port and with a 50 pF capacitive load.
The total delay through the cable and hub electronics must be a maximum of 70 ns. If the hub has a USB
standard detachable cable, then the delay through hub electronics must be a maximum of 40 ns to allow
for a worst case cable delay of 30 ns. The delay through this hub is measured in both upstream and
downstream directions as shown in Figure 7-19B, from data line crossover at the input port to data line
crossover at the output port.
50% Point of
Initial Sw i ng
VSS
Upstream
End of Cable
Upstream
End of Cable
Data Line
Crossover
Point
VSS
B . Upstream Hub DelayA. Downstream Hub Del ay
Downstream
Port Hub Delay
Downstream
70 ns ( max)
Data Line
Crossover
Point
Hub Delay
Downstream
70 ns ( max)
Data Line
Crossover
Point
Downstream
Port
Figure 7-19. Hub Propagation Delay of Full Speed Differential Signals
Low speed propagation delay for differential signals is measured in the same fashion as for full speed
signaling. The maximum low speed hub delay is 300 ns. This allows for the slower low speed buffer
propagation delay and rise and fall times.
When the hub acts as a repeater, it must reproduce the received signal accurately on its outputs. This
means that for differential signals, the propagation delays of a J to K state transition must match closely
to the delays of a K to J state transition. The maximum difference allowed between these two delays (as
measured in Figure 7-19) for a hub plus cable is ±3.0 ns. Similarly, the difference in delay between any
two J to K or K to J transitions through a hub should be less than ±1.0 ns.
An exception to this case is the skew that can be introduced in the SOP idle to K state transition (refer to
Section 7.1.4.2). In this case, the delay to the opposite port includes the time to enable the output buffer.
However, the delays should be closely matched to the normal hub delay and the maximum additional
delay difference over a normal J to K transition is -3.0 ns to +5.0 ns. This limits the maximum distortion
of the first bit in the packet. (Note: Because of this distortion of the SOP transition relative to the next K
to J state transition, the first sync field bit should not be used to synchronize the receiver to the data
stream.)
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The EOP has to be propagated through a hub in the same way as the differential signaling. The
propagation delay for sensing an SE0 must be no less than the greater of the J to K or K to J differential
data delay (to avoid truncating the last data bit in a packet) but not more than 15 ns greater than the larger
differential delay at full speed and 200 ns at low speed (to prevent creating a bit stuff error at the end of
the packet). EOP delays are shown in Figure 7-20.
Since the sense levels for the SE0 state are not at the midpoint of the signal swing, the width of the
single-ended 0 state will be changed as it passes through each hub. A hub may not change the length of a
full speed single-ended 0 state by more than ±15 ns as measured by the difference of the EOP- and EOP+
delays (see Figure 7-20). A single-ended 0 from a low speed device has long rise and fall times and is
subject to greater skew, but this conditions exists only on the cable from the low speed device to the port
to which it is connected. Thereafter, the signaling uses full speed buffers and their faster rise and fall
times. The single-ended 0 from the low speed device cannot be changed by more than ±300 ns as it
passes through the hub to which the device is connected. This time allows for some signal conditioning
in the low speed port to reduce its sensitivity to noise.
50% Point of
In itial Swing
VSS
Upstream
End of
Upstream
End of
B. Upstream EOP DelayA. Downstream EOP Delay
VSS
Downstream
Port
EOP+
Delay
EOP-
Delay
Diff.
Data
Delay
Crossover
Point
Extended
Crossover
Point
Extended
Crossover
Point
Extended
EOP+
Delay
EOP-
Delay
Diff.
Data
Delay
Downstream
Port
Figure 7-20. Timing of EOP
7.1.13 Receiver Data Jitter
The data receivers for all types of devices must be able to properly decode the differential data in the
presence of jitter. The more of the bit cell that any data edge can occupy and still be decoded, the more
reliable the data transfer will be. Data receivers are required to decode differential data transitions that
occur in a window plus and minus a nominal quarter bit cell from the nominal (centered) data edge
position. (A simple 4X oversampling state machine DPLL can be built that satisfies these requirements.)
Jitter will be caused by the delay mismatches discussed above and by mismatches in the source and
destination data rates (frequencies). The receive data jitter budgets for full and low speed are given in
Table 7-2 and Table 7-3. These tables give the value and totals for each source of jitter for both
consecutive (next) and paired transitions. Note that the jitter component related to the source or
destination frequency tolerance has been allocated to the appropriate device (i.e., the source jitter
includes bit shifts due to source frequency inaccuracy over the worst case data transition interval). The
output driver jitter can be traded off against the device clock accuracy in a particular implementation as
long as the jitter specification is met.
The low speed jitter budget table has an additional line in it because the jitter introduced by the hub to
which the low speed device is attached is different from all the other devices in the data path. The
remaining devices operate with full speed signaling conventions (though at low speed data rate).
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Table 7-2. Full Speed Jitter Budget
HUBS 5 Full Speed Freq. Tol. 0.0025
MAX BITS/TRANS 7
Jitter Source Full Speed
Next Transition
Paired Transition
Each (ns) Total (ns) Each (ns) Total (ns)
Source Driver Jitter 2.0 2.0 1.0 1.0
Source Freq. Tol. - Worst case 0.21/bit 1.5 0.21/bit 3.0
Source Jitter Total 3.5 4.0
Hub Jitter 3.0 15.0 1.0 5.0
Jitter Spec 18.5 9.0
Destination Freq. Tol. 0.21/bit 1.5 0.21/bit 3.0
Rcv Jitter Budget 20.0 12.0
Table 7-3. Low Speed Jitter Budget
HUBS 5 Full Speed Freq. Tol. 0.0025
MAX BITS/TRANS 7 Low Speed. Freq. Tol. 0.015
Jitter Source Low Speed - Upstream
Next Transition
Paired Transition
Each (ns) Total (ns) Each (ns) Total (ns)
Function Driver Jitter 25.0 25.0 10.0 10.0
Function Freq. Tol - Worst
Case 10.0/bit 70.0 10.0/bit 140.0
Source (Function) Jitter Total 95.0 150.0
Hub w/ Low Speed Device Jitter 45.0 45.0 45.0 45.0
Remaining (Full Speed) Hubs'
Jitter 3.0 12.0 1.0 4.0
Jitter Spec 152.0 200.0
Host Freq. Tol. 1.7/bit 12.0 1.7/bit 24.0
Host Rcv Jitter Budget 164.0 225.0
Universal Serial Bus Specification Revision 1.0
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Table 7-3. Low Speed Jitter Budget (continued)
Jitter Source Low Speed - Downstream
Next Transition
Paired Transition
Each (ns) Total (ns)
Each (ns)
Total (ns)
Host Driver Jitter 2.0 2.0
1.0
1.0
Host Freq. Tol. - Worst Case 1.7/bit 12.0
1.7/bit
24.0
Source (Host) Jitter Total 14.0
25.0
Hub w/ Low Speed Device Jitter 45.0 45.0
15.0
15.0
Remaining (Full Speed) Hubs'
Jitter 3.0 12.0
1.0
4.0
Jitter Spec 75.0
45.0
Function Freq. Tol. 10.0/bit 70.0
10.0/bit
140.0
Function Rcv Jitter Budget 150.0
185.0
Note: This table describes the host transmitting at low speed data rate using
full speed signaling to a low speed device through the maximum number of
hubs. When the host is directly connected to the low speed device, then it uses
low speed data rate and low speed signaling, and the host has to meet the
source jitter listed in the “Jitter Spec” row.
7.1.14 Cable Delay
Only one data transition is allowed on a USB cable at a time. A full speed signal edge has to transition,
propagate to the far end of the cable, return, and settle within one full speed bit time. Therefore, the
maximum allowed cable delay is 30 ns. Independent of cable velocity, the maximum cable length is
5.0 meters for full speed devices and 3.0 meters for low speed devices. The maximum one-way data
delay on a cable is measured as shown in Figure 7-21.
Data Line
Crossover
Point
50% Point of
Initial Swing
VSS
VSS
Driver End
of Cable
Round Trip
Cable Delay
80ns (max)
Receiver
End of Cable
One Way
Cable
Delay
30ns
(max)
Figure 7-21. Cable Delay
7.1.15 Bus Turnaround Time/Interpacket Delay
A new device may not begin driving the bus until the previous device has completed the EOP sequence
and has disabled its drivers. This is assured by not allowing the new device to drive the bus until it has
Universal Serial Bus Specification Revision 1.0
130
detected that the bus is in the J state after the SE0 in the EOP for at least two bit times. This minimum of
two bit times applies to all devices, including back to back host packet transmissions.
If a function is expected to provide a response to a host transmission, the response must be seen on the
upstream end of the cable within 7.5 bit times of the bus returning to the J state after the EOP as seen on
the upstream end of its cable. The maximum bus turnaround time for a function or hub without an
integrated cable is 6.5 bit times. This maximum bus turnaround time prevents a full speed receiving
agent from timing out after 16 bit times on a response in a maximum depth topology (refer to
Section 7.1.16). However, these timings apply to both full speed and low speed devices.
The maximum delay a host has to respond to a data packet sent by a function (if a handshake is required)
is 7.5 bit times, measured at the host’s port pins. There is no maximum delay between packets in
separate transactions.
7.1.16 Maximum End to End Signal Delay
A device expecting a response to a transmission will invalidate the transaction if it does not see the start
of packet (SOP) transition within the time-out period after the end of the transmission (after the SE0 to J
state transition in the EOP). This can occur between an IN token and the following data packet or
between a data packet and the handshake packet (refer to Chapter 8). The device expecting the response
will not time out before 16 bit times and not after 18 bit times measured at the data pins of the device
after the end of the EOP. The host will not begin transmission of the token for the next transaction before
18 bit times (measured at its data pins) after it sees the end of a packet with no response.
Figure 7-22 depicts the configuration of six signal hops (cables) that results in worst allowable case signal
delay. The maximum propagation delay from the upstream end of a hub’s cable to any downstream port
connector is 70 ns.
Host
Controller Function
Cable Delay + Hub delay ≤≤ 70ns (All F.S. Cables 5 meters max)
Hub 5 Hub 4 Hub 3 Hub 2
Hub 1
Figure 7-22. Worst Case End to End Signal Delay Model
Universal Serial Bus Specification Revision 1.0
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7.2 Power Distribution
This section describes the USB power distribution specification.
7.2.1 Classes of Devices
The power sourcing and sinking requirements of different device classes can be simplified with the
introduction of the concept of a unit load. A unit load is defined to be 100 mA.
USB supports a range of devices as categorized by their power consumption; these include:
• Bus-powered hubs - Draws all of the power to any internal functions and downstream ports from the
USB connector power pins. May draw up to one load upon power up and a total of five loads, which
is split between any embedded functions and external ports. External ports in a bus-powered hub can
supply only one load per port regardless of the current drawn on the other ports of that hub. The hub
must be able to supply this port current when the hub is in the active or suspended state.
• Self-powered hubs - Power for the internal functions and downstream ports does not come from
USB, although the USB interface may draw up to one load from its upstream connection to allow the
interface to function when the remainder of the hub is powered down. The hub must be able to
supply up to five unit loads on all of its external downstream ports even when the hub is in the
suspended state.
• Low power, bus-powered functions - All power to these devices comes from USB connector. They
may draw no more than one unit load at any time.
• High power, bus-powered functions - All power to these devices comes from USB connector.
They must draw no more than one unit load upon power up and may draw up to five unit loads after
being configured.
• Self-powered functions - May draw up to one load from their upstream connection to allow the
interface to function when the remainder of the hub is powered down. All other power comes from
an external (to USB) source.
The hub on the host in a desktop computer is a self-powered hub. The same hub in a notebook computer
can be defined to be either a self-powered or bus-powered hub.
All devices, whether they are bus-powered or self-powered, can only draw (sink) current from the bus.
They must not supply current upstream to a host or hub port. On power up, all devices need to insure that
their upstream port is not enabled, so that the device is able to receive the reset signaling, and that the
maximum operating current drawn by a device is one unit load. If a device draws power from the bus, its
internal supply derived from Vbus must be stable within 100 ms of Vbus reaching 4.4 V. All devices which
are drawing power from the bus must be able enter the suspend state and reduce their current
consumption from Vbus to less than 500 µA (refer to Section 7.1.4.4 and Section 9.2.5.1).
7.2.1.1 Bus-powered Hubs
The above requirements can be met for bus-powered hubs with a power control circuit such as that shown
in Figure 7-23. Bus-powered hubs often contain at least one embedded function. Power is always
available to the hub’s controller, which permits host access to power management and other
configuration registers during the enumeration process. An embedded function may require that its
power be switched, so that upon power-up the entire device (hub and embedded functions) draws no more
than one unit load. Power switching on any embedded function may be implemented either by removing
its power or by shutting off the clock. Switching on the embedded function is not required if the
aggregate power drawn by it and the hub controller is less than one unit load. The total current drawn by
an bus-powered device is the sum of the current to the hub controller, any embedded function(s), and the
downstream ports.
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Figure 7-23 shows the partitioning of power based upon the maximum upstream current of five loads:
one unit load for the hub controller and the embedded function, and one load for each of the downstream
ports. The maximum number of downstream ports that can be supported is limited to four. If more ports
are required, then the hub will need to be self-powered. If the embedded function(s) and hub controller
draw more than one unit load, then the number of ports must be appropriately reduced. Power control to
a bus-powered hub may require a regulator. If present, the regulator is always enabled to supply the hub
controller. The regulator can also power the embedded functions(s). Inrush current limiting must also be
incorporated into the regulator subsystem.
Embedded
Function
Hub Co ntroller Downstream
Data por t s
Downstrea m V bus
Upstrea m Vbus
Upstream
Data Po r t
On/Off
Regulator
Switch
On/Off
1 unit load/ p ort
Iportctrl
1 unit load - Iportctrl
5 unit loads
Figure 7-23. Compound Bus-powered Hub
Power to downstream ports must be switched. The hub controller supplies a software controlled on/off
signal from the host, which is in the “off” state when the device is powered up or after reset signaling.
When switched to the “on” state, the switch implements a soft turn-on function which prevents excessive
transient current from being drawn from the upstream port. The voltage drop across the upstream cable,
connectors, and switch in a bus-powered hub must not exceed 350 mV at maximum rated current.
7.2.1.2 Self-powered Hubs
Self-powered hubs have a local power supply that furnishes power to any embedded functions and to all
downstream ports, as shown in Figure 7-24. Power for the hub controller, however, may be supplied
from either the upstream port (a “hybrid” powered hub) or the local power supply. The advantage of
supplying the hub controller from the upstream supply is that communication from the host is possible
even if the device’s power supply remains off. This makes it possible to differentiate between a
disconnected and an unpowered device.
Universal Serial Bus Specification Revision 1.0
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Embedded
Function
Hub Cont roller
Downstream
Data ports
Downstream Vbus
Upstream
Data Port
Regulator
Curre nt Limit
Upstream Vbus
1 unit load (max)
Local Power
Supply Regulator
On/Off
Curre nt Limit
.
.
.
.
.
.
.
5 unit loads/port
Figure 7-24. Compound Self-powered Hub
Power is provided to all downstream ports from the local power supply. The number of ports that can be
supported is limited only by what the local supply can deliver and by safety concerns. A reasonable
maximum is seven ports. Each port must be capable of supplying at least five unit loads. However, no
single port can deliver more than 5.0 A in order to meet regulatory safety limits. Current limiting on the
downstream ports may need to be partitioned into two or more port subgroups in order to deliver
sufficient power to all ports without exceeding the current available through one port. If all seven ports
were wired in parallel, the available current at a given port would be 7 * 500 mA = 3.5 A. This is very
close to the safety limit. By implementing two current limit circuits, the maximum current that needs to
be supplied in operation is reduced to 1.5 A to 2.0 A, which gives a comfortable margin to the safety
limit.
7.2.1.2.1 Overcurrent Protection
The host and all self-powered hubs must implement overcurrent protection for safety reasons, and they
must have a way to detect the overcurrent condition and report it to the USB software. Should the
aggregate current drawn by a group of downstream ports exceed a preset value, the overcurrent protector
removes power from all downstream ports and reports the condition through the hub to host controller.
The preset value cannot exceed 5.0 A and should be sufficiently above the maximum allowable port
current such that power up or dynamic attach transient currents do not trip the overcurrent protector. If
an overcurrent condition occurs on any port, subsequent operation of the USB is not guaranteed, and once
the condition is removed, it may be necessary to reinitialize the bus as would be done upon power-up.
Overcurrent limiting methods can include poly fuses, standard fuses, or some type of solid state switch.
The only requirements are that current be limited to five unit loads per port and that the host is notified of
an overcurrent condition.
Current limiting should not occur even if illegal topologies are configured, due to the protection afforded
by power switching in high power functions and bus-powered hubs. Instead, the overcurrent circuits are
used to protect from catastrophic device failures, software errors that turn on devices when the current
budget has been exceeded, and user actions such as shorting out the connector pins.
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7.2. 1.2.2 Power Supply Isolation
Figure 7-24 is based on the assumption that the local power supply shares a common ground with the
upstream and downstream ports. Its Vbus, however, is isolated from the Vbus of the upstream port. There is
an additional requirement that the chassis ground (if one exists) of the self-powered hub be DC isolated
from the USB signal ground. Chassis ground connects to the ground of a 120 Vac power cable, and there
is no guarantee that AC grounds from two different outlets are at the same potential. Failure to observe
this precaution could result in large low frequency currents running through USB ground paths.
7.2.1.3 Low-power, Bus-powered Functions
A low power function is one that draws less than one unit load from the USB cable when fully
operational. The regulator block must both limit inrush current and supply the necessary voltage for the
proper signaling levels. Figure 7-25 shows a typical bus-powered low-power function, such as a mouse.
Low power regulation can be integrated into the function silicon. For higher currents, in the range of
20 mA to 100 mA, an IC linear regulator may be used. Low power functions must be capable of
operating with input Vbus voltages as low as 4.40 V measured at the plug end of the cable.
Function
Upstream Vbus
Upstream
Data Port
Regulator
1 unit load (max)
Figure 7-25. Low-power Function
7.2.1.4 High-power, Bus-powered Functions
A function is defined as being high powered if, when fully powered, it draws over one and a maximum of
five unit loads from the USB cable. A high-power function requires staged switching of power. It must
first come up in a reduced power state of less than one unit load. At bus enumeration time, its total
power requirements are obtained and compared against the available power budget. If sufficient power
exists in the power budget, the remainder of the function may be powered on. If insufficient power is
available, the remainder of the function is not powered and a power limit warning message is sent to the
client. A high-power function is shown in Figure 7-26. The function’s electronics have been partitioned
into two sections; the function controller contains the minimum amount of circuitry necessary to permit
enumeration and power budgeting. The remainder of the function resides in the function block. High
power functions must be capable of operating in their low power (one unit load) mode with an input
voltage as low as 4.40 V, so that it will work even when plugged into an bus-powered hub. They must
also be capable of operating at full power (up to five unit loads) with an input Vbus voltage of 4.75 V
measured at the upstream plug end of the cable.
FunctionFunction Controller
Ups tre am Vbus
Upstream
Data Por t
On/Off
Regulator
1 unit load
(max)
5 unit loads(max)
Figure 7-26. High-power, Bus-powered Function
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7.2.1.5 Self-powered Functions
Figure 7-27 shows a self-powered function. The function controller is powered either from the upstream
bus via a low power regulator or from the local power supply. The advantage of the former scheme is
that it permits detection and enumeration of a self-powered function whose local power supply is turned
off. When the function controller is externally powered, the maximum upstream power that it can draw
is one unit load, and the regulator block must implement inrush current limiting. The amount of power
that the function block may draw is limited only by the local power supply. Because the local power
supply is not required to power any downstream bus ports, it does not need to implement current limiting,
soft start, or power switching.
Self-powered functions must adhere to the same ground and Vbus isolation rules as self-powered hubs.
FunctionFunction Controller
Upstream Vbus
Upstream
Data Port
Regulator
1 unit load (max)
Local Power
Supply Regulator
Figure 7-27. Self-powered Function
7.2.2 Voltage Drop Budget
The voltage drop budget is determined from:
• The voltage supplied by host or powered hub ports is 4.75 V to 5.25 V.
• Bus-powered hubs can have a maximum drop of 350 mV from their cable plug where they attach to a
source of power to their output port connectors where they supply a source of power.
• All hubs and functions must be able to provide configuration information with as little as 4.40 V at
the connector end of their upstream cables. Only low power functions need to be able to be fully
operational with this minimum voltage.
• Functions drawing more than one unit load must operate with a 4.75 V minimum input voltage at the
connector end of their upstream cables.
Figure 7-28 shows the minimum allowable voltages in a worst case topology consisting of an bus-
powered hub driving an bus-powered function.
Host or
Powered H ub
4.75V (min)
Bus-Powered
Hub Bus-Powered
Function
4.40V (min)
Figure 7-28. Worst Case Voltage Drop Topology
These requirements place stringent restrictions on cable and connector IR drops (refer to Section 6.4) and
port switch drops. However, they should be achievable with existing technologies at a reasonable cost.
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7.2.3 Power Control During Suspend/Resume
When a device is put into the suspend mode, it must draw less than 500 µA from the bus. This current
includes power drawn through the bus pull-up and pull-down. For bus-powered hubs, this current does
not include the current drawn by powered devices connected to downstream ports.
When a hub is in the suspend mode, it still must be able to provide the maximum current per port (one
unit of current per port for bus-powered hubs and five units per port for self-powered hubs). This is
necessary to support remote wake-up capable devices which will power-up while the remainder of the
system is still suspended.
When devices wake-up, either by themselves (remote wake-up) or by seeing resume signaling, they must
limit the inrush current on Vbus. The target maximum droop in the hub Vbus is 330 mV or about 10% of
the nominal signal swing from the function. The device must have sufficient on-board bypass
capacitance or a controlled power-on sequence such that the current drawn from the hub does not exceed
the maximum current capability of the port at any time while the device is waking up.
7.2.4 Dynamic Attach and Detach
The act of plugging or unplugging a hub or function must not affect the functionality of another device
on other segments of the network. Unplugging a function will stop the transaction between that function
and the host. However, the hub to which this function was attached will recover from this condition and
will alert the host that the port has been disconnected.
7.2.4.1 Inrush Current Limiting
When a function or hub is plugged into the network, it has a certain amount of on-board capacitance
between Vbus and ground. Also, the regulator will supply current to its output bypass capacitance and to
the function as soon as power is applied. As a result, if no measures are taken to prevent it, there can be
a surge of current into the device sufficient to pull the Vbus on the hub below its minimum operating
level. Inrush currents can also occur when a high power function is switched into its high power mode.
This problem must be solved by limiting the inrush current and by providing sufficient capacitance in
each hub to prevent the power supplied to the other ports from going out of tolerance. An additional
motivation for limiting inrush current is to minimize contact arcing, thereby prolonging connector contact
life.
The target maximum droop in the hub Vbus is 330 mV or about 10% of the nominal signal swing from the
function. In order to meet this target, the following conditions must be met:
• The maximum load that can be placed at the downstream end of a cable is 10 µF in parallel with
44 Ω. The 10 µF capacitance represents any bypass capacitor directly connected across the Vbus
lines in the function plus any capacitive effects visible through the regulator in the device. The 44 Ω
resistor represents one unit current load generated by the device during connect.
• If more bypass capacitance is required in the device due to large swings in the load current, then the
device must incorporate some form of surge current limiting for current in the cable such that it
matches the characteristics of the above load.
• The hub port Vbus power lines must be bypassed with no less than a 120 µF tantalum capacitor
(equivalent aluminum capacitor values are under test). Good standard bypass methods should be
used to minimize inductances between the bypass capacitors and the connectors. The bypass
capacitors themselves should have a low dissipation factor to allow decoupling at higher frequencies.
The upstream port of a hub is also required to meet the above requirements. Furthermore, a bus-powered
hub must provide additional surge limiting in the form of a soft start circuit when it enables power to its
downstream ports.
Universal Serial Bus Specification Revision 1.0
137
Signal pins are protected from excessive currents during dynamic attach by being recessed in the
connector such that the power pins make contact first. This guarantees that the power rails to the
downstream device are referenced before the signal pins make contact. Also, the hub port signal lines are
disabled and in a high impedance state during connect, allowing no current to flow for standard signal
levels.
7.2.4.2 Dynamic Detach
When a device is detached from the network with power flowing in the cable, the inductance of the cable
will cause a large flyback voltage to occur on the open end of the device cable. This flyback voltage is
not destructive. Proper bypass measures on the hub ports will suppress any coupled noise. The
frequency range of this noise is inversely dependent on the length of the cable to a maximum of 60 MHz
for a one meter cable. This will require some low capacitance, very low inductance bypass capacitors on
each hub port connector. The flyback voltage and the noise it creates is also moderated by the bypass
capacitance on the device end of the cable. Also, there must be some minimum capacitance on the
device end of the cable to insure that the inductive flyback on the open end of the cable does not cause
the voltage on the device end to reverse polarity. A minimum of 1.0 µF is recommended for bypass
across Vbus.
Again, signal pins are protected from excessive voltages during dynamic detach by being recessed in the
connector such that they break contact before the power pins.
7.3 Physical Layer
The physical layer specifications are described in the following subsections.
7.3.1 Environmental
The operating environment for USB is 0 °C to 70 °C ambient.
USB must meet the following regulatory requirements;
EMI:
FCC part 15 class B
EN55022:1994 (Based on CISPR-22:1993)
EN5082-1:1992 (Generic Immunity standard)
VCCI (Japan version of CISPR-22)
Safety:
UL, CSA
Universal Serial Bus Specification Revision 1.0
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7.3.2 Bus Timing/Electrical Characteristics
Table 7-4. DC Electrical Characteristics
Parameter Symbol Conditions (Notes 1,2) Min. Max. Unit
Supply Voltage:
Powered (Host or Hub) Port VBUS 4.75 5.25 V
Bus-powered Hub Port VBUS 4.40 5.25 V
Supply Current:
Powered Host/Hub Port (out) ICCPRT 500 mA
Bus-powered Hub Port (out) ICCUPT 100 mA
High Power Function (in) ICCHPF 500 mA
Low Power Function (in) ICCLPF 100 mA
Unconfig. Function/Hub (in) ICCINIT 100 mA
Suspended Device ICCS 500 µA
Leakage Current:
Hi-Z State Data Line Leakage ILO 0 V< VIN<3.3 V -10 +10 µA
Input Levels:
Differential Input Sensitivity VDI |(D+)-(D-)|, and Figure 7-4 0.2 V
Differential Common Mode Range VCM Includes VDI range 0.8 2.5 V
Single Ended Receiver Threshold VSE 0.8 2.0 V
Output Levels:
Static Output Low VOL RL of 1.5 kΩ to 3.6 V 0.3 V
Static Output High VOH RL of 15 kΩ to GND 2.8 3.6 V
Capacitance:
Transceiver Capacitance CIN Pin to GND 20 pF
Downstream Hub Port Bypass
Capacitance CHPB Vbus to GND (tantalum) 120 µF
Root Port Bypass Capacitance CRPB Vbus to GND Note 9 1.0 10.0 µF
Terminations:
Bus Pull-up Resistor on Root Port RPU (1.5 kΩ ± 5%) 1.425 1.575 kΩ
Bus Pull-down Resistor on
Downstream Port RPD (15 kΩ ± 5%) 14.25 15.75 kΩ
Cable Impedance and Timing:
Cable Impedance (Full Speed) ZO(45 Ω ±15%) 38.75 51.75 Ω
Cable Delay (one way) TCBL Figure 7-21 30 ns
Universal Serial Bus Specification Revision 1.0
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Table 7-5. Full Speed Source Electrical Characteristics
Parameter Symbol Conditions (Notes 1,2,3) Min. Max. Unit
Driver Characteristics:
Transition Time:
Rise Time
Fall Time
TR
TF
Note 5, 6 and Figure 7-16
CL = 50 pF
CL = 50 pF
4
4
20
20
ns
ns
Rise / Fall Time Matching TRFM (TR/TF) 90 110 %
Output Signal Crossover Voltage VCRS 1.3 2.0 V
Driver Output Resistance ZDRV Steady State Drive 28 43 Ω
Data Source Timings:
Full Speed Data Rate TDRATE Ave. Bit Rate
(12 Mb/s ± 0.25%) 11.97 12.03 Mbs
Frame Interval TFRAME 1.0 ms ± 0.05% 0.9995 1.0005 ms
Source Differential Driver Jitter
To Next Transition
For Paired Transitions
TDJ1
TDJ2
Note 7, 8 and Figure 7-29
-3.5
-4.0
3.5
4.0
ns
ns
Source EOP Width TEOPT Note 8 and Figure 7-30 160 175 ns
Differential to EOP transition
Skew TDEOP Note 8 and Figure 7-30 -2 5 ns
Receiver Data Jitter Tolerance
To Next Transition
For Paired Transitions
TJR1
TJR2
Note 8 and Figure 7-31
-18.5
-9
18.5
9
ns
ns
EOP Width at receiver
Must reject as EOP
Must accept as EOP
TEOPR1
TEOPR2
Note 8 and Figure 7-30
40
82
ns
ns
Universal Serial Bus Specification Revision 1.0
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Table 7-6. Low Speed Source Electrical Characteristics
Parameter Symbol Conditions (Notes 1,2,4) Min. Max. Unit
Driver Characteristics:
Transition Time:
Rise Time
Fall Time
TR
TF
Note 5, 6 and Figure 7-16
CL = 50 pF
CL = 350 pF
CL = 50 pF
CL = 350 pF
75
75
300
300
ns
ns
ns
ns
Rise/Fall Time Matching TRFM (TR/TF) 80 120 %
Output Signal Crossover Voltage VCRS 1.3 2.0 V
Data Source Timings:
Low Speed Data Rate TDRATE Ave. Bit Rate
(1.5 Mb/s ± 1.5%) 1.4775 1.5225 Mbs
Source Differential Driver Jitter
Host (Downstream):
To Next Transition
For Paired Transitions
Function (Upstream):
To Next Transition
For Paired Transitions
TDDJ1
TDDJ2
TUDJ1
TUDJ2
Note 7, 8 and Figure 7-29
-75
-45
-95
-150
75
45
95
150
ns
ns
ns
ns
Source EOP Width TEOPT Note 8 and Figure 7-30 1.25 1.50 µs
Differential to EOP transition
Skew TDEOP Note 8 and Figure 7-30 -40 100 ns
Receiver Data Jitter Tolerance
At Host (Upstream):
To Next Transition
For Paired Transitions
At Function (Downstream):
To Next Transition
For Paired Transitions
TUJR1
TUJR2
TDJR1
TDJR2
Note 8 and Figure 7-31
-152
-200
-75
-45
152
200
75
45
ns
ns
ns
ns
EOP Width at receiver
Must reject as EOP
Must accept TEOPR1
TEOPR2
Note 8 and Figure 7-30
330
675 ns
ns
Universal Serial Bus Specification Revision 1.0
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Table 7-7. Hub/Repeater Electrical Characteristics
Parameter Symbol Conditions (Note 2) Min. Max. Unit
Hub Characteristics
(Full Speed):
Driver Characteristics
(Refer to Table 7-5) Root port and
Downstream ports
configured as Full Speed
Hub Differential Data Delay
(with cable)
(without cable) THDD1
THDD2
Note 3, 7, 8
and Figure 7-32 70
40 ns
ns
Hub Differential Driver Jitter
(including cable)
To Next Transition
For Paired Transitions THDJ1
THDJ2
Note 3, 7, 8
and Figure 7-32
-3
-1 3
1ns
ns
Data bit width distortion after SOP TSOP Note 3, 8 and Figure 7-32 -5 3
Hub EOP Delay Relative to tHDD TEOPD Note 3, 8 and Figure 7-33 0 15 ns
Hub EOP Output Width Skew THESK Note 3, 8 and Figure 7-33 -15 15 ns
Hub Timings (Low Speed):
Driver Characteristics
(Refer to Table 7-6) Downstream ports
configured as Low Speed
Hub Differential Data Delay TLHDD Note 4, 7, 8
and Figure 7-32 300 ns
Hub Differential Driver Jitter
(including cable)
Downstream:
To Next Transition
For Paired Transitions
Upstream:
To Next Transition
For Paired Transitions
TLDHJ1
TLDHJ2
TLUHJ1
TLUHJ2
Note 4, 7, 8
and Figure 7-32
-45
-15
-45
-45
45
15
45
45
ns
ns
ns
ns
Data bit width distortion after SOP TSOP Note 4, 8 and Figure 7-32 -60 45
Hub EOP Delay Relative to tHDD TLEOPD Note 4, 8 and Figure 7-33 0 200 ns
Hub EOP Output Width Skew TLHESK Note 4, 8 and Figure 7-33 -300 +300 ns
Note 1: All voltages measured from the local ground potential, unless otherwise specified.
Note 2: All timings use a capacitive load (CL) to ground of 50 pF, unless otherwise specified.
Note 3: Full Speed timings have a 1.5 kΩ pull-up to 2.8 V on the D+ data line.
Note 4: Low Speed timings have a 1.5 kΩ pull-up to 2.8 V on the D- data line.
Note 5: Measured from 10% to 90% of the data signal.
Note 6: The rising and falling edges should be smoothly transitioning (monotonic).
Note 7: Timing difference between the differential data signals.
Note 8: Measured at crossover point of differential data signals.
Note 9: The maximum load specification is the maximum effective capacitive load allowed that meets the target hub
VBUS droop of 330 mV.
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7.3.3 Timing Waveforms
TPERIOD
Differential
Data Lines
Crossover
Points
Paired
Transitions
N * TPERIOD + TxJR2
Consecutive
Transitions
N * TPERIOD + TxJR1
Figure 7-29. Differential Data Jitter
TPERIOD
Differential
Data Lines
Crossover
Point
Crossover
Point Extended
Source EOP W idth: TEOPT
Re ceiver EOP Width: TEOPR1, TEOPR2
Diff. Data to
SE0 Skew
N * TPERIOD + TDEOP
Figure 7-30. Differential to EOP Transition Skew and EOP Width
Differential
Data Lines
Paired
Transitions
N * TPERIOD + TJR2
TPERIOD
Consecutive
Transitions
N * TPERIOD + TJR1
TJR TJR1 TJR2
Figure 7-31. Receiver Jitter Tolerance
Refer to Section 7.1.11 for the definition of TPERIOD
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50% Point of
Initial Swing
VSS
Upstream
End of
Upstream
End of
VSS
B. Upstream Hub Delay
Hub Differential Jitter:
THDJ1 = THDDx(J) - THDDx(K) or THDDx(K) - THDDx(J) Consecutive Transitions
THDJ2 = THDDx(J) - THDDx(J) or THDDx(K) - THDDx(K) Paired Transitions
Bit after SOP Width Distortion: (Same as data jitter for SOP and next J transition.)
THDJ1 = THDDx(SOP) - THDDx(next J)
Low Speed timings ar e determined in the same way for:
TLHDD, TLDHJ1, TLDJH2, TLUHJ1, TLUJH2, and TLSOP .
A. Downstream Hub Delay
Downstream
Port Hub Delay
Downstream
THDD1
Crossover
Point
Crossover
Point
Crossover
Point
Hub Del ay
Upstream
THDD1
THDD2
Downstream
Port
Figure 7-32. Hub Differential Delay, Differential Jitter, and SOP Distortion
50% Point of
Initial Swing
VSS
Upstream
End of Cable
B. Upstream EOP DelayA. Downstream EOP Delay
EOP Delay
TEOPD = TEOP-
EOP Skew
THESK = TEOP+ - TEOP-
Low Speed timings are determined in the same way for:
TLEOPD and TLHESK
VSS
TEOP+
Crossover
Point
Extended
Crossover
Point
Extended
Crossover
Point
Extended
Downstream
Port
TEOP- TEOP- TEOP+
Upstream
End of
Cable
Downstream
Port
Figure 7-33. Hub EOP Delay and EOP Skew
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Chapter 8
Protocol Layer
This chapter presents a bottom-up view of the USB protocol starting with field and packet definitions.
This is followed by a description of packet transaction formats for different transaction types. Link layer
flow control and transaction level fault recovery are then covered. The chapter finishes with a discussion
of retry synchronization, babble, and loss of bus activity recovery.
8.1 Bit Ordering
Bits are sent out onto the bus LSB first, followed by next LSB, through to MSB last. In the following
diagrams, packets are displayed such that both individual bits and fields are represented (in a left to right
reading order) as they would move across the bus.
8.2 SYNC Field
All packets begin with a synchronization (SYNC) field, which is a coded sequence that generates a
maximum edge transition density. The SYNC field appears on the bus as IDLE followed by the binary
string “KJKJKJKK,” in its NRZI encoding. It is used by the input circuitry to align incoming data with
the local clock and is defined to be eight bits in length. SYNC serves only as a synchronization
mechanism and is not shown in the following packet diagrams (refer to Section 7.1.7). The last two bits
in the SYNC field are a marker that is used to identify the first bit of the PID. All subsequent bits in the
packet must be indexed from this point.
8.3 Packet Field Formats
Field formats for the token, data, and handshake packets are described in the following section. Packet
bit definitions are displayed in unencoded data format. The effects of NRZI coding and bit stuffing have
been removed for the sake of clarity. All packets have distinct start and end of packet delimiters. The
start of packet (SOP) is part of the SYNC field, and the end of packet (EOP) delimiter is described in
Chapter 7.
8.3.1 Packet Identifier Field
A packet identifier (PID) immediately follows the SYNC field of every USB packet. A PID consists of a
four bit packet type field followed by a four-bit check field as shown in Figure 8-1. The PID indicates
the type of packet and, by inference, the format of the packet and the type of error detection applied to
the packet. The four-bit check field of the PID insures reliable decoding of the PID so that the remainder
of the packet is interpreted correctly. The PID check field is generated by performing a ones complement
of the packet type field.
(MSb)
(LSb)
PID 2PID 3
PID 10
PID PID 0PID 1PID 2PID 3
Figure 8-1. PID Format
The host and all functions must perform a complete decoding of all received PID fields. Any PID
received with a failed check field or which decodes to a non-defined value is assumed to be corrupted
and it, as well as the remainder of the packet, is ignored by the packet receiver. If a function receives an
otherwise valid PID for a transaction type or direction that it does not support, the function must not
respond. For example, an IN only endpoint must ignore an OUT token. PID types, codings, and
descriptions are listed in Table 8-1.
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Table 8-1. PID Types
PID Type PID Name PID[3:0] Description
Token OUT
IN
SOF
SETUP
b0001
b1001
b0101
b1101
Address + endpoint number in host -> function
transaction
Address + endpoint number in function -> host
transaction
Start of frame marker and frame number
Address + endpoint number in host -> function
transaction for setup to a control endpoint
Data DATA0
DATA1
b0011
b1011
Data packet PID even
Data packet PID odd
Handshake ACK
NAK
STALL
b0010
b1010
b1110
Receiver accepts error free data packet
Rx device cannot accept data or Tx device cannot send
data
Endpoint is stalled
Special PRE b1100 Host-issued preamble. Enables downstream bus traffic
to low speed devices.
PIDs are divided into four coding groups: token, data, handshake, and special, with the first two
transmitted PID bits (PID<1:0>) indicating which group. This accounts for the distribution of PID codes.
8.3.2 Address Fields
Function endpoints are addressed using two fields: the function address field and the endpoint field. A
function needs to fully decode both address and endpoint fields. Address or endpoint aliasing is not
permitted, and a mismatch on either field must cause the token to be ignored. Accesses to non-initialized
endpoints will also cause the token to be ignored.
8.3.2.1 Address Field
The function address (ADDR) field specifies the function, via its address, that is either the source or
destination of a data packet, depending on the value of the token PID. As shown in Figure 8-2, a total of
128 addresses are specified as ADDR<6:0>. The ADDR field is specified for IN, SETUP, and OUT
tokens. By definition, each ADDR value defines a single function. Upon reset and power-up, a
function’s address defaults to a value of 0 and must be programmed by the host during the enumeration
process. The 0 default address is reserved for default and cannot be assigned for normal operation.
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(LSb) (MSb)
Addr
4 Addr
5 Addr6
Addr
3
Addr
2
Addr
1
Addr
0
Figure 8-2. ADDR Field
8.3.2.2 Endpoint Field
An additional four-bit endpoint (ENDP) field, shown in Figure 8-3, permits more flexible addressing of
functions in which more than one sub-channel is required. Endpoint numbers are function specific. The
endpoint field is defined for IN, SETUP, and OUT token PIDs only. All functions must support one
control endpoint at 0. Low speed devices support a maximum of two endpoint addresses per function: 0
plus one additional endpoint. Full speed functions may support up to the maximum of 16 endpoints.
Endp 2 Endp 3
Endp1
Endp0
(LSb) (MSb)
Figure 8-3. Endpoint Field
8.3.3 Frame Number Field
The frame number field is an 11-bit field that is incremented by the host on a per frame basis. The frame
number field rolls over upon reaching its maximum value of x7FF, and is sent only for SOF tokens at the
start of each frame.
8.3.4 Data Field
The data field may range from 0 to 1023 bytes and must be an integral numbers of bytes. Figure 8-4
shows the format for multiple bytes. Data bits within each byte are shifted out LSB first.
(MSb) (LSb)
(MSb)
(LSb)
Byte N Byte N+ 1
D0D5
D4
D3
D2
D1D6D7D0
D7
Byte N-1
Figure 8-4. Data Field Format
Data packet size varies with the transfer type as described in Chapter 5.
8.3.5 Cyclic Redundancy Checks
Cyclic redundancy checks (CRCs) are used to protect the all non-PID fields in token and data packets. In
this context, these fields are considered to be protected fields. The PID is not included in the CRC check
of a packet containing a CRC. All CRCs are generated over their respective fields in the transmitter
before bit stuffing is performed. Similarly, CRCs are decoded in the receiver after stuffed bits have been
removed. Token and data packet CRCs provide 100% coverage for all single and double bit errors. A
failed CRC is considered to indicate that one or more of the protected fields is corrupted and causes the
receiver to ignore those fields, and, in most cases, the entire packet.
For CRC generation and checking, the shift registers in the generator and checker are seeded with an all
ones pattern. For each data bit sent or received, the high order bit of the current remainder is XORed
with the data bit and then the remainder is shifted left one bit and the low order bit set to 0. If the result
of that XOR is 1, then the remainder is XORed with the generator polynomial.
When the last bit of the checked field is sent, the CRC in the generator is inverted and sent to the checker
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MSB first. When the last bit of the CRC is received by the checker and no errors have occurred, the
remainder will be equal to the polynomial residual.
Bit stuffing requirements must be met for the CRC, and this includes the need to insert a zero at the end
of a CRC if the preceding six bits were all ones.
8.3.5.1 Token CRCs
A five-bit CRC field is provided for tokens and covers the ADDR and ENDP fields of IN, SETUP, and
OUT tokens or the time stamp field of an SOF token. The generator polynomial is:
G(X) = X5 + X2 + 1
The binary bit pattern that represents this polynomial is 00101. If all token bits are received without
error, the five-bit residual at the receiver will be 01100.
8.3.5.2 Data CRCs
The data CRC is a 16-bit polynomial applied over the data field of a data packet. The generating
polynomial is:
G(X) = X16 + X15 + X2 + 1
The binary bit pattern that represents this polynomial is 1000000000000101. If all data and CRC bits are
received without error, the 16-bit residual will be 1000000000001101.
8.4 Packet Formats
This section shows packet formats for token, data, and handshake packets. Fields within a packet are
displayed in the order in which bits are shifted out onto the bus in the order shown in the figures.
8.4.1 Token Packets
Figure 8-5 shows the field formats for a token packet. A token consists of a PID, specifying either IN,
OUT, or SETUP packet type, and ADDR and ENDP fields. For OUT and SETUP transactions, the
address and endpoint fields uniquely identify the endpoint that will receive the subsequent data packet.
For IN transactions, these fields uniquely identify which endpoint should transmit a data packet. Only
the host can issue token packets. IN PIDs define a data transaction from a function to the host. OUT and
SETUP PIDs define data transactions from the host to a function.
ADDRPID
8 bits 7 bits
ENDP
4 bits
CRC5
5 bits
Figure 8-5. Token Format
Token packets have a five-bit CRC which covers the address and endpoint fields as shown above. The
CRC does not cover the PID, which has its own check field. Token and SOF packets are delimited by an
EOP after three bytes of packet field data. If a packet decodes as an otherwise valid token or SOF but
does not terminate with an EOP after three bytes, it must be considered invalid and ignored by the
receiver.
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8.4.2 Start of Frame Packets
Start of Frame (SOF) packets are issued by the host at a nominal rate of once every 1.00 ms ±0.05. SOF
packets consist of a PID indicating packet type followed by an 11-bit frame number field as illustrated in
Figure 8-6.
Frame Number
PID
8 bits 11 bits
CRC5
5 bits
Figure 8-6. SOF Packet
The SOF token comprises the token-only transaction that distributes a start of frame marker and
accompanying frame number at precisely timed intervals corresponding to the start of each frame. All
full speed functions, including hubs, must receive and decode the SOF packet. The SOF token does not
cause any receiving function to generate a return packet; therefore, SOF delivery to any given function
cannot be guaranteed. The SOF packet delivers two pieces of timing information. A function is
informed that a start of frame has occurred when it detects the SOF PID. Frame timing sensitive
functions, which do not need to keep track of frame number, need only decode the SOF PID; they can
ignore the frame number and its CRC. If a function needs to track frame number, it must comprehend
both the PID and the time stamp.
8.4.3 Data Packets
A data packet consists of a PID, a data field, and a CRC as shown in Figure 8-7. There are two types of
data packets, identified by differing PIDs: DATA0 and DATA1. Two data packet PIDs are defined to
support data toggle synchronization (refer to Section 8.6).
PID CRC16
16 bits
DATA
0-1023 bytes
8 bits
Figure 8-7. Data Packet Format
Data must always be sent in integral numbers of bytes. The data CRC is computed over only the data
field in the packet and does not include the PID, which has its own check field.
8.4.4 Handshake Packets
Handshake packets, as shown in Figure 8-8, consist of only a PID. Handshake packets are used to report
the status of a data transaction and can return values indicating successful reception of data, flow control,
and stall conditions. Only transaction types that support flow control can return handshakes. Handshakes
are always returned in the handshake phase of a transaction and may be returned, instead of data, in the
data phase. Handshake packets are delimited by an EOP after one byte of packet field. If a packet
decodes as an otherwise valid handshake but does not terminate with an EOP after one byte, it must be
considered invalid and ignored by the receiver.
PID
8 bits
Figure 8-8. Handshake Packet
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There are three types of handshake packets:
• ACK indicates that the data packet was received without bit stuff or CRC errors over the data field and
that the data PID was received correctly. ACK may be issued either when sequence bits match and the
receiver can accept data or when sequence bits mismatch and the sender and receiver must resynchronize
to each other (refer to Section 8.6 for details). An ACK handshake is applicable only in transactions in
which data has been transmitted and where a handshake is expected. ACK can be returned by the host
for IN transactions and by a function for OUT transactions.
• NAK indicates that a function was unable to accept data from the host (OUT) or that a function has no
data to transmit to the host (IN). NAK can only be returned by functions in the data phase of IN
transactions or the handshake phase of OUT transactions. The host can never issue a NAK. NAK is used
for flow control purposes to indicate that a function is temporarily unable to transmit or receive data, but
will eventually be able to do so without need of host intervention. NAK is also used by interrupt
endpoints to indicate that no interrupt is pending.
• STALL is returned by a function in response to an IN token or after the data phase of an OUT (see
Figure 8-9 and Figure 8-13). STALL indicates that a function is unable to transmit or receive data, and
that the condition requires host intervention to remove the stall. Once a function’s endpoint is stalled, the
function must continue returning STALL until the condition causing the stall has been cleared through
host intervention. The host is not permitted to return a STALL under any condition.
8.4.5 Handshake Responses
Transmitting and receiving functions must return handshakes based upon an order of precedence detailed
in Table 8-2 through Table 8-4. Not all handshakes are allowed, depending on the transaction type and
whether the handshake is being issued by a function or the host.
8.4.5.1 Function Response to IN Transactions
Table 8-2 shows the possible responses a function may make in response to an IN token. If the function
is unable to send data, due to a stall or a flow control condition, it issues a STALL or NAK handshake,
respectively. If the function is able to issue data, it does so. If the received token is corrupted, the
function returns no response.
Table 8-2. Function Responses to IN Transactions
Token Received
Corrupted Function Tx
Endpoint Stalled Function Can
Transmit Data Action Taken
Yes Don’t care Don’t care Return no response
No Yes Don’t care Issue STALL handshake
No No No Issue NAK handshake
No No Yes Issue data packet
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8.4.5.2 Host Response to IN Transactions
Table 8-3 shows the host response to an IN transaction. The host is able to return only one type of
handshake, an ACK. If the host receives a corrupted data packet, it discards the data and issues no
response. If the host cannot accept data from a function, (due to problems such as internal buffer
overrun) this condition is considered to be an error and the host returns no response. If the host is able to
accept data and the data packet is received error free, the host accepts the data and issues an ACK
handshake.
Table 8-3. Host Responses to IN Transactions
Data Packet
Corrupted Host Can
Accept Data Handshake Returned by Host
Yes N/A Discard data, return no response
No No Discard data, return no response
No Yes Accept data, issue ACK
8.4.5.3 Function Response to an OUT Transaction
Handshake responses for an OUT transaction are shown in Table 8-4. A function, upon receiving a data
packet, may return any one of the three handshake types. If the data packet was corrupted, the function
returns no handshake. If the data packet was received error free and the function’s receiving endpoint is
stalled, the function returns a STALL handshake. If the transaction is maintaining sequence bit
synchronization and a mismatch is detected (refer to Section 8.6 for details), then the function returns
ACK and discards the data. If the function can accept the data and has received the data error free, it
returns an ACK handshake. If the function cannot accept the data packet due to flow control reasons, it
returns a NAK.
Table 8-4. Function Responses to OUT Transactions in Order of Precedence
Data Packet
Corrupted Receiver
Stalled Sequence Bits
Mismatch Function Can
Accept Data Handshake Returned
by Function
Yes N/A N/A N/A None
No Yes N/A N/A STALL
No No Yes N/A ACK
No No No Yes ACK
No No No No NAK
8.4.5.4 Function Response to a SETUP Transaction
Setup defines a special type of host to function data transaction which permits the host to initialize an
endpoint’s synchronization bits to those of the host. Upon receiving a Setup transaction, a function must
accept the data. Setup transactions cannot be stalled or NAKed and the receiving function must accept
the Setup transfer’s data. If a non-control endpoint receives a SETUP PID, it must ignore the transaction
and return no response.
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8.5 Transaction Formats
Packet transaction format varies depending on the endpoint type. There are four endpoint types: bulk,
control, interrupt, and isochronous.
8.5.1 Bulk Transactions
Bulk transaction types are characterized by the ability to guarantee error free delivery of data between the
host and a function by means of error detection and retry. Bulk transactions use a three phase transaction
consisting of token, data, and handshake packets as shown in Figure 8-9. Under certain flow control and
stall conditions, the data phase may be replaced with a handshake resulting in a two phase transaction in
which no data is transmitted.
Token
Data
FunctionHost
IN OUT
Idle
DATA0/
DATA1
ACK
DATA0/
DATA1
Idle
ACK
NAK STALL
Idle
STALL
NAK
Figure 8-9. Bulk Transaction Format
When the host wishes to receive bulk data, it issues an IN token. The function endpoint responds by
returning either a DATA packet or, should it be unable to return data, a NAK or STALL handshake. A
NAK indicates that the function is temporarily unable to return data, while a STALL indicates that the
endpoint is permanently stalled and requires host software intervention. If the host receives a valid data
packet, it responds with an ACK handshake. If the host detects an error while receiving data, it returns
no handshake packet to the function.
When the host wishes to transmit bulk data, it first issues an OUT token packet followed by a data
packet. The function then returns one of three handshakes. ACK indicates that the data packet was
received without errors and informs the host that that it may send the next packet in the sequence. NAK
indicates that the data was received without error but that the host should resend the data because the
function was in a temporary condition preventing it from accepting the data at this time (e.g., buffer full).
If the endpoint was stalled, STALL is returned to indicate that the host should not retry the transmission
because there is an error condition on the function. If the data packet was received with a CRC or bit
stuff error, no handshake is returned.
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Figure 8-10 shows the sequence bit and data PID usage for bulk reads and writes. Data packet
synchronization is achieved via use of the data sequence toggle bits and the DATA0/DATA1 PIDs. Bulk
endpoints must have their toggle sequence bits initialized via a separate control endpoint.
OUT (0) OUT (0/1)
IN (0) IN (1) IN (0/1)
DATA1
...
...
DATA0
DATA0 DATA1
DATA0/1
DATA0/1
OUT (1)
Bulk
Write
Bulk
Read
Figure 8-10. Bulk Reads and Writes
The host always initializes the first transaction of a bus transfer to the DATA0 PID. The second
transaction uses a DATA1 PID, and successive data transfers alternate for the remainder of the bulk
transfer. The data packet transmitter toggles upon receipt of ACK, and the receiver toggles upon receipt
and acceptance of a valid data packet (refer to Section 8.6).
8.5.2 Control Transfers
Control transfers minimally have two transaction stages: Setup and Status. A control transfer may
optionally contain a data stage between the setup and status stages. During the Setup stage, a Setup
transaction is used to transmit information to the control endpoint of a function. Setup transactions are
similar in format to an OUT, but use a SETUP rather than on OUT PID. Figure 8-11 shows the Setup
transaction format. A Setup always uses a DATA0 PID for the data field of the Setup transaction. The
function receiving a Setup must accept the Setup data and respond with an ACK handshake or, if the data
is corrupted, discard the data and return no handshake.
FunctionHost
Idle
DATA0
ACK
SETUP
Idle
Token
Data
Handshake
Figure 8-11. Control Setup Transaction
The Data stage, if present, of a control transfer consists of one or more IN or OUT transactions and
follows the same protocol rules as bulk transfers. All the transactions in the Data stage must be in the
same direction, i.e., all INs or all OUTs. The amount of data to be sent during the data phase and its
direction are specified during the Setup stage. If the amount of data exceeds the prenegotiated data
packet size, the data is sent in multiple transactions (INs or OUTs) which carry the maximum packet size.
Any remaining data is sent as a residual in the last transaction.
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The Status stage of a control transfer is the last operation in the sequence. A Status stage is delineated by
a change in direction of data flow from the previous stage and always uses a DATA1 PID. If, for
example, the Data stage consists of OUTs, the status is a single IN transaction. If the control sequence
has no data stage, then it consists of a Setup stage followed by a Status stage consisting of an IN
transaction. Figure 8-12 shows the transaction order, the data sequence bit value, and the data PID types
for control read and write sequences. The sequence bits are displayed in parentheses.
SETUP (0) OUT (1) OUT (0/1)
IN (0) IN (0/1)
Setup
Stage
Data
Stage
Control
Write
Status
Stage
DATA0 DATA0
...
...
DATA1 DATA1
DATA1
IN (1)
OUT (1)
DATA0
DATA0/1
DATA0/1
OUT (0)
IN (1)
Control
Read DATA0 DATA1
SETUP (0)
IN (1)
No-data
Control DATA0 DATA1
SETUP (0)
Setup
Stage
Status
Stage
Figure 8-12. Control Read and Write Sequences
8.5.2.1 Reporting Status Results
The Status stage reports to the host the outcome of the previous Setup and Data stages of the transfer.
Three possible results may be returned:
• The command sequence completed successfully.
• The command sequence failed to complete.
• The function is still busy completing command.
Status reporting is always in the function to host direction. The following table summarizes the type of
responses required for each. Control write transfers return status information on the data phase of the
transfer. Control read transfers return status information on the handshake phase after the host has issued
a zero length data packet during the previous data phase.
Table 8-5. Status Phase Responses
Status Response Control Write Transfer
(sent during data phase) Control Read Transfer
(send during handshake phase)
Function completes 0 length data packet ACK handshake
Function has an error STALL handshake STALL handshake
Function is busy NAK handshake NAK handshake
For control reads, the host sends a zero length data packet to the control endpoint. The endpoint’s
handshake response indicates the completion status. NAK indicates that the function is still processing
the command and that the host should continue the status phase. ACK indicates that the function has
completed the command and is ready to accept a new command and STALL indicates that the function
has an error that prevents it from completing the command.
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For control writes, the function responds with either a handshake or a zero length data packet to indicate
its status. A NAK indicates that the function is still processing the command and that the host should
continue the status phase, return of a zero length packet indicates normal completion of the command,
and STALL indicates that the function has an error that prevents it from completing the command.
Control write transfers which return a zero length data packet during the data phase always cause the host
to return an ACK handshake to the function.
If, during a Data or Status stage, a command endpoint is sent more data or is requested to return more
data than was indicated in the Setup stage, it should return a STALL. If a control endpoint returns
STALL during the Data stage, there will be no Status stage for that control transfer.
8.5.2.2 Error Handling on the Last Data Transaction
If the ACK handshake on an IN transaction is corrupted, the function and the host will temporarily
disagree on whether the transaction was successful. If the transaction is followed by another IN, the
toggle retry mechanism will detect the mismatch and recover from the error. If the ACK was on the last
IN of a control transfer, the toggle retry mechanism cannot be used and an alternative scheme must be
used.
The host that successfully received the data of the last IN, issues an OUT setup transfer, and the function,
upon seeing that the token direction has toggled, interprets this action as proof that the host successfully
received the data. In other words, the function interprets the toggling of the token direction as implicit
proof of the host’s successful receipt of the last ACK handshake. Therefore, when the function sees the
OUT setup transaction, it advances to the status phase.
Control writes do not have this ambiguity. The host, by virtue of receiving the handshake, knows for sure
if the last transaction was successful. If an ACK handshake on an OUT gets corrupted, the host does not
advance to the status phase and retries the last data instead. A detailed analysis of retry policy is
presented in Section 8.6.4.
8.5.3 Interrupt Transactions
Interrupt transactions consist solely of IN as shown in Figure 8-13. Upon receipt of an IN token, a
function may return data, NAK, or STALL. If the endpoint has no new interrupt information to return,
i.e., no interrupt is pending, the function returns a NAK handshake during the data phase. A stalled
interrupt endpoint causes the function to return a STALL handshake if it is permanently stalled and
requires software intervention by the host. If an interrupt is pending, the function returns the interrupt
information as a data packet. The host, in response to receipt of the data packet, issues either an ACK
handshake if data was received error free or returns no handshake if the data packet was received
corrupted.
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DeviceHost
Token
Handshake
Idle
Data
IN
Idle
ACK
DATA0/
DATA1 NAK
Idle
STALL
Figure 8-13. Interrupt Transaction Format
When an endpoint is using the interrupt transfer mechanism for actual interrupt data, the data toggle
protocol must be followed. This allows the function to know that the data has been received by the host
and the event condition may be cleared. This “guaranteed" delivery of events allows the function to only
send the interrupt information until it has been received by the host rather than having to send the
interrupt data every time the function is polled and until host software clears the interrupt condition.
When used in the toggle mode, an interrupt endpoint is initialized to the DATA0 PID and behaves the
same as the bulk IN transaction shown in Figure 8-10.
An interrupt endpoint may also be used to communicate rate feedback information for certain types of
isochronous functions. When used in this mode, the data toggle bits should be changed after each data
packet is sent to the host without regard to the presence or type of handshake packet.
8.5.4 Isochronous Transactions
ISO transactions have a token and data phase, but no handshake phase, as shown in Figure 8-14. The
host issues either an IN or an OUT token followed by the data phase in which the endpoint (for INs) or
the host (for OUTs) transmits data. ISO transactions do not support a handshake phase or retry
capability.
Token
Idle
Idle
IN OUT
DATA0/
DATA1 Data
DATA0/
DATA1
FunctionHost
Figure 8-14. Isochronous Transaction Format
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ISO transactions do not support toggle sequencing, and the data PID is always DATA0. The packet
receiver does not examine the data PID.
8.6 Data Toggle Synchronization and Retry
USB provides a mechanism to guarantee data sequence synchronization between data transmitter and
receiver across multiple transactions. This mechanism provides a means of guaranteeing that the
handshake phase of a transaction was interpreted correctly by both the transmitter and receiver.
Synchronization is achieved via use of the DATA0 and DATA1 PIDs and separate data toggle sequence
bits for the data transmitter and receiver. Receiver sequence bits toggle only when the receiver is able to
accept data and receives an error free data packet with the correct data PID. Transmitter sequence bits
toggle only when the data transmitter receives a valid ACK handshake. The data transmitter and receiver
must have their sequence bits synchronized at the start of a transaction. The synchronization mechanism
used varies with the transaction type. Data toggle synchronization is not supported for ISO transfers.
8.6.1 Initialization via SETUP Token
Control transfers use the SETUP token for initializing host and function sequence bits. Figure 8-15
shows the host issuing a SETUP packet to a function followed by an OUT. The numbers in the circles
represent the transmitter and receiver sequence bits. The function must accept the data and ACK the
transaction. When the function accepts the transaction, it must reset its sequence bit so that both the
host’s and function’s sequence bits are equal to 1 at the end of the SETUP transaction.
Tx
(1) Rx
(X->1)
Tx
(1) Rx
(1)
Rx
(X)
SETUP
DATA0
ACK
Accept
data
Host Device
Tx
(X-1)
Figure 8-15. SETUP Initialization
8.6.2 Successful Data Transactions
Figure 8-16 shows the case where two successful transactions have occurred. For the data transmitter,
this means that it toggles its sequence bit upon receipt of an ACK. The receiver toggles its sequence bit
only if it receives a valid data packet and the packet’s data PID matches the receiver’s sequence bit.
During each transaction, the receiver compares the transmitter sequence bit (encoded in the data packet
PID as either DATA0 or DATA1) with its receiver sequence bit. If data cannot be accepted, the receiver
must issue a NAK. If data can be accepted and the receiver’s sequence bit matches the PID sequence bit,
then data is accepted. Sequence bits may only change if a data packet is transmitted. Two-phase
transactions in which there is no data packet leave the transmitter and receiver sequence bits unchanged.
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Tx
(0) Rx
(0->1)
Tx
(0->1) Rx
(1)
DATA0
ACK
Tx
(1) Rx
(1->0)
Tx
(1->0) Rx
(0)
DATA1
ACK
Transfer i Transfer i + 1
Accept
data
Accept
data
Figure 8-16. Consecutive Transactions
8.6.3 Data Corrupted or Not Accepted
If data cannot be accepted or the received data packet is corrupted, the receiver will issue a NAK or
STALL handshake, or time out, depending on the circumstances, and the receiver will not toggle its
sequence bit. Figure 8-17 shows the case where a transaction is NAKed and then retried. Any non-ACK
handshake or time out will generate similar retry behavior. The transmitter, having not received an ACK
handshake, will not toggle its sequence bit. As a result, a failed data packet transaction leaves the
transmitter’s and receiver’s sequence bits synchronized and untoggled. The transaction will then be
retried and, if successful, will cause both transmitter and receiver sequence bits to toggle.
Tx
(0) Rx
(0->0)
Tx
(0->0) Rx
(0)
DATA0
NAK
Tx
(0) Rx
(0->1)
Tx
(0->1) Rx
(1)
DATA0
ACK
Transfer i Retry
Transfer i
Accept
data
Reject
data
Figure 8-17. NAKed Transaction with Retry
8.6.4 Corrupted ACK Handshake
The transmitter is the last and only agent to know for sure whether a transaction has been successful, due
to its receiving an ACK handshake. A lost or corrupted ACK handshake can lead to a temporary loss of
synchronization between transmitter and receiver as shown in Figure 8-18. Here the transmitter issues a
valid data packet, which is successfully acquired by the receiver; however, the ACK handshake is
corrupted.
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Tx
(0) Rx
(0->1)
Tx
(0->0) Rx
(1)
DATA0
Fai led ACK
Tx
(0) Rx
(1)
Tx
(0->1) Rx
(1)
DATA0
ACK
Transfer i Transfer i
(retried)
Tx
(1) Rx
(1->0)
Tx
(1->0) Rx
(0)
DATA1
ACK
Transfer i + 1
Ignore
data
Accept
data
Figure 8-18. Corrupted ACK Handshake with Retry
At the end of transaction <i>, there is a temporary loss of coherency between transmitter and receiver, as
evidenced by the mismatch between their respective sequence bits. The receiver has received good data,
but the transmitter does not know whether it has successfully sent data. On the next transaction, the
transmitter will resend the previous data using the previous DATA0 PID. The receiver’s sequence bit
and the data PID will not match, so the receiver knows that it has previously accepted this data.
Consequently, it discards the incoming data packet and does not toggle its sequence bit. The receiver
then issues an ACK, which causes the transmitter to regard the retried transaction as successful. Receipt
of ACK causes the transmitter to toggle its sequence bit. At the beginning of transaction <i+1>, the
sequence bits have toggled and are again synchronized.
The data transmitter must guarantee that any retried data packet be the same length as that sent in the
original transaction. If the data transmitter is unable, because of problems such as a buffer underrun
condition, to transmit the identical amount of data as was in the original data packet, it must abort the
transaction by generating a bit stuffing violation. This causes a detectable error at the receiver and
guarantees that a partial packet will not be interpreted as a good packet. The transmitter should not try to
force an error at the receiver by sending a known bad CRC. A combination of a bad packet with a “bad”
CRC may be interpreted by the receiver as a good packet.
8.6.5 Low Speed Transactions
USB supports signaling at two speeds: full speed signaling at 12.0 Mbs and low speed signaling at
1.5 Mbs. Hubs disable downstream bus traffic to all ports to which low speed devices are attached during
full speed downstream signaling. This is required both for EMI reasons and to prevent any possibility
that an low speed device might misinterpret downstream a full speed packet as being addressed to it.
Figure 8-19 shows an IN low speed transaction in which the host issues a token and handshake and
receives a data packet.
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AAAA
A
AAA
A
AAA
A
AAA
A
AAA
A
AAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
A
AAA
A
AAA
A
AAA
A
AAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAA
AA
A
AA
A
AA
A
AA
A
AAA
SYNC PID SYNC PID
Token sent at low speed
. . .
ENDP EOP
Hub disables low
speed port outputs
SYNC PID SYNC PID
Preamble
sent at full speed
Handshake sent at low speed
EOP
Data packet sent at low speed
SYNC PID CRC EOPDATA
Hub disables low
speed port outputs
Hub enables low
speed port outputs
Hub enables low
speed port outputs
Hub setup
Hub setup
Preamble
sent at full speed
Figure 8-19. Low Speed Transaction
All downstream packets transmitted to low speed devices require a preamble. The preamble consists of a
SYNC followed by a PID, both sent at full speed. Hubs must comprehend the PRE PID; all other USB
devices must ignore it and treat it as undefined. After the end of the preamble PID, the host must wait at
least four full speed bit times during which hubs must complete the process of configuring their repeater
sections to accept low speed signaling. During this hub setup interval, hubs must drive their full speed
and low speed ports to their respective idle states. Hubs must be ready to accept low speed signaling
from the host before the end of the hub setup interval. Low speed connectivity rules are summarized
below:
1. Low speed devices are identified during the connection process and the hub ports to which they are
connected are identified as low speed.
2. All downstream low speed packets must be prefaced with a preamble (sent at full speed) which turns
on the output buffers on low speed hub ports.
3. Low speed hub port output buffers are turned off upon receipt of EOP and are not turned on again
until a preamble PID is detected.
4. Upstream connectivity is not affected by whether a hub port is full or low speed.
Low speed signaling begins with the host issuing SYNC at low speed, followed by the remainder of the
packet. The end of packet is identified by End of Packet (EOP), at which time all hubs tear down
connectivity and disable any ports to which low speed devices are connected. Hubs do not switch ports
for upstream signaling; low speed ports remain enabled in the upstream direction for both low speed and
full speed signaling.
Low speed and full speed transactions maintain a high degree of protocol commonality. However, low
speed signaling does have certain limitations which include:
• Data payload limited to eight bytes, maximum.
• Low speed only supports interrupt and control types of transfers.
• The SOF packet is not received by low speed devices.
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8.7 Error Detection and Recovery
USB permits reliable end to end communication in the presence of errors on the physical signaling layer.
This includes the ability to reliably detect the vast majority of possible errors and to recover from errors
on a transaction type basis. Control transactions, for example, require a high degree of data reliability;
they support end to end data integrity using error detection and retry. ISO transactions, by virtue of their
bandwidth and latency requirements, do not permit retries and must tolerate a higher incidence of
uncorrected errors.
8.7.1 Packet Error Categories
USB employs three error detection mechanisms: bit stuff violations, PID check bits, and CRCs. A bit
stuff violation exists if a packet receiver detects seven or more consecutive bit times without a
differential (J -> K or K -> J) transition, as detected on the physical D+ and D- lines, between the start
and end of a packet. A PID error exists if the four PID check bits are not complements of their respective
packet identifier bits. A CRC error exists if the computed checksum remainder at the end of a packet
reception is not zero.
With the exception of the SOF token, any packet that is received corrupted causes the receiver to ignore
it and discard any data or other field information that came with the packet. Table 8-6 lists error
detection mechanisms, the types of packets to which they apply, and the appropriate packet receiver
response.
Table 8-6. Packet Error Types
Field Error Action
PID PID Check, Bit Stuff Ignore packet
Address Bit Stuff, Address CRC Ignore token
Frame Number Bit Stuff, Frame Number CRC Ignore Frame Number field
Data Bit Stuff, Data CRC Discard data
8.7.2 Bus Turnaround Timing
The host and USB function need to keep track of how much time has elapsed from when the transmitter
completes sending a packet until it begins to receive a packet back. This time is referred to as the bus
turnaround time and is tracked by the packet transmitter’s bus turnaround timer. The timer starts
counting on the SE0 to IDLE transition of the EOP strobe and stops counting when the IDLE to K SOP
transition is detected. Both devices and the host require turnaround timers. The device bus turnaround
time is defined by the worst case round trip delay plus the maximum device response delay (refer to
Section 7.1.14). USB devices cannot time out earlier than 16 bit times after the end of the previous EOP
and they must time out by 18 bit times. If the host wishes to indicate an error condition via a timeout, it
must wait at least 18 bit times before issuing the next token to insure that all downstream devices have
timed out.
As shown in Figure 8-20, the device uses its bus turnaround timer between token and data or data and
handshake phases. The host uses its timer between data and handshake or token and data phases.
If the host receives a corrupted data packet, it must wait before sending out the next token. This wait
interval guarantees that the host does not attempt to issue a token immediately after a false EOP.
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OUT/SETUP
IN
Data
Data
Handshake
Handshake
device waits
device waitshost waits
host waits
Figure 8-20. Bus Turnaround Timer Usage
8.7.3 False EOPs
False EOPs must be handled in a manner which guarantees that the packet currently in progress
completes before the host or any other device attempts to transmit a new packet. If such an event were to
occur, it would constitute a bus collision and have the ability to corrupt up to two consecutive
transactions. Detection of false EOP relies upon the fact that a packet into which a false EOP has been
inserted will appear as a truncated packet with a CRC failure. (The last 16 bits of the packet will have a
very low probability of appearing to be a correct CRC.)
The host and devices handle false EOP situations differently. When a device sees a corrupted data
packet, it issues no response and waits for the host to send the next token. This scheme guarantees that
the device will not attempt to return a handshake while the host may still be transmitting a data packet. If
a false EOP has occurred, the host data packet will eventually end, and the device will be able to detect
the next token. If a device issues a data packet that gets corrupted with a false EOP, the host will ignore
the packet and not issue the handshake. The device, expecting to see a handshake from the host, will
time out.
If the host receives a corrupted data packet, it assumes that a false EOP may have occurred and waits for
16 bit times to see if there is any subsequent upstream traffic. If no bus transitions are detected within
the 16 bit interval and the bus remains in the IDLE state, the host may issue the next token. Otherwise,
the host waits for the device to finish sending the remainder of its packet. Waiting 16 bit times
guarantees two conditions. The first condition is to make sure that the device has finished sending its
packet. This is guaranteed by a time-out interval (with no bus transitions) greater than the worst case 6-
bit time bit stuff interval. The second condition is that the transmitting device’s bus turnaround timer
must be guaranteed to expire. Note that the time-out interval is transaction speed sensitive. For full
speed transactions, the host must wait 16 full speed bit times; for low speed transactions, it must wait 16
low speed bit times.
If the host receives a data packet with a valid CRC, it assumes that the packet is complete and need not
delay in issuing the next token.
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8.7.4 Babble and Loss of Activity Recovery
USB must be able to detect and recover from conditions which leave it waiting indefinitely for an end of
packet or which leave the bus in something other than the idle state at the end of a frame. Loss of
activity (LOA) is characterized by SOP followed by lack of bus activity (bus remains in J or K state) and
no EOP at the end of a frame. Babble is characterized by an SOP followed by the presence of bus
activity past the end of a frame. LOA and babble have the potential to either deadlock the bus or force
out the beginning of the next frame. Neither condition is acceptable, and both must be prevented from
occurring. As the USB component responsible for controlling connectivity, hubs are responsible for
babble/LOA detection and recovery. All USB devices that fail to complete their transmission at the end
of a frame are prevented from transmitting past a frame’s end by having the nearest hub disable the port
to which the offending device is attached. Details of the hub babble/LOA recovery mechanism appear in
Section 11.4.3.
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Chapter 9
USB Device Framework
A USB device may be divided into three layers. The bottom layer is a bus interface that transmits and
receives packets. The middle layer handles routing data between the bus interface and various endpoints
on the device. An endpoint is the ultimate consumer or provider of data. It may be thought of as a
source or sink for data. The top layer is the functionality provided by the serial bus device; for instance,
a mouse or ISDN interface.
This chapter describes the common attributes and operations of the middle layer of a USB device. These
attributes and operations are used by the function-specific portions of the device to communicate through
the bus interface and ultimately with the host.
9.1 USB Device States
A USB device has several possible states. Some of these states are visible to the USB and the host and
others are internal to the USB device. This section describes those states.
9.1.1 Visible Device States
This section describes USB device states that are externally visible (see Figure 9-1). Note: USB devices
perform a reset operation in response to a Reset request to the upstream port from the host. When reset
signaling has completed, the USB device is reset.
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Powered
Default
Hub
Configured
Address
Configured
Device
Configured
Device
Deconfigured
Suspended
Bus
Inactive
Bus Activity
Bus Activity
Attached
Hub Reset
or
Deconfigured
Reset
Suspended
Suspended
Bus
Inactive
Bus
Inactive
Bus Activity
Address
Assigned
Reset
Suspended
Bus Activity
Bus
Inactive
Power
Interruption
Figure 9-1. Device State Diagram
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Table 9-1. Visible Device States
Attached Powered Default Address Configured Suspended State
No -- -- -- -- -- Device is not attached to
USB. Other attributes
are not significant.
Yes No -- -- -- -- Device is attached to
USB, but is not powered.
Other attributes are not
significant.
Yes Yes No -- -- -- Device is attached to
USB and powered, but
has not been reset.
Yes Yes Yes No -- -- Device is attached to
USB and powered and
has been reset, but has
not been assigned a
unique address. Device
responds at the default
address.
Yes Yes Yes Yes No -- Device is attached to
USB, powered, has been
reset, and a unique
device address has been
assigned. Device is not
configured.
Yes Yes Yes Yes Yes No Device is attached to
USB, powered, has been
reset, has unique
address, is configured,
and is not suspended.
Host may now use the
function provided by the
device.
Yes Yes Yes Yes Yes Yes Device is, at minimum,
attached to USB, has
been reset, and is
powered at the minimum
suspend level. It may
also have a unique
address and be
configured for use.
However, since the
device is suspended, the
host may not use the
device’s function.
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9.1.1.1 Attached
A USB device may be attached or detached from the USB. The state of a USB device when it is
detached from the USB is not defined by this specification. This specification only addresses required
operations and attributes once the device is attached.
9.1.1.2 Powered
USB devices may obtain power from an external source and/or from USB through the hub to which they
are attached. Externally powered USB devices are termed self-powered. These devices may already be
powered before they are attached to the USB. A device may support both self-powered and bus-powered
configurations. Some device configurations support either power source. Other device configurations
may only be available if the device is externally powered. Devices report their power source capability
through the Configuration Descriptor. The current power source is reported as part of a device’s status.
Devices may change their power source at any time; e.g., from self- to USB-powered. If a configuration
is capable of supporting both power modes, the power maximum reported for that configuration is the
maximum the device will draw in either mode. The device must observe this maximum, regardless of its
mode. If a configuration supports only one power mode and the power source of the device changes, the
device will loose its current configuration and address and return to the attached state.
A hub port must be powered in order to detect port status changes, including attach and detach. Hubs do
not provide any downstream power until they are configured, at which point they will provide power as
allowed by their configuration and power source. A USB device must be able to be addressed within a
specified time period from when power is initially applied (refer to Chapter 7). After an attachment to a
port has been detected, the host shall enable the port, which will also reset the device attached to the port.
9.1.1.3 Default
After the device has been powered, it must not respond to any bus transactions until it has received a
reset from the bus. After receiving a reset, the device is then addressable at the default address.
9.1.1.4 Address Assigned
All USB devices use the default address when initially powered or after the device has been reset. Each
USB device is assigned a unique address by the host after attachment or after reset. A USB device
maintains its assigned address while suspended.
A USB device responds to requests on its default pipe whether the device is currently assigned a unique
address or is using the default address.
9.1.1.5 Configured
Before the USB device’s function may be used, the device must be configured. From the device’s
perspective, configuration involves writing a non-zero value to the device configuration register.
Configuring a device or changing an alternate setting causes all of the status and configuration values
associated with endpoints in the affected interfaces to be set to their default values. This includes setting
the data toggle of any endpoint using data toggles to the value DATA0.
9.1.1.6 Suspended
In order to conserve power, USB devices automatically enter the Suspended state when the device has
observed no bus traffic for a specified period (refer to Chapter 7). When suspended, the USB device
maintains any internal status including its address and configuration.
All devices must suspend if bus activity has not been observed for the length of time specified in
Chapter 7. Attached devices must be prepared to suspend at any time they are powered, whether they
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have been assigned a non-default address or are configured. Bus activity may cease due to the host
entering a suspend mode of its own. In addition, a USB device shall also enter the suspended state when
the hub port it is attached to is disabled. This is referred to as selective suspend.
A USB device exits suspend mode when there is bus activity. A USB device may also request the host
exit suspend mode or a selective suspend by using electrical signaling to indicate remote wakeup. The
ability of a device to signal remote wakeup is optional. If a USB device is capable of remote wakeup
signaling, the device must support the ability of the host to enable and disable this capability.
9.1.2 Bus Enumeration
When a USB device is attached to or removed from the USB, the host uses a process known as bus
enumeration to identify and manage the device state changes necessary. When a USB device is attached,
the following actions are undertaken:
1. The hub to which the USB device is now attached informs the host of the event via a reply on its
status change pipe (refer to Chapter 11 for more information). At this point, the USB device is in the
attached state and the port to which it is attached is disabled.
2. The host determines the exact nature of the change by querying the hub.
3. Now that the host knows the port to which the new device has been attached, the host issues a port
enable and reset command to that port.
4. The hub maintains the reset signal to that port for 10 ms. When the reset signal is released, the port
has been enabled and the hub provides 100 mA of bus power to the USB device. The USB device is
now in the powered state. All of its registers and state have been reset and it answers to the default
address.
5. Before the USB device receives a unique address, it’s default pipe is still accessible via the default
address. The host reads the device descriptor to determine what actual maximum data payload size
this USB device’s default pipe can use.
6. The host assigns a unique address to the USB device, moving the device to the addressed state.
7. The host reads the configuration information from the device by reading each configuration zero to
n. This process may take several frames to complete.
8. Based on the configuration information and how the USB device will be used, the host assigns a
configuration value to the device. The device is now in the configured state and all of the endpoints
in this configuration have taken on their described characteristics. The USB device may now draw
the amount of Vbus power described in its configuration descriptor. From the device’s point of view
it is now ready for use.
When the USB device is removed, the hub again sends a notification to the host. Detaching a device
disables the port to which it had been attached. Upon receiving the detach notification, the host will
update its local topological information.
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9.2 Generic USB Device Operations
All USB devices support a common set of operations. This section describes those operations.
9.2.1 Dynamic Attachment and Removal
USB devices may be attached and removed at any time. The hub that provides the attachment point or
port is responsible for reporting any change in the state of the port.
The host enables the hub port where the device is attached upon detection of an attachment, which also
has the effect of resetting the device. A reset USB device has the following characteristics:
• Responds to the default USB address
• Is unconfigured
• Is not initially suspended
When a device is removed from a hub port, the host is notified of the removal. The host responds by
disabling the hub port where the device was attached.
9.2.2 Address Assignment
When a USB device is attached, the host is responsible for assigning a unique address to the device after
the device has been reset by the host and the hub port where the device is attached has been enabled.
9.2.3 Configuration
A USB device must be configured before its function may be used. The host is responsible for
configuring a USB device. The host typically requests configuration information from the USB device to
determine the device’s capabilities.
As part of the configuration process, the host sets the device configuration and, where necessary, sets the
maximum packet size for endpoints that require such limitation.
Within a single configuration, a device may support multiple interfaces. An interface is a related set of
endpoints that present a single feature or function of the device to the host. The protocol used to
communicate with this related set of endpoints and the purpose of each endpoint within the interface may
be specified as part of a device class or vendor specific class definition.
In addition, an interface within a configuration may have alternate settings that redefine the number or
characteristics of the associated endpoints. If this is the case, the device shall support the Get Interface
and Set Interface requests to report or select a specific alternative setting for a specific interface.
Within each configuration, each interface descriptor contains fields that identify the interface number and
the alternate setting. Interfaces are numbered from zero to one less than the number of concurrent
interfaces supported by the configuration. Alternate settings range from zero to one less than the number
of alternate settings for a specific interface. The default setting when a device is initially configured is
alternate setting zero.
In support of adaptive device drivers that are capable of managing a related group of USB devices, the
device and interface descriptors contain Class, SubClass, and Protocol fields. These fields are used to
identify the function(s) provided by a USB device and the protocols used to communicate with the
function(s) on the device. A Class code is assigned to a related class of devices that has been defined as a
part of the USB specification. A class of devices is further subdivided into subclasses and within a class
or subclass a protocol code defines how host software communicates with the device.
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9.2.4 Data Transfer
Data may be transferred between a USB device endpoint and the host in one of four ways. Refer to
Chapter 5 for the definition of the four types of transfers. Some endpoints may be capable of different
types of data transfers. However, once configured, a USB device endpoint uses only one data transfer
method.
9.2.5 Power Management
Power management on USB devices involves the issues described in the following sections.
9.2.5.1 Power Budgeting
For bus-powered devices, power is a limited resource. When a host detects the attachment of a bus-
powered USB device, the host needs to evaluate the power requirements of the device. If USB device
power requirements exceed available power, the device is not configured.
No USB device may require more than 100 mA when first attached. A configured bus-powered USB
device attached to a self-powered hub may use up to 500 mA; however, some ports may not be able
supply this much power and thus the device will not be usable.
All USB devices must support a suspended mode that requires less than 500 µA. A USB device
automatically suspends when the bus is inactive, as previously described.
9.2.5.2 Remote Wakeup
Remote wakeup allows a suspended USB device to signal a host that may also be suspended. This
notifies the host that it should resume from its suspended mode, if necessary, and service the external
event that triggered the suspended USB device to signal the host. A USB device reports its ability to
support remote wakeup in a configuration descriptor. If a device supports remote wakeup, it must also
allow the capability to be enabled and disabled using the standard USB requests.
Remote wakeup is accomplished using electrical signaling described elsewhere in this document.
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9.3 USB Device Requests
All USB devices respond to requests from the host on the device’s default pipe. These requests are made
using control transfers. The request and the request’s parameters are sent to the device in the setup
packet. The host is responsible for establishing the values passed in the following fields. Every setup
packet has eight bytes, used as follows:
Offset Field Size Value Description
0
bmRequestType
1 Bit-map Characteristics of request
D7 Data xfer direction
0 = Host to device
1 = Device to host
D6..5 Type
0 = Standard
1 = Class
2 = Vendor
3 = Reserved
D4..0 Recipient
0 = Device
1 = Interface
2 = Endpoint
3 = Other
4..31 = Reserved
1
bRequest
1 Value Specific request (refer to Table 9-2)
2
wValue
2 Value Word-sized field that varies according to
request
4
wIndex
2 Index or
Offset Word sized field that varies according to
request - typically used to pass an index
or offset
6
wLength
2 Count Number of bytes to transfer if there is a
data phase
9.3.1 bmRequestType
This bit-mapped field identifies the characteristics of the specific request. In particular, this field
identifies the direction of data transfer in the second phase of the control transfer. The state of the
direction bit is ignored if the wLength field is zero, signifying there is no data phase.
The USB Specification defines a series of Standard requests that all devices must support. In addition, a
device class may define additional requests. A device vendor may also define requests supported by the
device.
Requests may be directed to the device, an interface on the device, or a specific endpoint on a device.
This field also specifies the intended recipient of the request. When an interface or endpoint is specified,
the wIndex field identifies the interface or endpoint.
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9.3.2 bRequest
This field specifies the particular request. The Type bits in the bmRequestType field modify the meaning
of this field. This specification only defines values for the bRequest field when the bits are reset to zero
indicating a standard request (refer to Table 9-2).
9.3.3 wValue
The contents of this field vary according to the request. It is used to pass a parameter to the device
specific to the request.
9.3.4 wIndex
The contents of this field vary according to the request. It is used to pass a parameter to the device
specific to the request.
9.3.5 wLength
This field specifies the length of the data transferred during the second phase of the control transfer. The
direction of data transfer (host to device or device to host) is indicated by the Direction bit of the
bRequestType field. If this field is zero, there is no data transfer phase.
9.4 Standard Device Requests
This section describes the standard device requests defined for all USB devices (refer to Table 9-2).
USB devices must respond to standard device requests whether the device has been assigned a non-
default address or the device is currently configured.
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Table 9-2. Standard Device Requests
bmRequestType bRequest wValue wIndex wLength Data
00000000B
00000001B
00000010B
CLEAR_FEATURE Feature
Selector Zero
Interface
Endpoint
Zero None
10000000B GET_CONFIGURATION Zero Zero One Configuration
Value
10000000B GET_DESCRIPTOR Descriptor
Type and
Descriptor
Index
Zero or
Language
ID
Descriptor
Length Descriptor
10000001B GET_INTERFACE Zero Interface One Alternate
Interface
10000000B
10000001B
10000010B
GET_STATUS Zero Zero
Interface
Endpoint
Two Device,
Interface, or
Endpoint
Status
00000000B SET_ADDRESS Device
Address Zero Zero None
00000000B SET_CONFIGURATION Configuration
Value Zero Zero None
00000000B SET_DESCRIPTOR Descriptor
Type and
Descriptor
Index
Zero or
Language
ID
Descriptor
Length Descriptor
00000000B
00000001B
00000010B
SET_FEATURE Feature
Selector Zero
Interface
Endpoint
Zero None
00000001B SET_INTERFACE Alternate
Setting Interface Zero None
10000010B SYNCH_FRAME Zero Endpoint Two Frame Number
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Table 9-3. Standard Request Codes
bRequest Value
GET_STATUS 0
CLEAR_FEATURE 1
Reserved for future use 2
SET_FEATURE 3
reserved for future use 4
SET_ADDRESS 5
GET_DESCRIPTOR 6
SET_DESCRIPTOR 7
GET_CONFIGURATION 8
SET_CONFIGURATION 9
GET_INTERFACE 10
SET_INTERFACE 11
SYNCH_FRAME 12
Table 9-4. Descriptor Types
Descriptor Types Value
DEVICE 1
CONFIGURATION 2
STRING 3
INTERFACE 4
ENDPOINT 5
Feature selectors are used when enabling or setting features, such as remote wakeup, specific to a device,
interface or endpoint. The values for the feature selectors are given below.
Feature Selector Recipient Value
DEVICE_REMOTE_WAKEUP Device 1
ENDPOINT_STALL Endpoint 0
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If an unsupported or invalid request is made to a USB device, the device responds by indicating a stall
condition on the pipe used for the request. Control pipes, including the default pipe, must accept a setup
transaction even if they are stalled. The ClearStall request is used to clear a stalled pipe. After the stall
condition is cleared by the host, system software continues normal accesses to the control pipe. If for any
reason, the device becomes unable to communicate via its default pipe due to an error condition, the
device must be reset to clear the condition and restart the default pipe.
9.4.1 Clear Feature
This request is used to clear or disable a specific feature
bmRequestType bRequest wValue wIndex wLength Data
00000000B
00000001B
00000010B
CLEAR_FEATURE Feature
Selector Zero
Interface
Endpoint
Zero None
Feature selector values in wValue must be appropriate to the recipient. Only device feature selector
values may be used when the recipient is a device, only interface feature selector values may be used
when the recipient is an interface, and only endpoint feature selector values may be used when the
recipient is an endpoint.
Refer to Section 9.4 for a definition of which feature selector values are defined for which recipients.
A ClearFeature request that references a feature that cannot be cleared or that does not exist will cause a
stall.
9.4.2 Get Configuration
This request returns the current device configuration.
bmRequestType bRequest wValue wIndex wLength Data
10000000B GET_CONFIGURATION Zero Zero One Configuration
Value
If the returned value is zero, the device is not configured.
9.4.3 Get Descriptor
This request returns the specified descriptor if the descriptor exists.
bmRequestType bRequest wValue wIndex wLength Data
10000000B GET_DESCRIPTOR Descriptor
Type and
Descriptor
Index
Zero or
Language ID
(refer to
Section 9.6.5)
Descriptor
Length Descriptor
The wValue field specifies the descriptor type in the high byte and the descriptor index in the low byte
(refer to Table 9-4). The wIndex field specifies the Language ID for string descriptors or is reset to zero
for other descriptors. The wLength field specifies the number of bytes to return. If the descriptor is
longer than the wLength field, only the initial bytes of the descriptor are returned. If the descriptor is
shorter than the wLength field, the device indicates the end of the control transfer by sending a short
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packet when further data is requested. A short packet is defined as a packet shorter than the maximum
payload size or a NULL data packet (refer to Chapter 5).
The standard request to a device supports three types of descriptors: DEVICE, CONFIGURATION, a n d
STRING. A request for a configuration descriptor returns the configuration descriptor, all interface
descriptors, and endpoint descriptors for all of the interfaces in a single request. The first interface
descriptor immediately follows the configuration descriptor. The endpoint descriptors for the first
interface follow the first interface descriptor. If there are additional interfaces, their interface descriptor
and endpoint descriptors follow the first interface’s endpoint descriptors.
All devices must provide a device descriptor and at least one configuration descriptor. If a device does
not support a requested descriptor, it responds by stalling the pipe used for the request. A non-zero value
as the first byte of a descriptor indicates the buffer contains a valid descriptor.
9.4.4 Get Interface
This request returns the selected alternate setting for the specified interface.
bmRequestType bRequest wValue wIndex wLength Data
10000001B GET_INTERFACE Zero Interface One Alternate
Setting
Some USB devices have configurations with interfaces that have mutually exclusive settings. This
request allows the host to determine the currently selected alternative setting.
9.4.5 Get Status
This request returns status for the specified recipient.
bmRequestType bRequest wValue wIndex wLength Data
10000000B
10000001B
10000010B
GET_STATUS Zero Zero
Interface
Endpoint
Two Device,
Interface, or
Endpoint
Status
The Recipient bits of the bRequestType field specify the desired recipient. The data returned is the
current status of the specified recipient.
A GetStatus request to a device returns the following information in little-endian order:
D7
D6 D5 D4 D3 D2 D1 D0
Reserved (Reset to zero) Remote
Wakeup Self
Powered
D15
D14 D13 D12 D11 D10 D9 D8
Reserved (Reset to zero)
The Self Powered field indicates whether the device is currently bus-powered or self-powered. If D0 is
reset to zero, the device is bus-powered. If D0 is set to one, the device is self-powered. The Self
Powered field may not be changed by the SetFeature or ClearFeature requests.
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The Remote Wakeup field indicates whether the device is currently enabled to request remote wakeup.
The default mode for devices that support remote wakeup is disabled. If D1 is reset to zero, the ability of
the device to signal remote wakeup is disabled. If D1 is set to one, the ability of the device to signal
remote wakeup is enabled. The Remote Wakeup field can be modified by the SetFeature and
ClearFeature requests using the DEVICE_REMOTE_WAKEUP feature selector. This field is reset to
zero when the device is reset.
A GetStatus request to an interface returns the following information in little-endian order:
D7
D6 D5 D4 D3 D2 D1 D0
Reserved (Reset to zero)
D15
D14 D13 D12 D11 D10 D9 D8
Reserved (Reset to zero)
If the request is made to an endpoint, then the endpoint must be specified in the wIndex field. The upper
byte of xIndex is reset to zero and the lower byte contains the endpoint number as follows:
D7 D6 D5 D4 D3 D2
D1
D0
Direction Reserved (Reset to zero) Endpoint Number
For IN endpoints, D7 is set to one. For OUT endpoints, D7 is reset to zero.
A GetStatus request to an endpoint returns the following information:
D7
D6 D5 D4 D3 D2 D1 D0
Reserved (Reset to zero) Stall
D15
D14 D13 D12 D11 D10 D9 D8
Reserved (Reset to zero)
If the endpoint is currently stalled, the Stall field is set to one. Otherwise the Stall field is reset to zero.
The Stall field may be changed with the SetFeature and ClearFeature requests with the
ENDPOINT_STALL feature selector. When set by the SetFeature request, the endpoint exhibits the
same stall behavior as if the field had been set by a hardware condition. If the condition causing a stall
has been removed, clearing the stall field results in the endpoint no longer returning a stall status. For
this endpoints using a data toggle, clearing a stalled endpoint results in the data toggle being reinitialized
to DATA0. This field is reset to zero after either the SetConfiguration or SetInterface request.
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9.4.6 Set Address
This request sets the device address for all future device accesses.
bmRequestType bRequest wValue wIndex wLength Data
00000000B SET_ADDRESS Device
Address Zero Zero None
The wValue field specifies the device address to use for all subsequent accesses.
As noted elsewhere, requests actually may result in up to three stages. In the first stage, the setup packet
is sent to the device. In the optional second stage, data is transferred between the host and the device. In
the final stage, status is transferred between the host and the device. The direction of data and status
transfer depends on whether the host is sending data to the device or the device is sending data to the
host. The status stage transfer is always in the opposite direction of the data stage. If there is no data
stage, the status stage is from the device to the host.
Stages after the initial setup packet assume the same device address as the setup packet. The USB device
does not change its device address until after the status stage of this request is completed successfully.
Note that this is a difference between this request and all other requests. For all other requests, the
operation indicated must be completed before the status stage.
9.4.7 Set Configuration
This request sets the device configuration.
bmRequestType bRequest wValue wIndex wLength Data
00000000B SET_CONFIGURATION Configuration
Value Zero Zero None
The wValue field specifies the desired configuration. This value must be zero or match a configuration
value from a configuration descriptor. If the value is zero, the device is placed in its unconfigured state.
9.4.8 Set Descriptor
This request is optional. If a device supports this request, existing descriptors may be updated or new
descriptors may be added.
bmRequestType bRequest wValue wIndex wLength Data
00000000B SET_DESCRIPTOR Descriptor
Type and
Descriptor
Index
Language ID
(refer to
Section 9.6.5)
Descriptor
Length Descriptor
The wValue field specifies the descriptor type in the high byte and the descriptor index in the low byte
(refer to Table 9-4). The wIndex field specifies the Language ID for string descriptors or is reset to zero
for other descriptors. The wLength field specifies the number of bytes to transfer from the host to the
device.
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9.4.9 Set Feature
This request is used to set or enable a specific feature.
bmRequestType bRequest wValue wIndex wLength Data
00000000B
00000001B
00000010B
SET_FEATURE Feature
Selector Zero
Interface
Endpoint
Zero None
Feature selector values in wValue must be appropriate to the recipient. Only device feature selector
values may be used when the recipient is a device; only interface feature selector values may be used
when the recipient is an interface, and only endpoint feature selector values may be used when the
recipient is an endpoint.
Refer to Section 9.4 for a definition of which feature selector values are defined for which recipients. A
SetFeature request that references a feature that cannot be set or that does not exist causes a stall.
9.4.10 Set Interface
This request allows the host to select an alternate setting for the specified interface.
bmRequestType bRequest wValue wIndex wLength Data
00000001B SET_INTERFACE Alternative
Setting Interface Zero None
Some USB devices have configurations with interfaces that have mutually exclusive settings. This
request allows the host to select the desired alternate setting.
9.4.11 Synch Frame
This request is used to set and then report an endpoint’s synchronization frame.
bmRequestType bRequest wValue wIndex wLength Data
00000010B SYNCH_FRAME Zero Endpoint Two Frame
Number
When an endpoint supports isochronous transfers, the endpoint may also require per frame transfers to
vary in size according to a specific pattern. The host and the endpoint must agree on which frame the
repeating pattern begins. This request causes the endpoint to begin monitoring the SOF frame number to
track the position of a given frame in its pattern. The number of the frame in which the pattern began is
returned to the host. This frame number is the one conveyed to the endpoint by the last SOF prior to the
first frame of the pattern.
This value is only used for isochronous data transfers using implicit pattern synchronization. If the
endpoint is not isochronous or is not using this method, this request is not supported by the endpoint and
returns a stall.
The starting frame is reset to zero by a device reset or by configuring the endpoint either via a
SetConfiguration or a SetInterface request.
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9.5 Descriptors
USB devices report their attributes using descriptors. A descriptor is a data structure with a defined
format. Each descriptor begins with a byte-wide field that contains the total number of bytes in the
descriptor followed by a byte-wide field that identifies the descriptor type.
Using descriptors allows concise storage of the attributes of individual configurations because each
configuration may reuse descriptors or portions of descriptors from other configurations that have the
same characteristics. In this manner, the descriptors resemble individual data records in a relational
database.
Where appropriate, descriptors contain references to string descriptors that provide displayable
information describing a descriptor in human-readable form. The inclusion of string descriptors is
optional. However, the reference fields within descriptors are mandatory. If a device does not support
string descriptors, string reference fields must be reset to zero to indicate no string descriptor is available.
If a descriptor returns with a value in its length field that is less than defined by this specification, the
descriptor is invalid and should be rejected by the host. If the descriptor returns with a value in its length
field that is greater than defined by this specification, the extra bytes are ignored by the host, but the next
descriptor is located using the length returned rather than the length expected.
Class and vendor specific descriptors may be returned in one of two ways. Class and vendor specific
descriptors that are related to standard descriptors are returned in the same data buffer as the standard
descriptor immediately following the related standard descriptor.
If, for example, a class or vendor specific descriptor is related to an interface descriptor, the related class
or vendor specific descriptor is placed between the interface descriptor and the interface’s endpoint
descriptors in the buffer returned in response to a GET_CONFIGURATION_DESCRIPTOR request. The
length of a standard descriptor is not increased to accommodate device class or vendor specific
extensions. Class or vendor specific descriptors follow the same format as standard descriptors with the
length and type fields as the first two bytes of the specific descriptor.
Class or vendor specific descriptors that are not related to a standard descriptor are returned using class or
vendor specific requests.
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9.6 Standard USB Descriptor Definitions
9.6.1 Device
A device descriptor describes general information about a USB device. It includes information that
applies globally to the device and all of the device’s configurations. A USB device has only one device
descriptor.
All USB devices have an endpoint zero used by the default pipe. The maximum packet size of a device’s
endpoint zero is described in the device descriptor. Endpoints specific to a configuration and its
interface(s) are described in the configuration descriptor. A configuration and its interface(s) do not
include an endpoint descriptor for endpoint zero. Other than the maximum packet size, the
characteristics of endpoint zero are defined by this specification and are the same for all USB devices.
The bNumConfigurations field identifies the number of configurations the device supports.
Offset Field Size Value Description
0
bLength
1 Number Size of this descriptor in bytes
1
bDescriptorType
1 Constant DEVICE Descriptor Type
2
bcdUSB
2 BCD USB Specification Release Number in
Binary-Coded Decimal (i.e., 2.10 is
0x210). This field identifies the release of
the USB Specification that the device and
its descriptors are compliant with.
4
bDeviceClass
1 Class Class code (assigned by USB).
If this field is reset to 0, each interface
within a configuration specifies its own
class information and the various
interfaces operate independently.
If this field is set to a value between 1
and 0xFE, the device supports different
class specifications on different interfaces
and the interfaces may not operate
independently. This value identifies the
class definition used for the aggregate
interfaces. (For example, a CD-ROM
device with audio and digital data
interfaces that require transport control to
eject CDs or start them spinning.)
If this field is set to 0xFF, the device class
is vendor specific.
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Offset Field Size Value Description
5
bDeviceSubClass
1 SubClass Subclass code (assigned by USB).
These codes are qualified by the value of
the
bDeviceClass
field.
If the
bDeviceClass
field is reset to 0, this
field must also be reset to 0.
If the
bDeviceClass
field is not set to
0xFF, all values are reserved for
assignment by USB.
6
bDeviceProtocol
1 Protocol Protocol code (assigned by USB).
These codes are qualified by the value of
the
bDeviceClass
and the
bDeviceSubClass
fields. If a device
supports class-specific protocols on a
device basis as opposed to an interface
basis, this code identifies the protocols
that the device uses as defined by the
specification of the device class.
If this field is reset to 0, the device does
not use class specific protocols on a
device basis. However, it may use class
specific protocols on an interface basis.
If this field is set to 0xFF, the device uses
a vendor specific protocol on a device
basis.
7
bMaxPacketSize0
1 Number Maximum packet size for endpoint zero
(only 8, 16, 32, or 64 are valid)
8
idVendor
2 ID Vendor ID (assigned by USB)
10
idProduct
2 ID Product ID (assigned by the
manufacturer)
12
bcdDevice
2 BCD Device release number in binary-coded
decimal
14
iManufacturer
1 Index Index of string descriptor describing
manufacturer
15
iProduct
1 Index Index of string descriptor describing
product
16
iSerialNumber
1 Index Index of string descriptor describing the
device’s serial number
17
bNumConfigurations
1 Number Number of possible configurations
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9.6.2 Configuration
The configuration descriptor describes information about a specific device configuration. The descriptor
contains a bConfigurationValue field with a value that, when used as a parameter to the Set
Configuration request, causes the device to assume the described configuration.
The descriptor describes the number of interfaces provided by the configuration. Each interface may
operate independently. For example, an ISDN device might be configured with two interfaces, each
providing 64 kBs bi-directional channels that have separate data sources or sinks on the host. Another
configuration might present the ISDN device as a single interface, bonding the two channels into one
128 kBs bi-directional channel.
When the host requests the configuration descriptor, all related interface and endpoint descriptors are
returned (refer to Section 9.4.2).
A USB device has one or more configuration descriptors. Each configuration has one or more interfaces
and each interface has one or more endpoints. An endpoint is not shared among interfaces within a single
configuration unless the endpoint is used by alternate settings of the same interface. Endpoints may be
shared among interfaces that are part of different configurations without this restriction.
Once configured, devices may support limited adjustments to the configuration. If a particular interface
has alternate settings, an alternate may be selected after configuration. Within an interface, an
isochronous endpoint’s maximum packet size may also be adjusted.
Offset Field Size Value Description
0
bLength
1 Number Size of this descriptor in bytes
1
bDescriptorType
1 Constant CONFIGURATION
2
wTotalLength
2 Number Total length of data returned for this
configuration. Includes the combined length
of all descriptors (configuration, interface,
endpoint, and class or vendor specific)
returned for this configuration.
4
bNumInterfaces
1 Number Number of interfaces supported by this
configuration
5
bConfigurationValue
1 Number Value to use as an argument to Set
Configuration to select this configuration
6
iConfiguration
1 Index Index of string descriptor describing this
configuration
7
bmAttributes
1 Bitmap Configuration characteristics
D7 Bus Powered
D6 Self Powered
D5 Remote Wakeup
D4..0 Reserved (reset to 0)
A device configuration that uses power from
the bus and a local source sets both D7 and
D6. The actual power source at runtime may
be determined using the Get Status device
request.
If a device configuration supports remote
wakeup, D5 is set to 1.
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Offset Field Size Value Description
8
MaxPower
1 mA Maximum power consumption of USB device
from the bus in this specific configuration
when the device is fully operational.
Expressed in 2 mA units (i.e., 50 = 100 mA).
Note: A device configuration reports whether
the configuration is bus-powered or self-
powered. Device status reports whether the
device is currently self-powered. If a device is
disconnected from its external power source,
it updates device status to indicate that it is no
longer self-powered.
A device may not increase its power draw
from the bus, when it loses its external power
source, beyond the amount reported by its
configuration.
If a device can continue to operate when
disconnected from its external power source,
it continues to do so. If the device cannot
continue to operate, it fails operations it can
no longer support. Host software may
determine the cause of the failure by checking
the status and noting the loss of the device’s
power source.
9.6.3 Interface
This descriptor describes a specific interface provided by the associated configuration. A configuration
provides one or more interfaces, each with its own endpoint descriptors describing a unique set of
endpoints within the configuration. When a configuration supports more than one interface, the
endpoints for a particular interface immediately follow the interface descriptor in the data returned by the
Get Configuration request. An interface descriptor is always returned as part of a configuration
descriptor. It cannot be directly accessed with a Get or Set Descriptor request.
An interface may include alternate settings that allow the endpoints and/or their characteristics to be
varied after the device has been configured. The default setting for an interface is always alternate
setting zero. The Set Interface request is used to select an alternate setting or to return to the default
setting. The Get Interface request returns the selected alternate setting.
Alternate settings allow a portion of the device configuration to be varied while other interfaces remain
in operation. If a configuration has alternate settings for one or more of its interfaces, a separate interface
descriptor and its associated endpoints are included for each setting.
If a device configuration supported a single interface with two alternate settings, the configuration
descriptor would be followed by an interface descriptor with the bInterfaceNumber and bAlternateSetting
fields set to zero and then the endpoint descriptors for that setting, followed by another interface
descriptor and its associated endpoint descriptors. The second interface descriptor’s bInterfaceNumber
field would also be set to zero, but the bAlternateSetting field of the second interface descriptor would be
set to one.
If an interface only uses endpoint zero, no endpoint descriptors follow the interface descriptor and the
interface identifies a request interface that uses the default pipe attached to endpoint zero. In this case,
the bNumEndpoints field shall be set to zero.
An interface descriptor never includes endpoint zero in the number of endpoints.
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Offset Field Size Value Description
0
bLength
1 Number Size of this descriptor in bytes
1
bDescriptorType
1 Constant INTERFACE Descriptor Type
2
bInterfaceNumber
1 Number Number of interface. Zero-based value
identifying the index in the array of
concurrent interfaces supported by this
configuration.
3
bAlternateSetting
1 Number Value used to select alternate setting for
the interface identified in the prior field
4
bNumEndpoints
1 Number Number of endpoints used by this
interface (excluding endpoint zero). If
this value is 0, this interface only uses
endpoint zero.
5
bInterfaceClass
1 Class Class code (assigned by USB)
If this field is reset to 0, the interface
does not belong to any USB specified
device class.
If this field is set to 0xFF, the interface
class is vendor specific.
All other values are reserved for
assignment by USB.
6
bInterfaceSubClass
1 SubClass Subclass code (assigned by USB).
These codes are qualified by the value of
the
bInterfaceClass
field.
If the
bInterfaceClass
field is reset to 0,
this field must also be reset to 0.
If the
bInterfaceClass
field is not set to
0xFF, all values are reserved for
assignment by USB.
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Offset Field Size Value Description
7
bInterfaceProtocol
1 Protocol Protocol code (assigned by USB). These
codes are qualified by the value of the
bInterfaceClass
and the
bInterfaceSubClass
fields. If an interface
supports class-specific requests, this
code identifies the protocols that the
device uses as defined by the
specification of the device class.
If this field is reset to 0, the device does
not use a class specific protocol on this
interface.
If this field is set to 0xFF, the device uses
a vendor specific protocol for this
interface.
8
iInterface
1 Index Index of string descriptor describing this
interface
9.6.4 Endpoint
Each endpoint used for an interface has its own descriptor. This descriptor contains the information
required by the host to determine the bandwidth requirements of each endpoint. An endpoint descriptor
is always returned as part of a configuration descriptor. It cannot be directly accessed with a Get or Set
Descriptor request. There is never an endpoint descriptor for endpoint zero.
Offset Field Size Value Description
0
bLength
1 Number Size of this descriptor in bytes
1
bDescriptorType
1 Constant ENDPOINT Descriptor Type
2
bEndpointAddress
1 Endpoint The address of the endpoint on the USB
device described by this descriptor. The
address is encoded as follows:
Bit 0..3: The endpoint number
Bit 4..6: Reserved, reset to 0
Bit 7: Direction, ignored for
control endpoints
0 OUT endpoint
1 IN endpoint
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Offset Field Size Value Description
3
bmAttributes
1 Bit Map This field describes the endpoint’s
attributes when it is configured using the
bConfigurationValue
.
Bit 0 .. 1: Transfer Type
00 Control
01 Isochronous
10 Bulk
11 Interrupt
All other bits are reserved
4
wMaxPacketSize
2 Number Maximum packet size this endpoint is
capable of sending or receiving when this
configuration is selected.
For isochronous endpoints, this value is
used to reserve the bus time in the
schedule, required for the per frame data
payloads. The pipe may, on an ongoing
basis, actually use less bandwidth than
that reserved. The device reports, if
necessary, the actual bandwidth used via
its normal, non-USB defined
mechanisms.
For interrupt, bulk, and control endpoints
smaller data payloads may be sent, but
will terminate the transfer and may or
may not require intervention to restart.
Refer to Chapter 5 for more information.
6
bInterval
1 Number Interval for polling endpoint for data
transfers. Expressed in milliseconds.
This field is ignored for bulk and control
endpoints. For isochronous endpoints
this field must be set to 1. For interrupt
endpoints, this field may range from 1 to
255.
9.6.5 String
String descriptors are optional. As noted previously, if a device does not support string descriptors, all
references to string descriptors within device, configuration, and interface descriptors must be reset to
zero.
String descriptors use UNICODE encodings as defined by The Unicode Standard, Worldwide Character
Encoding, Version 1.0, Volumes 1 and 2, The Unicode Consortium, Addison-Wesley Publishing
Company, Reading, Massachusetts. The strings in a USB device may support multiple languages. When
requesting a string descriptor, the requester specifies the desired language using a sixteen-bit language ID
(LANGID) defined by Microsoft for Windows as described in Developing International Software for
Windows 95 and Windows NT, Nadine Kano, Microsoft Press, Redmond, Washington. String index 0 for
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all languages returns an array of two-byte LANGID codes supported by the device. A USB device may
omit all string descriptors.
The UNICODE string descriptor is not NULL terminated. The string length is computed by subtracting
two from the value of the first byte of the descriptor.
Offset Field Size Value Description
0
bLength
1 Number Size of this descriptor in bytes
1
bDescriptorType
1 Constant STRING Descriptor Type
2
bString
N Number UNICODE encoded string
9.7 Device Class Definitions
All devices must support the above registers and descriptor definitions. Most devices provide additional
registers and possibly, descriptors for device-specific extensions. In addition, devices may provide
extended services which are common to a group of devices. In order to define a class of devices, the
following information must be provided to completely define the appearance and behavior of the device
class.
9.7.1 Descriptors
If the class requires any specific definition of the standard descriptors, the class definition must include
those requirements as part of the class definition. In addition, if the class defines a standard extended set
of descriptors, they must also be fully defined in the class definition. Any extended descriptor definitions
should follow the approach used for standard descriptors; for example, all descriptors should begin with a
length field.
9.7.2 Interface(s) and Endpoint Usage
When a class of devices is standardized, the interfaces used by the devices including how endpoints are
used must be included in the device class definition. Devices may further extend a class definition with
proprietary features as long as they meet the base definition of the class.
9.7.3 Requests
All of the requests specific to the class must be defined.
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9.8 Device Communications
The USB communications model characterizes data and control traffic between the host and a given
device across the USB interconnect. The host and the device are divided into the distinct layers shown in
Figure 9-2.
Client
USB Bus
Interface
USB Device
Function
Host DeviceInterconnect
USB Bus
Interface
USB System
Actual communicati ons flow
Logical communications flow
Figure 9-2. Interlayer Communications Model
The actual communication on the host, as indicated by vertical arrows, takes place via SPIs. The
interlayer relationships on the device are implementation-specific. Between the host and device, all
communications must ultimately occur on the physical USB wire. However, there are logical host-device
interfaces between horizontal layers. Between client software, resident on the host, and the function
provided by the device, communications are typified by a contract based on the needs of the application
currently using the device and the capabilities provided by the device. This client-function interaction
creates the requirements for all of the underlying layers and their interfaces.
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This section describes the communications model from the point of view of the device and its layers.
Figure 9-3 illustrates, based on the overall view introduced in Chapter 8, the device’s view of its
communication with the host.
SIE
USB Bus
Interface
USB Device
a collection of
endpoints
Device
USB Wire
Data Per
Endpoint
Interface
Specific
Function
a collection of
interfaces
Default Pipe
to Endpoint Zero
Pi pe Bun dl e
to an interface
Pipe, represents connection abstraction between two horizontal layers
Data transport mechanism
USB-relevant format of tran sported data
USB
Framed
Data
No USB
Format
Interface x
Endpoint
Zero
Figure 9-3. Device Communications
The USB bus interface handles interactions among the electrical and protocol layers (refer to Chapters 7
and 8). The USB device layer presents a uniform abstraction of the USB device to the host. It is this
layer that is primarily described here. The function layer uses the capabilities provided by the USB
device layer, combined into a given interface, to support the requirements of a host-based application.
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A USB device acts as a collection of endpoints, each capable of supporting different types of pipes. Each
pipe can support one of the following transfer types at a time:
• Control
• Isochronous
• Interrupt
• Bulk
These transfer types are described in more detail in Chapter 5. Each of the transfer types, however,
requires the associated endpoint to behave in a certain fashion. A given endpoint may support a variety
of transfer types. However, once a pipe is associated with an endpoint, the endpoint only uses a single
transfer type. In this section, when discussing the behavior of an endpoint for a given transfer type, it is
assumed that the endpoint has been associated with a pipe supporting that specific transfer type. The
basic communication mechanisms used by endpoints are:
• Pipe mode
• Start of Frame (SOF) synchronization
• Handshakes
• Data toggles
The mode of a pipe indicates whether the data flow across the pipe is stream or message mode. Devices
may use the SOF as generated by the host to synchronize their internal clocks. Devices may use
handshakes and data toggles to implement error and flow control.
Traffic between a client and a function may require a certain transport rate. The client, USB and the
function all will be using, at best, slightly different clock rates. To ensure that all of the required data can
be delivered with minimum buffering required, the various clocks must be synchronized. Refer to
Chapter 5 for a discussion of the synchronization options. Additionally, in order to support the just-in-
time delivery capability implied by clock synchronization, the size of the data packets transmitted
between the host and the device will be normalized such that variations in size over time are minimized.
To support data flows in which the loss of data is acceptable as long as the loss can be accurately
communicated, sample headers may be used by the host and the device to communicate the expected
transmission volumes. Refer to Chapter 5 for the definition of sample headers.
These basic communication mechanisms are described, from the device’s point of view, in greater detail
below. Each of the different transfer types uses these basic communication mechanisms in different
ways.
9.8.1 Basic Communication Mechanisms
This section describes in more detail the basic communication mechanisms as supported by the USB
Device layer.
9.8.1.1 Pipe Mode
A pipe supports either stream or message mode transfers. In stream mode, the data flow is considered to
be a unidirectional serial stream of samples. In message mode, data is delivered as a related set of bytes.
Message mode pipes are always considered to be capable of being bi-directional.
A stream mode pipe is always unidirectional. When in the stream mode, the endpoint expects to receive
a token either requesting the endpoint to send data or alerting it that data will be sent to it. The amount
of data sent will always be equal to or less than the current MaxPacketSize for the endpoint.
Message transfers begin with a command from the host to the device. The device may respond to the
command with data, the host may follow the command with data for the device, or the command may
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require no data to be transmitted in which case a NULL data packet will be sent. In message mode, an
endpoint must keep track of where it is in the phase sequence defined by the mode. An endpoint expects
the first transaction of a message sequence to be a setup for the subsequent communication. After setup
is received, an endpoint usually expects to receive a token requesting the endpoint to send data (IN token)
or alerting it that data will be sent to the endpoint (OUT Token). The endpoint will know what the
direction of the subsequent transactions will be, based on the setup command that started the series of
transactions. Some setup commands do not require subsequent transactions to or from an endpoint.
Setup transactions are always eight bytes or less. The subsequent transactions will always be of a size
equal to or less than the current MaxPacketSize for the endpoint.
9.8.1.2 Synchronization
The host provides a special SOF token to the bus at regularly timed intervals. The interval between
SOF’s, within error tolerances (refer to Chapter 7), is 1 ms. Endpoints may use the receipt of this token
to synchronize their associated clock to the USB clock. This enables endpoints to match their rate of data
consumption or production to the host’s rate.
Not all endpoints require SOF synchronization. Some endpoints requiring synchronization have clocks
which cannot be synchronized to the 1 ms bus clock provided by USB. Such devices have two choices.
They can attempt to have the entire USB synchronize to them or such devices may periodically adjust
their transfer rate as they compensate for the difference between the USB clock and their own clock.
USB provides for a maximum of one client per USB instance to adjust the host’s SOF generation. This
client performs the adjustment based on feedback provided by an associated device. The rate of SOF
token generation remains 1 ms, however. Refer to Chapter 10 for a complete discussion of this
adjustment mechanism. If an endpoint has not been configured to adjust the USB clock using the SOF
handshake, or if the endpoint is not capable of so adjusting the clock, then the endpoint must continually
adjust its data flow.
Therefore, as noted above, there are three possible types of synchronization interaction for an endpoint
with regard to SOF. The endpoint may:
1. Synchronize its clock exactly to the existing USB clock.
2. Adjust the bus clock.
3. Synchronize with the host by adjusting its data flow.
It is important to note that an endpoint requiring synchronization, that cannot implement the type of
synchronization described in (1) but can implement the type described in (2), must also implement the
type described in (3). This is because such an endpoint cannot be guaranteed that it will be chosen as the
endpoint to adjust the bus clock. Only one device on the entire USB will be used to adjust the SOF.
9.8.1.3 Handshakes
Endpoints use the handshake phase of USB transactions to communicate error and data flow needs to the
host. Endpoints may also receive handshakes from the host to communicate error conditions. The types
of handshakes used by the endpoint vary according to the transfer type supported. These handshakes are
described in detail in Chapter 8.
9.8.1.4 Data Toggles
When an error or flow control situation occurs, some pipes are allowed to skip the frame in which the
condition occurred and transfer the data scheduled for that frame during a subsequent frame. In some
cases, it is possible that the receiver of the data had indicated to the transmitter that the data had been
successfully received, but that the transmitter believes, due to a bus error, that the data was not received
successfully. The transmitter will then retransmit the same data. The receiver needs some mechanism to
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understand that the data it is now receiving is a retransmission of data it has already received and not new
data.
USB provides this information by using data toggles which are the PIDs for the data phase of
transactions. Depending on the transfer type, the endpoint needs to understand data toggles and generate
or process data PIDs accordingly. Refer to Chapter 8 for a more complete discussion of data toggles.
Table 9-5 summarizes the USB communication mechanisms.
Table 9-5. USB Communications Mechanisms
Control Isochronous Interrupt Bulk
Pipe Mode Message Stream Stream Stream
Synchronization None Bus, external, or
software None None
Handshakes Yes Not used Yes Yes
Data Toggles Yes Ignored Yes Yes
Required Buffering Minimum of eight
bytes Twice frame
traffic Single transaction Single transaction
Error and Status
Handling Guaranteed
delivery reports
fatal errors only
Reports missing
or corrupt data -
no retries
Guaranteed
delivery reports
fatal errors only
Guaranteed delivery
reports fatal errors
only
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Chapter 10
USB Host: Hardware and Software
The USB interconnect supports data traffic between a host and a USB device. This chapter describes the
host interfaces necessary to facilitate USB communication between a software client, resident on the
host, and a function implemented on a device.
10.1 Overview of the USB Host
The basic flow and interrelationships of the USB communications model are shown in Figure 10-1.
Client
USB Bus
Interface
USB Device
Function
Host DeviceInterconnect
USB Bus
Interface
USB System
Actual communicati ons flow
Logical communications flow
Figure 10-1. Interlayer Communications Model
The host and the device are divided into the distinct layers depicted in Figure 10-1. The actual
communication on the host is indicated by vertical arrows. The corresponding interfaces on the device
are implementation-specific. All communications between the host and device ultimately occur on the
physical USB wire. However, there are logical host-device interfaces between each horizontal layer.
These communications, between client software resident on the host and the function provided by the
device, are typified by a contract based on the needs of the application currently using the device and the
capabilities provided by the device.
This client-function interaction creates the requirements for all of the underlying layers and their
interfaces.
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This chapter describes this model from the point-of-view of the host and its layers. Figure 10-2
describes, based on the overall view introduced in Chapter 5, the host’s view of its communication with
the device.
SIE
Host
Controller
USB Bus
Interface
USB System
ma nages pi pes
InterconnectHost
USB Wire
IRPs
Default Pipe
to Endpoint Zero
Pi pe Bun dl e
to an interface
Pipe, represents connection abstraction
between two horiz ontal layers
Interprocess Communication
HC
Defined
Client
manages interfaces
USB Driver Host
Software
HC Driver
Configuration
HW Defined
Optional
Component
Figure 10-2. Host Communications
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There is only one host for each USB. The major layers of a host are:
• USB bus interface
• USB system
• Client
The USB bus interface handles interactions for the electrical and protocol layers (refer to Chapter 7 and
Chapter 8). From the interconnect point-of-view, a similar USB bus interface is provided by both the
USB device and the host, as exemplified by the Serial Interface Engine (SIE). On the host, however, the
USB bus interface has additional responsibilities due to the unique role of the host on the USB and is
implemented as the host controller. The host controller also has an integrated, or root, hub providing
attachment points to the host.
The USB system uses the host controller to manage data transfers between the host and the USB device.
The interface between the USB system and the host controller is dependent on the hardware definition of
the host controller. The USB system, in concert with the host controller, performs the translation
between the client’s view of data transfers and the USB transactions appearing on the interconnect. This
includes the addition of any USB feature support such as protocol wrappers. The USB system is also
responsible for managing USB resources; e.g., bandwidth, such that client access to USB is possible.
The USB system has three basic components:
• Host Controller Driver
• USB Driver
• Host Software
The Host Controller Driver (HCD) exists to more easily map the various possible host controller
implementations into the USB system, such that a client can interact with its device without knowing to
which host controller the device is connected. The USB Driver (USBD) provides the basic host interface
(USBDI) for clients to USB devices. The interface between the HCD and the USBD is known as the
Host Controller Driver Interface (HCDI). This interface is never available directly to clients and thus is
not defined by the USB specification. A particular HCDI is, however, defined by each operating system
that supports various host controller implementations.
The USBD provides data transfer mechanisms in the form of I/O Request Packets (IRPs), which consist
of a request to transport data across a specific pipe. In addition to providing data transfer mechanisms,
the USBD is responsible for presenting to its clients an abstraction of a USB device which can be
manipulated for configuration and state management. As part of this abstraction, the USBD owns the
default pipe (see Chapter 5 and Chapter 9) through which all USB devices are accessed for the purposes
of standard USB control. This default pipe represents a logical communication between the USBD and
the abstraction of a USB device as shown in Figure 10-2.
In some operating systems, additional non-USB host software is available which provides configuration
and loading mechanisms to device drivers. In such operating systems, the device driver shall use the
provided interfaces instead of directly accessing the USBDI mechanisms.
The client layer describes all of those software entities which are responsible for directly interacting with
their peripherals. When each device is independently attached to the system, these clients might interact
directly with the peripheral hardware. The shared characteristics of USB place a USB software stack
between the client and its device; that is, a client cannot directly access the device’s hardware.
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Overall, the host layers provide the following capabilities:
• Detecting the attachment and removal of USB devices
• Managing USB standard control flow between the host and USB devices
• Managing data flow between the host and USB devices
• Collecting status and activity statistics
• Controlling the electrical interface between the host controller and USB devices, including the
provision of a limited amount of power
The following sections describe these responsibilities and the requirements placed on the USBDI in
greater detail. The actual interfaces used for a specific combination of host platform and operating
system are described in the appropriate Operating System Environment Guide.
All hubs provide a status change pipe (see Chapter 11) on which status changes for hubs and their ports
are reported. This includes a notification of when a USB device is attached to or removed from one of
their ports. A USBD client generically known as the hub driver receives these notifications as owner of
the hub’s status change pipe. For device attachments, the hub driver then initiates the device
configuration process. In some systems, this hub driver is a part of the host software provided by the
operating system for managing devices.
10.1.2 Control Mechanisms
Control information may be passed between the host and a USB device using in-band or out-of-band
signaling. In-band signaling mixes control information with data in a pipe outside the awareness of the
host. Out-of-band signaling places control information in a separate pipe.
There is a message pipe called the default pipe for each attached USB device. This logical association
between a host and a USB device is used for USB standard control flow such as device enumeration and
configuration. The default pipe provides a standard interface to all USB devices. The default pipe may
also be used for device-specific communications, as mediated by the USBD which owns the default pipes
of all of the USB devices.
A particular USB device may allow the use of additional message pipes to transfer device-specific
control information. These pipes use the same communications protocol as the default pipe, but the
information transferred is specific to the USB device and is not standardized by the Universal Serial Bus
Specification.
The USBD supports the sharing of the default pipe, which it owns and uses, with its clients. It also
provides access to any other control pipes associated with the device.
10.1.3 Data Flow
The host controller is responsible for transferring streams of data between the host and USB devices.
These data transfers are treated as a continuous stream of bytes. USB supports four basic types of data
transfers:
• Control transfers
• Isochronous transfers
• Interrupt transfers
• Bulk transfers
For additional information on transfer types, refer to Chapter 5.
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Each device presents one or more interfaces which a client may use to communicate with it. Each
interface is composed of zero or more pipes which individually transfer data between the client and a
particular endpoint on the device. The USBD establishes interfaces and pipes at the explicit request of
host software. The host controller provides service based on parameters provided by the host software
when the configuration request is made.
A pipe has several characteristics based on the delivery requirements of the data to be transferred.
Examples of these characteristics are: the rate at which data needs to be transferred, whether data is
provided at a steady rate or sporadically, how long data may be delayed before delivery, and whether the
loss of data being transferred is catastrophic.
A USB device endpoint describes the characteristics required for a specific pipe. Endpoints are described
as part of a USB device’s characterization information. For additional details, refer to Chapter 9.
10.1.4 Collecting Status and Activity Statistics
As a common communicant for all control and data transfers between the host and USB devices, the
USB system and the host controller is well positioned to track status and activity information. Such
information is provided upon request to host software allowing that software to manage status and
activity information.
10.1.5 Electrical Interface Considerations
The host provides power to USB devices attached to the root hub. The amount of power provided by a
port is specified in the discussion of hubs in Chapter 11.
10.2 Host Controller Requirements
In all implementations, host controllers perform the same basic duties with regard to the USB and its
attached devices. These basic duties are described below.
The host controller has requirements from both the host and the USB. The following is a brief overview
of the functionality provided. Each capability is discussed in detail in subsequent subsections.
State Handling As a component of the host, the host controller reports and manages
its states.
Serializer/Deserializer For data transmitted from the host, the host controller converts
protocol and data information from its native format to a bit stream
transmitted on USB. For data being received into the host, the
reverse operation is performed.
Frame Generation The host controller produces SOF tokens at a period of 1 ms.
Data Processing The host controller processes requests for data transmission to and
from the host.
Protocol Engine The host controller supports the protocol specified by USB.
Transmission Error
Handling All host controllers exhibit the same behavior when detecting and
reacting to the defined error categories.
The following subsections present a more detailed discussion of the required capabilities of the host
controller.
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10.2.1 State Handling
As a normal component in the host, the host controller has a series of states which the USB system
manages. Additionally, the host controller has two areas of USB-relevant state:
• Root hub
• State change propagation
The root hub presents to the hub driver the same standard states as other USB devices. The host
controller supports these states and their transitions for the hub. For detailed discussions of USB states,
including their interrelations and transitions, refer to Chapter 9.
The overall state of the host controller is inextricably linked with that of the root hub and of the overall
USB. Any host controller state changes which are visible to attached devices must be reflected in
corresponding device state changes such that the resulting host controller and device states are consistent.
USB devices request a wakeup through the use of resume signaling (refer to Chapter 11), which causes
hubs to tear down connectivity, and devices to return to their configured state. The host controller itself
may cause a resume event through the same signaling method. The host controller must notify the rest of
the host of a resume event through a mechanism or mechanisms specific to that system’s implementation.
10.2.2 Serializer/Deserializer
The actual transmission of data across the physical USB takes places as a serial bit stream. A Serial
Interface Engine (SIE), whether implemented as part of the host or a USB device, handles the
serialization and deserialization of USB transmissions. On the host, this SIE is part of the host controller.
10.2.3 Frame Generation
It is the host controller’s responsibility to partition USB time into 1 ms quantities called “frames.”
Frames are created by the host controller through issuing Start of Frame (SOF) tokens at 1.00 ms
intervals as shown in Figure 10-3. The SOF token is the first transmission in the frame period. After
issuing a SOF token, the host controller is free to transmit other transactions for the remainder of the
frame period. When the host controller is in its normal operating state, SOF tokens must be continuously
generated at the 1 ms periodic rate, regardless of the other bus activity or lack thereof. If the host
controller enters a state where it is not providing power on the bus, it must not generate SOFs. Also, if
the host controller is not generating SOFs, it may enter a power-reduced state.
SOF
Frame N+1
SOFSOF
Frame N
SOF
Frame N-1
EOF Interval (Frame N-1) EOF Interval (Frame N) EOF Interval (Frame N+1)
Figure 10-3. Frame Creation
The SOF token holds the highest priority access to the bus. Babble circuitry in hubs electrically isolates
any active transmitters during the End of Frame (EOF) interval, providing an idle bus for the SOF
transmission.
• The host controller must allow the length of the USB frame to be adjusted by ±1 bit time (refer to
Section 10.5.3.2.2). The host controller maintains the current frame number which may be read by
the USB system. The current frame number is used to uniquely identify one frame from another.
• Incremented at the end of every frame period.
• Valid through the subsequent frame.
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The host transmits the lower 11-bits of the current frame number in each SOF token transmission. When
requested from the host controller, the current frame number is the frame number in existence at the time
the request was fulfilled. The current frame number as returned by the host (host controller or HCD) is at
least 32 bits, although the host controller itself is not required to maintain more than 11 bits.
The host controller shall cease transmission during the EOF interval. When the EOF interval begins, any
transactions scheduled specifically for the frame which has just passed are retired. If the host controller
is executing a transaction at the time the EOF interval is encountered, the host controller terminates the
transaction.
10.2.4 Data Processing
The host controller is responsible for receiving data from the USB system and sending it to the USB and
for receiving data from the USB and sending it to the USB system. The particular format used for the
data communications between the USB system and the host controller is implementation specific, within
the rules for transfer behavior described in Chapter 5.
10.2.5 Protocol Engine
The host controller manages the USB protocol level interface. It inserts the appropriate protocol
information for outgoing transmissions. It also strips and interprets, as appropriate, the incoming
protocol information.
10.2.6 Transmission Error Handling
The host controller must be capable of detecting the following transmission error conditions, which are
defined from the host’s point-of-view:
• Time-out conditions after a host-transmitted token or packet. These errors occur when the addressed
endpoint is unresponsive or when the structure of the transmission is so badly damaged that the
targeted endpoint does not recognize it.
• Data errors resulting in missing or invalid transmissions.
− The host controller sends or receives a packet shorter than that required for the transmission; for
example, a transmission extending beyond EOF or a lack of resources available to the host
controller.
− An invalid CRC field on a received data packet.
• Protocol errors.
− An invalid handshake PID. For example, a malformed or inappropriate handshake.
− A false EOP.
− A bit stuffing error.
For each bulk, command, and interrupt transaction, the host must maintain an error count tally. Errors
result from the types of conditions described above, not as a result of an endpoint NAKing a request.
This value reflects the number of times the transaction has encountered a transmission error. If the error
count tally for a given transaction reaches three, the host retires the transfer. When a transfer is retired
due to excessive errors, the last error type will be indicated. Isochronous transactions are attempted only
once, regardless of outcome, and, therefore, no error count is maintained for this type.
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10.3 Overview of Software Mechanisms
The HCD and the USBD present software interfaces based on different levels of abstraction. They are,
however, expected to operate together in a specified manner to satisfy the overall requirements of the
USB system (see Figure 10-2). The requirements for the USB software stack are expressed primarily as
requirements on the USBDI. The division of duties between the USBD and the HCD is not defined.
However, the one requirement of the HCDI which must be met is that it support, in the specified
operating system context, multiple definitions of host controllers.
The HCD provides an abstraction of the host controller and an abstraction of the host controller’s view of
data transfer across USB. The USBD provides an abstraction of the USB device and of the data transfers
between the client of the USBD and the function on the USB device. Overall, the USB software stack
acts as a facilitator for transmitting data between the client and the function and as a control point for the
USB-specific interfaces of the USB device. As part of facilitating data transfer, the USB software
provides buffer management capabilities and allows the synchronization of the data transmittal to the
needs of the client and the function.
The specific requirements on the USBDI are described later in this chapter. The exact functions which
fulfill these requirements are described in the relevant Operating System Environment Guide for the
HCDI and the USBDI. The procedures involved in accomplishing data transfers via the USBDI are
described below.
10.3.1 Device Configuration
Different operating system environments perform device configuration using different software
components and different sequences of events. A specific operating system method is not assumed by the
USB system. However, there are some basic requirements that must be fulfilled by any USB system
implementation. In some operating systems, these requirements are met by existing host software. In
others, the USB system provides the capabilities.
The USB system assumes a specialized client of the USBD, called a hub driver, which acts as a clearing
house for addition of devices to and removal of devices from a particular hub. Once the hub driver
receives such notifications, it will employ additional host software and other USBD clients, in an
operating system specific manner, to recognize and configure the device. This model, shown in Figure
10-4 is the basis of the following discussion.
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Device
Driver
HCD
USBD
Host So ftware
Configuration
Support
Configuration
Control
Optional
Configuration
Control
Hub
Driver
Optional
Component
Figure 10-4. Configuration Interactions
When a device is attached, the hub driver receives a notification from the hub detecting the change. The
hub driver, using the information provided by the hub, requests a device identifier from the USBD. The
USBD in turn sets up the default pipe for that device and returns a device identifier to the hub driver.
The device is now ready to be configured for use. For each device, there are three types of configuration
which must be complete before that device is ready for use:
1. Device configuration. This includes setting up all of the device’s USB parameters and allocating all
USB host resources which are visible to the device. This is accomplished by setting the
configuration value on the device. A limited set of configuration changes, such as alternate setting,
are allowed without totally reconfiguring the device. Once the device is configured, it is, from its
point of view, ready for use.
2. USB configuration. In order to actually create a USBD pipe ready for use by a client, additional
USB information, not visible to the device, must be specified by the client. This information, known
as the Policy for the pipe, describes how the client will use the pipe. This includes such items as the
maximum amount of data the client will transfer with one IRP, the maximum service interval the
client will use, the client’s notification identification, and so on.
3. Function configuration. Once configuration types 1 and 2 have been accomplished, the pipe is
completely ready for use from the USB’s point of view. However, additional vendor- or class-
specific setup may be required before the pipe can actually be used by the client. This configuration
is a private matter between the device and the client and is not standardized by the USBD.
The following paragraphs describe the device and USB configuration requirements.
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The actual device configuration is performed by the responsible configuring software. Depending on the
particular operating system implementation, the software responsible for configuration can include:
• The hub driver
• Other host software
• A device driver
The configuring software first reads the device descriptor, then requests the description for each possible
configuration. It may use the information provided to load a particular client, such as a device driver,
which initially interacts with the device. The configuring software, perhaps with input from that device
driver, chooses a configuration for the device. Setting the device configuration sets up all of the
endpoints on the device and returns a collection of interfaces to be used for data transfer by USBD
clients. Each interface is a collection of pipes owned by a single client.
This initial configuration uses the default settings for interfaces and the default bandwidth for each
endpoint. A USBD implementation may additionally allow the client to specify alternate interfaces when
selecting the initial configuration. The USB system will verify that the resources required for the support
of the endpoint are available and, if so, will allocate the bandwidth required. Refer to Section 10.3.2 for
a discussion of resource management.
The device is now configured, but the created pipes are not yet ready for use. The USB configuration is
accomplished when the client initializes each pipe by setting a policy to specify how it will interact with
the pipe. Among the information specified is the client’s maximum service interval and notification
information. Among the actions taken by the USB system, as a result of setting the policy, is determining
the amount of buffer working space required beyond the data buffer space provided by the client. The
size of the buffers required is based upon the usage chosen by the client and upon the per transfer needs
of the USB system.
The client receives notifications when IRPs complete, successfully or due to errors. The client may also
wake up independently of USB notification to check the status of pending IRPs.
The client may also choose to make configuration modifications such as enabling an alternate setting for
an interface or changing the bandwidth allocated to a particular pipe. In order to perform these changes,
the interface or pipe, respectively, must be idle.
10.3.2 Resource Management
Whenever a pipe is setup by the USBD for a given endpoint, the USB system must determine if it can
support the pipe. The USB system makes this determination based on the requirements stated in the
endpoint descriptor. One of the endpoint requirements which must be supported in order to create a pipe
for an endpoint is the bandwidth necessary for that endpoint’s transfers. There are two stages to check
for available bandwidth. First the maximum execution time for a transaction is calculated. Then, the
frame schedule is consulted to determine if the indicated transaction will fit.
The allocation of the guaranteed bandwidth for isochronous and interrupt pipes, and the determination of
whether a particular control or bulk transaction will fit into a given frame, can be determined by a
software heuristic in the USB system. If the actual transaction execution time in the host controller
exceeds the heuristically determined value, the host controller is responsible for ensuring that frame
integrity is maintained (refer to Section 10.2.3). The following discussion describes the requirements on
the USB system heuristic.
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In order to determine if bandwidth can be allocated, or if a transaction can be fit into a particular frame,
the maximum transaction execution time must be calculated. The calculation of the maximum
transaction execution time requires that the following information be provided: (note that some of this
information may be provided by an agent other than the client)
• Number of data bytes (MaxPacketSize) to be transmitted.
• Transfer type.
• Depth in the topology. If less precision is allowed, the maximum topology depth may be assumed.
This calculation must include the bit transmission time, the signal propagation delay through the
topology, and any implementation specific delays such as preparation or recovery time required by the
host controller itself. Refer to Chapter 5 for examples of formulas that can be used for such calculations.
10.3.3 Data Transfers
The basis for all client-function communication is the interface: a bundle of related pipes associated with
a particular USB device.
A given interface is managed by exactly one client on the host. The client initializes each pipe of an
interface by setting the policy for that pipe. This includes the maximum amount of data to be transmitted
per IRP and the maximum service interval for the pipe. A service interval is stated in milliseconds and
describes the interval over which an IRP’s data will be transmitted for an isochronous pipe. It describes
the polling interval for an interrupt pipe. The client is notified when a specified request is completed.
The client manages the size of each IRP such that its duty cycle and latency constraints are maintained.
Additional policy information includes the notification information for the client.
The client provides the buffer space required to hold the transmitted data. The USB system uses the
policy to determine the additional working space it will require .
The client views its data as a contiguous serial stream, which it manages in a similar manner to those
streams provided over other types of bus technologies. Internally, the USB system may, depending on its
own policy and any host controller constraints, break the client request down into smaller requests to be
sent across the USB. However, two requirements must be met whenever the USB system chooses to
undertake such division:
• The division of the data stream into smaller chunks is not visible to the client.
• USB samples are not split across bus transactions. Refer to Chapter 9 for a definition of USB sample
and its relationship to the pipe’s natural sample size.
When a client wishes to transfer data, it will send an IRP to the USBD. Depending on the direction of
data transfer, a full or empty data buffer will be provided. When the request is complete (successfully or
due to an error condition), the IRP and its status is returned to the client. Where relevant, this status is
also provided on a per transaction basis.
10.3.4 Common Data Definitions
In order to allow the client to receive request results as directly as possible from its device, it is desirable
to minimize the amount of processing and copying required between the device and the client. To
facilitate this, some control aspects of the IRP are standardized such that the information provided by the
client may be directly used by different layers in the stack. The particular format for this data is
dependent on the actualization of the USBDI in the operating system. Some data elements may in fact
not be directly visible to the client at all, but are generated as a result of the client request.
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The following data elements define the relevant information for a request:
• Identification of the pipe associated with the request. Identifying this pipe also describes information
such as transfer type for this request.
• Notification identification for the particular client.
• Location and length of data buffer which is to be transmitted or received.
• Completion status for the request. Both the summary status, and, as required, detailed per-
transaction status must be provided.
• Location and length of working space. This is implementation dependent.
The actual mechanisms used to communicate requests to the USBD are operating system specific.
However, beyond the requirements stated above for what request-related information must be available,
there are also requirements on how requests will be processed. The basic requirements are described in
Chapter 5. Additionally, the USBD provides a mechanism to designate a group of isochronous IRPs for
which the transmission of the first transaction of each IRP will occur in the same frame. The USBD also
provides a mechanism for designating an uninterruptable set of vendor- or class-specific requests to a
default pipe. No other requests to that default pipe, including standard, class, or vendor request may be
inserted in the execution flow for such an uninterruptable set. If any request in this set fails, the entire set
is retired.
10.4 Host Controller Driver
The Host Controller Driver (HCD) is an abstraction of host controller hardware and the host controller’s
view of data transmission over the USB. The HCDI meets the following requirements:
• Provides an abstraction of the host controller hardware.
• Provides an abstraction for data transfers by the host controller across the USB interconnect.
• Provides an abstraction for the allocation (and de-allocation) of host controller resources, to support
guaranteed service to USB devices.
• Presents the root hub and its behavior according to the hub class definition. This includes supporting
the root hub such that the hub driver interacts with the root hub exactly as it would for any hub. In
particular, even though a root hub can be implemented in a combination of hardware and software,
the root hub responds initially to the default device address (from a client perspective), returns
descriptor information, supports having its device address set, and supports the other hub class
requests. However, bus transactions may or may not need to be generated to accomplish this
behavior given the close integration possible between the host controller and the root hub.
The HCD provides a software interface (HCDI) which implements the required abstractions. The
function of the HCD is to provide an abstraction which hides the details of the host controller hardware.
Below the host controller hardware is the physical USB and all the attached USB devices.
The HCD is the lowest tier in the USB software stack. The HCD has only one client: the Universal
Serial Bus Driver (USBD). The USBD maps requests from many clients to the appropriate HCD. A
given HCD may manage many host controllers.
The HCDI is not directly accessible from a client. Therefore, the specific interface requirements for the
HCDI are not discussed here.
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10.5 Universal Serial Bus Driver
A Universal Serial Bus Driver (USBD) provides a collection of mechanisms that operating system
components, typically device drivers, use to access USB devices. The only access to USB devices is that
provided by the USBD. The USBD implementations are operating system specific. The mechanisms
provided by the USBD are implemented using as appropriate and augmenting as necessary the
mechanisms provided by the operating system environment in which it runs. The following discussion
centers on the basic capabilities required for all USBD implementations. For specifics of the USBD
operation within a specific environment, see the relevant Operating System Environment Guide for the
USBD. A single instance of the USBD directs accesses to one or more HCDs that in turn connect to one
or more USB host controllers. If allowed, how USBD instancing is managed is dependent upon the
operating system environment. However, from the client’s point-of-view, the USBD with which it
communicates manages all of the attached USB devices.
10.5.1 Overview
Clients of USBD direct commands to devices or move streams of data to or from pipes. The USBD
presents two groups of software mechanisms to clients: command mechanisms and pipe mechanisms.
Command mechanisms allow clients to configure and control USBD operation as well as to configure
and generically control a USB device. In particular, command mechanisms provide all access to the
device’s default pipe.
Pipe mechanisms allow a USBD client to manage device specific data and control transfers. Pipe
mechanisms do not allow a client to directly address the device’s default pipe.
Figure 10-5 presents an overview of the USBD structure.
Services
Host
Controller
Driver
Host
Controller
Driver
USB Host
Controller USB Host
Controller
Device Data
access
Power Control
Bus and Device
Management
Pipe Interfaces Command Interfaces
Universal Serial Bus Driver
Configuration
Management
(Hub)
Interrupt transfer
Message
and
Stream
Pipe
Access
Figure 10-5. Universal Serial Bus Driver Structure
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10.5.1.1 Initialization
Specific USBD initialization is operating system dependent. When a particular USB managed by USBD
is initialized, the management information for that USB is also created. Part of this management
information is the default address device and its default pipe.
Whenever a device is attached to a USB, it responds to a special address known as the default address
(refer to Chapter 9) until its unique address is assigned by the hub driver. In order for the USB system to
interact with the new device, the default address device for that particular USB and that default address
device’s default pipe must always be available to the hub driver whenever a device is attached. The
default address device and its default pipe are created by the USB system when a new USB is initialized.
10.5.1.2 USBD Pipe Usage
Pipes are the method by which a device endpoint is associated with a host software entity. Pipes are
owned by exactly one such entity on the host. Although the basic concept of a pipe is the same no matter
who the owner, some distinction of capabilities provided to the USBD client occurs between two groups
of pipes:
• Default pipes, which are owned and managed by the USBD
• All other pipes, which are owned and managed by clients of the USBD
Default pipes are never directly accessed by clients, although they are often used to fulfill some part of
client requests relayed via command mechanisms.
10.5.1.2.1 Default Pipes
The USBD is responsible for allocating and managing appropriate buffering to support transfers on the
default pipe which are not directly visible to the client such as setting a device address. For those
transfers which are directly visible to the client, such as sending vendor and class commands or reading a
device descriptor, the client must provide the required buffering.
10.5.1.2.2 Client Pipes
Any pipe not owned and managed by the USBD can be owned and managed by a USBD client. From the
USBD viewpoint, the pipe is owned by a single client. In fact, a cooperative group of clients can manage
the pipe provided they behave as a single coordinated entity when using the pipe.
The client is responsible for providing the amount of buffering it needs to service the data transfer rate of
the pipe within a service interval attainable by the client. Additional buffering requirements for working
space are specified by the USB system.
10.5.1.3 Service Capabilities
The USBD provides services in the following categories:
• Configuration via command mechanisms
• Transfer services via both command and pipe mechanisms
• Event notification
• Status reporting and error recovery
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10.5.2 USBD Command Mechanism Requirements
USBD command mechanisms allow a client generic access to a USB device. Generally these commands
allow the client to make read or write accesses to one of potentially several device data and control
spaces. The client provides as little as a device identifier and the relevant data or empty buffer pointer.
USBD command transfers do not require that the USB device be configured. Many of the device
configuration facilities provided by the USBD are command transfers.
Following are the specific requirements on the command mechanisms provided.
10.5.2.1 Interface State Control
USBD clients must be able to set a specified interface to any settable pipe state. Setting an interface state
results in all of the pipes in that interface moving to that state. Additionally, all of the pipes in an
interface may be reset or aborted.
10.5.2.2 Pipe State Control
USBD pipe state has two components:
• Host status
• Reflected endpoint status
Whenever the pipe status is reported, the value for both components will be identified. The pipe status
reflected from the endpoint is the result of the endpoint being in a particular state. The USBD client
manages the pipe state as reported by the USBD. For any pipe state reflected from the endpoint, the
client must also interact with the endpoint to change the state.
A USBD pipe is in exactly one of the following states:
• Active. The pipe’s policy has been set and the pipe is able to transmit data. The client can query as
to whether any IRPs are outstanding for a particular pipe. Pipes for which there are no outstanding
IRPs are still considered to be in the active state as long as they are able to accept new IRPs.
• Stalled. An error has occurred on the pipe. This state may also be a reflection of the corresponding
stalled endpoint on the device.
• Idle. The pipe will not accept further IRPs. The endpoint must also be in the idle state.
USBD clients must be able to set a specified pipe to the idle or active state from any of the states
described above. Clients also must be able to set an endpoint to either the active or idle state.
Additionally, clients manipulate pipe state in the following ways:
• Aborting a pipe. All of the IRPs scheduled for a pipe are retired immediately and returned to the
client with a status indicating they have been aborted. Neither the host state nor the reflected
endpoint state of the pipe is affected.
• Resetting a pipe. The pipe’s IRPs are aborted. The host state is moved to active. If the reflected
endpoint state needs to be changed, that must be commanded explicitly by the USBD client.
10.5.2.3 Getting Descriptors
The USBDI must provide a mechanism to retrieve standard device, configuration and string descriptors,
as well as any class- or vendor-specific descriptors.
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10.5.2.4 Getting Current Configuration Settings
The USBDI must provide a facility to return, for any specified device, the current configuration
descriptor. If the device is not configured, no configuration descriptor is returned. This action is
equivalent to returning the configuration descriptor for the current configuration by requesting the
specific configuration descriptor. It does not, however, require the client to know the identifier for the
current configuration. This will return all of the configuration information, including:
• All of the configuration descriptor information as stored on the device including all of the alternate
settings for all of the interfaces
• Indicators for which of the alternate settings for interfaces are active
• Pipe handles for endpoints in the active alternate settings for interfaces
• Actual MaxPacketSizes for endpoints in the active alternate settings for interfaces
Additionally, for any specified pipe, the USBDI must provide a facility to return the MaxPacketSize
which is currently being used by the pipe.
10.5.2.5 Adding Devices
The USBDI must provide a mechanism for the hub driver to inform USBD of the addition of a new
device to a specified USB and to retrieve the USBD ID of the new USB device. The USBD tasks include
assigning the device address and preparing the device’s default pipe for use.
10.5.2.6 Removing Devices
The USBDI must provide a facility for the hub driver to inform the USBD that a specific device has been
removed.
10.5.2.7 Managing Status
The USBDI must provide a mechanism for obtaining and clearing device-based status, on a device,
interface, or pipe basis.
10.5.2.8 Sending Class Commands
This USBDI mechanism is used by a client, typically a class specific or adaptive driver, to send one or
more class specific commands to a device.
10.5.2.9 Sending Vendor Commands
This USBDI mechanism is used by a client to send one or more vendor specific commands to a device.
10.5.2.10 Establishing Alternate Settings
The USBDI must provide a mechanism to change the alternate setting for a specified interface. As a
result, the pipe handles for the previous setting are released and new pipe handles for the interface are
returned. For this request to succeed, the interface must be idle; i.e., all pipes in the interface must be in
the idle state.
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10.5.2.11 Establishing a Configuration
Configuring software requests a configuration by passing a buffer containing a configuration descriptor to
the USBD. The USBD requests resources for the endpoints in the configuration, and if all resource
requests succeed, the USBD sets the device configuration and returns interface handles with
corresponding pipe handles for all of the active endpoints. The default values are used for all alternate
settings for interfaces and for the MaxPacketSize for endpoints. These default values may be
subsequently modified.
Note: the specific interface implementing configuration may require specific alternate settings to be
identified.
10.5.2.12 Setting Descriptors
For devices supporting this behavior, the USBDI allows existing descriptors to be updated or new
descriptors to be added.
10.5.2.13 Establishing the Maximum Packet Size for a Pipe
The USBDI must provide a mechanism to modify a pipe’s transfer characteristics. The USBD adjusts
requested resources and sets the current maximum data payload size per bus transaction for the specified
pipe. This service may only apply to a pipe which has been created by establishing a configuration and
which is currently idle.
10.5.3 USBD Pipe Mechanisms
This part of the USBDI offers clients the highest-speed, lowest overhead data transfer services possible.
Higher performance is achieved by shifting some pipe management responsibilities from the USBD to the
client. As a result, the pipe mechanisms are implemented at a more primitive level than the data transfer
services provided by the USBD command mechanisms. Pipe mechanisms do not allow access to a
device’s default pipe.
USBD pipe transfers are available only after both the device and USB configuration have completed
successfully. At the time the device is configured, the USBD requests the resources required to support
all device pipes in the configuration. Clients are allowed to modify the configuration, constrained by
whether the specified interface or pipe is idle.
Clients provide full buffers to outgoing pipes and retrieve transfer status information following the
completion of a request. The transfer status returned for an outgoing pipe allows the client to determine
the success or failure of the transfer.
Clients provide empty buffers to incoming pipes and retrieve the filled buffers and transfer status
information from incoming pipes following the completion of a request. The transfer status returned for
an incoming pipe allows a client to determine the amount and the quality of the data received.
10.5.3.1 Supported Pipe Types
The four types of pipes supported, based on the four transfer types, are described below.
10.5.3.1.1 Isochronous Data Transfers
Each buffer queued for an isochronous pipe is required to be viewable as a stream of samples. As with
all pipe transfers, the client establishes a policy for using this isochronous pipe, including the relevant
service interval for this client. Lost or missing bytes, which are detected on input, and transmission
problems, which are noted on output, are indicated to the client.
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The client queues a first buffer, starting the pipe streaming service. To maintain the continuous
streaming transfer model used in all isochronous transfers, the client queues an additional buffer before
the current buffer is retired.
The USBD is required to be able to provide a sample stream view of the client’s data stream. In other
words, using the client’s specified method of synchronization, the precise packetization of the data is
hidden from the client. Additionally, a given transaction is always contained completely within some
client data buffer.
For an output pipe, the client provides a buffer of data. The USBD allocates the data across the frames
for the service period using the client’s chosen method of synchronization.
For an input pipe, the client must provide an empty buffer large enough to hold the maximum number of
bytes the client’s device will deliver in the service period. The USB system strips USB defined
packaging information from the stream such that bytes are contiguous in the client’s buffer. Where
missing or invalid bytes are indicated, the USBD leaves the space which the bytes would have occupied
in place in the buffer and identifies the error. One of the consequences of using no synchronization
method, is that this reserved space is assumed to be the maximum packet size. The buffer-retired
notification occurs when the IRP completes. Note that the input buffer need not be full when returned to
the client.
The USBD may optionally provide additional views of isochronous data streams. The USBD is also
required to be able to provide a packet stream view of the client’s data stream.
10.5.3.1.2 Interrupt Transfers
Interrupt transfers originate in a USB device and are delivered to a client of the USB Driver.
The client queues a buffer large enough to hold the interrupt transfer data (typically a single USB
transaction). When all of the data is transferred, or if the error threshold is exceeded, the IRP is returned
to the client.
10.5.3.1.3 Bulk Transfers
Bulk transfers may originate either from the device or the client. No periodicity or guaranteed latency is
assumed. When all of the data is transferred, or if the error threshold is exceeded, the IRP is returned to
the client.
10.5.3.1.4 Control Transfers
All message pipes transfer data in both directions. In all cases, the client outputs a setup stage to the
device endpoint. The optional data stage may be either an input or an output and the final status is
always logically presented to the host. For details of the defined message protocol, refer to Chapter 8.
The client prepares a buffer specifying the command phase and any optional data or empty buffer space.
The client receives a buffer-retired notification when all phases of the control transfer are complete, or an
error notification, if the transfer is aborted due to transmission error.
10.5.3.2 USBD Pipe Mechanism Requirements
The following pipe mechanisms are provided.
10.5.3.2.1 Aborting IRPs
The USBDI must allow IRPs for a particular pipes to be aborted.
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10.5.3.2.2 Adjusting the Start-Of-Frame
The USBDI must allow a master client to change the number of bit times in a USB frame. A client
wishing to adjust the SOF must already have received master client status from the USBD (refer to
Section 10.5.3.2.5). Invoking this change more frequently than once every 6 ms has undefined results.
10.5.3.2.3 Managing Pipe Policy
The USBDI must allow a client to set and clear the policy for an individual pipe or for an entire interface.
Any IRPs made by the client prior to successfully setting a policy are rejected by the USBD.
10.5.3.2.4 Queuing IRPs
The USBDI must allow clients to queue IRPs for a given pipe. When IRPs are returned to the client, the
request status is also returned. A mechanism is provided by the USBD to identify a group of isochronous
IRPs whose first transactions will all occur in the same frame.
10.5.3.2.5 Being a Master Client
The USBDI must allow a client to request becoming a master client for a given USB and to release this
capability when it is no longer required. Only master clients may adjust the SOF for a given USB.
A client requesting master status identifies itself with an interface handle for the device from which it is
mastering.
10.5.4 Managing the USB via the USBD Mechanisms
Using the provided USBD mechanisms, the following general capabilities are supported by any USB
system.
10.5.4.1 Configuration Services
Configuration Services operate on a per device basis. The USBD performs device configuration at the
direction of the configuring software. A hub driver has a special role in device management and provides
at least the following capabilities:
• Device attach/detach recognition, driven by an interrupt pipe owned by the hub driver
• Device reset, accomplished by the hub driver by resetting the hub port upstream of the device
• Directs the USBD to perform device address assignment
The USBDI additionally provides the following configuration facilities, which may be used by the hub
driver or other configuring software available on the host:
• Device identification and access to configuration information (via access to descriptors on the
device)
• Device configuration via command mechanisms
When the hub driver informs the USBD of a device attachment, the USBD establishes the default pipe for
the new device.
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10.5.4.1.1 Configuration Management
Configuration Management services are provided primarily as a set of specific interface commands which
generate USB transactions on the default pipe. The notable exception is the use of an additional interrupt
pipe that delivers hub status directly to the hub driver.
Every hub initiates an interrupt transfer when there is a change in the state of one of the hub ports.
Generally, the port state change will be the connection or removal of a downstream USB device. (Refer
to Chapter 11 for more information.)
10.5.4.1.2 Initial Device Configuration
The device configuration process begins when a hub reports via its status change pipe the connection of a
new USB device.
Configuration Management services allow configuring software to select a USB device configuration
from the set of configurations listed in the device. The USBD verifies that the data transfer rates given
for all endpoints in the configuration do not exceed the capabilities of the USB with the current schedule
before setting the device configuration.
10.5.4.1.3 Modifying a Device Configuration
Configuration Management services allow configuring software to replace a USB device configuration
with another configuration from the set of configurations listed in the device. The operation succeeds if
the data transfer rates given for all end points in the new configuration fit within the capabilities of the
USB with the current schedule. If the new configuration is rejected, the previous configuration remains.
Configuration Management services allow configuring software to return a USB device to a not
configured state.
10.5.4.1.4 Device Removal
Error recovery and/or device removal processing begins when a hub reports via its status change pipe that
communication with a USB device has ceased.
10.5.4.2 Bus and Device Management
Bus and Device Management services allow a client to become the master client on a USB, and as the
master client, to adjust the number of bit times in a frame on that bus. A master client can explicitly
release master status, or the client’s master status will be automatically released when the device
containing the referenced interface is reset or detached. There can be at most one master client on a
USB. The new master client will be awarded a special master handle to be used when adjusting the SOF.
A master client may add or subtract one bit time to the current USB frame. The client must reference an
interface identifying the USB being adjusted and allowing the client’s master status to be validated.
Adjusting SOF more frequently than once every 6 ms has undefined results.
10.5.4.3 Power Control
The USB system will provide power management in a system-specific manner. No USB requirements
beyond those described in Chapter 9 are placed on the USB system.
No standard commands are provided for use with all USB devices. Individual devices may choose to
offer a power control interface via vendor specific control(s). Device classes may define class-specific
power control capabilities.
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All USB devices must however support the Powered Down state (refer to Chapter 9). Basic control of the
power provided to a device is exercised via control of the hub port to which the device is attached.
10.5.4.4 Event Notifications
USBD clients receive several kinds of event notifications through a number of sources:
• Completion of an action initiated by a client.
• Interrupt transfers over stream pipes can deliver notice of device events directly to USBD clients.
For example, hubs use an interrupt pipe to deliver events corresponding to changes in hub status.
• Event data can be embedded by devices in streams.
• Standard device interface commands, device class commands, vendor specific commands, and even
general control transfers over message pipes can all be used to poll devices for event conditions.
10.5.4.5 Status Reporting and Error Recovery Services
The command and pipe mechanisms both provide status reporting on individual requests as they are
invoked and completed.
Additionally, USB device status is available to USBD clients using the command mechanisms.
The USBD provides clients with pipe error recovery mechanisms by allowing pipes to be reset or
aborted.
10.6 Operating System Environment Guides
As noted previously, the actual interfaces between USB software and host software are specific to the
host platform and operating system. A companion specification is required for each combination of
platform and operating system with USB support. These specifications describe the specific interfaces
used to integrate USB into the host. Each operating system provider for USB software identifies a
compatible Universal USB Specification revision.
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Chapter 11
Hub Specification
This chapter describes the architectural requirements for the USB hub. It contains a description of the
two principal sub-blocks: the hub repeater and the hub controller. The chapter also describes the hubs
operation for error recovery, reset, and suspend/resume. The second half of the chapter defines hub
request behavior and hub descriptors.
The hub specification supplies sufficient information to permit an implementer to design a USB hub
which conforms to the USB specification.
11.1 Overview
Hubs provide the electrical interface between USB devices and the host and are directly responsible for
supporting many of the attributes that make USB user friendly and hide its complexity from the user.
Listed below are the major aspects of USB functionality that hubs must support:
• Connectivity behavior
• Power management
• Device connect/disconnect detection
• Bus fault detection and recovery
• Full/Low speed device support
A hub consists of two components, the hub repeater and the hub controller. The repeater is responsible
for connectivity setup and tear-down. It also supports exception handling such as bus fault detection and
recovery and connect/disconnect detect. The hub controller provides the mechanism for host to hub
communication. Hub specific status and control commands permit the host to configure a hub and to
monitor and control its individual downstream ports.
11.2 Device Characteristics
11.2.1 Hub Architecture
Figure 11-1 shows a hub and the locations of its root and downstream ports. A hub consists of a repeater
section and a hub controller section. The repeater is responsible for managing connectivity on a per
packet basis, while the hub controller provides status and control and permits host access to the hub.
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Port 1 Port 2 Port N
...
Downstream Ports
Port 0
Root port
Hub
Controller
Hub
Repeater
Figure 11-1. Hub Architecture
11.2.2 Hub Connectivity
Hubs display differing connectivity behavior depending on whether they are propagating packet traffic or
resume signaling, or are in the idle state.
11.2.2.1 Packet Signaling Connectivity
The hub repeater contains one port that must always connect in the upstream direction (referred to as the
root port) and one or more downstream ports. Upstream connectivity is defined as being towards the
host, and downstream connectivity is defined as being towards a device. Figure 11-2 shows the packet
signaling connectivity behavior for hubs in the upstream and downstream directions. A hub also has an
idle state during which the hub makes no connectivity. When in the idle state all of the hub’s ports are in
the receive mode waiting for the start of the next packet.
Downstream
Connectivity
Downstream
ports
Root
port
disabled
port
Upstream
Connectivity
disabled
port
Idle
(No connectivity)
disabled
port
Figure 11-2. Hub Connectivity
If a downstream hub port is enabled (i.e., in a state where it can propagate signaling through the hub) and
the hub detects an SOP on that port, connectivity is established in an upstream direction to the root port
of that hub, but not to any other downstream ports. This means that when a device or a hub transmits a
packet upstream, only those hubs in line between the transmitting device and the host will see the packet.
When SOP on an upstream port is detected, all other downstream ports are locked. This guarantees that
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hub connectivity will not be modified until the next EOP is detected or until the hub times out at the end
of the frame.
In the downstream direction, hubs operate in a broadcast mode. When a hub detects an SOP on its root
port, it establishes connectivity to all enabled downstream ports. If a port is not enabled, it does not
receive any packet traffic activity from the root port and does not propagate packet signaling
downstream.
11.2.2.2 Resume Connectivity
Hubs exhibit differing connectivity behaviors for upstream and downstream directed resume signaling. A
hub which is in the suspend state reflects resume signaling from its root port to all of its enabled
downstream ports. Figure 11-3 illustrates hub upstream and downstream resume connectivity.
Downstream
Connectivity Upstream
Connectivity
Downstream
ports
Root
port
disabled
port disabled
port
Root
port
Figure 11-3. Resume Connectivity
If a hub is in the suspend state and detects resume signaling from a downstream port, the hub reflects that
signaling upstream to its root and also to all of its enabled downstream ports, including the port which
initiated the resume sequence. Resume signaling is not reflected to disabled ports. A detailed discussion
of resume connectivity appears in Section 11.5
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11.2.3 Hub Port States
A hub downstream port may be in one of four or five possible states and must comprehend such features
as connect/disconnect detect, port enable/disable, suspend/resume, reset, and, optionally, power
switching. Hubs must support independent port state machines on a per downstream port basis.
Figure 11-4 illustrates the states for a non-power switched port, and Figure 11-5 shows the states for a
port with power switching. Hub port states apply only to downstream ports.
reset_on_root_port
Disconnected else
conn_det
Disabled else
Enabled
PORT_DISABLE or
fr_error
PORT_SUSPEND
Suspended else
else PORT_RESUME or
PORT_RESET
disc_det
disc_det
disc_det
PORT_RESET or
PORT_ENABLE
Figure 11-4. Non-power Switched Hub Port States
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SetPortPower = OFF or
reset_on_root_port
Disconnected else
conn_det
Disabled else
Enabled
disc_det
PORT_DISABLE
or fr_error
Powered off
SetPortPower = ON
else
PORT_SUSPEND
Suspended else
else
PORT_RESUME or
PORT_RESET
disc_det
disc_det
PORT_ENABLE or
PORT_RESET
Figure 11-5. Power Switched Hub Port States
Each of the states is described below.
Powered off: This state is only supported for ports that have power switching as shown in Figure 11-5.
A port transitions to its powered off state when the hub receives a ClearPortFeature(PORT_POWER)
request, or when the hub detects root port reset signaling. A hub’s downstream ports also transition to the
powered off state when power is initially applied to a hub. A powered off port supplies no power
downstream, and its signal outputs buffers are placed in the Hi-Z state. A powered off port ignores all
upstream directed bus activity on that port.
Disconnected: A downstream port of a hub supporting power switching transitions from the powered off
to the disconnected state when power is applied to the port via a SetPortFeature(PORT_POWER) request.
If a hub does not support power switching, it transitions to the disconnected state upon power up or upon
receipt of a root port reset. In the disconnected state, a port cannot propagate any signaling in either the
upstream or downstream direction. However, the port can detect a connect event (conn_det), which sets
a status field in the hub controller and causes the port state to transition to the disabled state.
Conn_det is asserted if a downstream port is in the disconnect state and detects 2.5 µs of continuous non
SE0 signaling on the bus. When conn_det is first asserted, the idle bus state can be driven from either a
low or full speed device, and may be either a DIFF0 or a DIFF1. The hub automatically determines the
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device type (low or full speed) by determining whether D+ or D- is pulled high. Device speed
determination is completed before the hub transitions to the disabled state.
Disabled: A port transitions to the disabled state from the disconnected state when it detects a connect
event (conn_det). This requires that the port first be power switched on if the hub supports power
switching. A port in the disabled state can only propagate downstream directed signaling arising from a
SetPortFeature(RESET) request; at all other times, the port’s output buffers are in the Hi-Z state. In the
upstream direction, a port in the disabled state does not propagate any signaling through the hub to the
root port when the hub is awake. However, certain events, such as disconnect will cause upstream
resume signaling to be propagated to the root port if the hub is in the suspend state. A disconnect event
(disc_det) will cause the port to return to the disconnect state and will set a status field in the hub
controller. Disc_det is asserted whenever the port detects 2.5 µs of continuous SE0 when the port is not
propagating downstream traffic. Before a disconnect event can be timed the hub suspended hub must first
wake up.
Enabled: A port transitions to the enabled state upon receipt of a SetPortFeature(PORT_ENABLE) or a
SetPortFeature(PORT_RESET) request. In the enabled state the port propagates all downstream
signaling, full speed packet traffic and reset signaling; low speed packet traffic is propagated downstream
when preceded by the preamble PID. In the enabled state, the port propagates all upstream signaling
including full speed and low speed packets and resume signaling. A port transitions to the disabled state
if it receives a ClearPortFeature(PORT_ENABLE) request or if a frame error (fr_error) occurs. The
ClearPortFeature(PORT_ENABLE) request may be issued at any time by the host, and the hub must
respond by immediately placing the port in the disabled state. An enabled port will transition to the
disconnected state if a disconnect detect occurs.
Suspended: The hub selectively suspends all devices downstream of a port when it receives a
SetPortFeature(PORT_SUSPEND) request. This request must not cause the now suspended port to stop
propagating in the middle of a transaction; i.e., the current transaction must complete before the port
enters the suspend state. A port displays different suspend connectivity behavior depending on whether
the hub is awake or is itself suspended. If the hub is awake, no upstream or downstream connectivity can
propagate through the port. However, if the hub is suspended, an idle to resume or an idle to SE0
transition on the port is reflected onto all other non-disabled ports as an idle to resume transition. This
behavior makes it possible to suspend multiple hubs in series and still have a device at the bottom be able
to wake up the entire bus.
If a hub is suspended and bus activity occurs on a suspended port, the hub first wakes up. The termination
of a resume request to the port causes a status field to be set in the hub. In response, the host polls the hub
to read the status field change and determine which on which port the resume occurred. The hub port
transitions to the enabled state when the resume is complete. Details of the hub port signaling for
selective resume are described in Section 11.5.2.
A disconnect event to a suspended hub causes the hub state to transition to the disconnected state and sets
the hub controller status field to indicate that a disconnect has occurred. It is not possible to place a
disconnected port directly into the suspend mode, since the port never exits the disconnected state.
Table 11-1 summarizes hub port behavior in different port states for different types of signaling. Hub
behavior during resume signaling when the hub itself is in the suspend state constitutes a special case,
and is discussed in Section 11.5.2.1.
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Table 11-1. Port Behavior vs. Signaling
Signaling\State Powered-off Disconnected Disabled Enabled Suspended
Reset on root port
(hub with power
switching)
Stay in
Powered-off Go to Powered off Go to Powered
off Go to Powered
off Go to Powered
off
Reset on root port
(hub without power
switching)
N.A. Go to
Disconnected Go to
Disconnected Go to
Disconnected Go to
Disconnected
ClearPortFeature(P
ORT_POWER)(hub
with power
switching)
Stay in powered-
off Go to Powered off Go to Powered
off Go to Powered
off Go to Powered
off
SetPortFeature(PO
RT_POWER)(hub
with power
switching)
Go to
Disconnected N.A. N.A. N.A. N.A.
SetPortFeature(PO
RT_RESET)
request
Stay in Powered
-off Go to Enabled Go to Enabled Stay in Enabled Go to Enabled
SetPortFeature(PO
RT_ENABLE)
request
Ignore Ignore Go to Enabled Stay in Enabled Ignore
ClearPortFeature(P
ORT_ENABLE)
request
Ignore Ignore Stay at
Disabled Go to Disabled Ignore
Downstream packet
traffic (hub awake) Do not propagate Do not propagate Do not
propagate Propagate traffic Do not
propagate
Upstream packet
traffic (hub awake) Do not propagate Do not propagate Do not
propagate Propagate traffic Set status field,
do not
propagate
SetPortFeature(PO
RT_SUSPEND)requ
est
Ignore Ignore Ignore Go to Suspend Ignore
ClearPortFeature(P
ORT_SUSPEND)
request
Ignore Ignore Ignore Ignore Go to Resume
Disconnect detect Ignore Ignore Go to
Disconnected Go to
Disconnected Go to
Disconnected
Connect Event Ignore Go to Disabled N.A. N.A. N.A.
11.2.3.1 Device Detach Detection
A hub is able to detect a detach event by means of a continuous SE0 persisting for at least 2.5 µs detected at a
downstream port. In response to a detach event, the hub places the port into the Disconnected state and floats
its output buffers to a Hi-Z. Device detach can only be detected while there is no downstream traffic on the
bus. If power is removed from a port, it must respond by registering a detach event and placing the port in the
Powered Off state. This guarantees if a device is detached and a new one added that the attach event will be
acknowledged.
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11.2.4 Bus State Evaluation
Bus state evaluation is done at the end of the frame and is able to discriminate between the SE0, the
differential 1 and 0 bus states. When no device is connected to a downstream hub port, its pull-down
resistors pull both D+ and D- below VSE(min).
Connect/Disconnect detect can only be performed after Vbus is applied to the downstream port. (This
requirement only affects hubs whose downstream ports are power switched.) When a device is
connected, the bus state changes from the disconnected to the attach detect state. Low speed devices pull
up D- to an SE1 and leave D+ at SE0. Full speed devices pull up D+ to an SE1 and leave D- at SE0.
Each downstream hub port must be capable of detecting and differentiating between low speed and full
speed device connections once a device is connected. The differential J and K states are undefined until
a device is attached and the device’s speed has been ascertained.
When a connect or disconnect occurs, it must be reflected in the hub status by the end of the frame in
which the event occurred unless the hub is in the reset or suspend modes. A hub in the suspend mode is
awakened by a connect or disconnect event and must be capable of reporting the event upon completion
of resume. Upon coming out of reset, a hub must detect which downstream ports have devices connected
to them. Connect and disconnect changes are reported on a per-port basis.
11.2.5 Full vs. Low Speed Behavior
Hubs must differentiate between full speed and low speed devices when a device is connected to the bus
or at power-up. Hubs detect whether a device is full or low speed when the hub port transitions from its
disconnected to its disabled state. Devices attached to a hub are determined to be either full speed or low
speed by detecting which data line (D- or D+) is pulled high. Low speed devices pull D- high, and full
speed devices pull D+ high. Full speed signaling must not be transmitted to low speed devices. Doing so
could cause low speed devices to mistakenly respond to full speed signaling and create a bus conflict.
Communication between the host and the hub controller are always done using full speed signaling.
If a device is detected to be low speed, the hub port’s output buffers are configured to operate at the slow
slew rate (75-300 ns), and the port will not propagate downstream directed traffic unless it is prefaced
with a preamble PID. Low speed signaling immediately follows the PID and is propagated to both low
and full speed devices. Full speed devices will never misinterpret low speed traffic because no low speed
data pattern can generate a valid full speed PID. When low speed signaling is enabled, a hub continues
to propagate downstream signaling to all enabled ports until a downstream EOP is detected, at which
time the output drivers for the low speed ports are turned off and will not be turned on again until the hub
receives another PRE PID. If a port is disabled, no signaling is propagated to the port. Hubs must enable
their low speed port drivers within four full speed bit times of having received the last bit of the PRE
PID, and at that time they must drive a low speed J onto the bus.
If a downstream port is enabled, it propagates upstream directed bus signaling independently of whether
the port was configured as low speed or full speed. Hubs implement slew rate selectable output buffers
only in the downstream direction on their downstream ports; in the upstream direction, they transparently
propagate both low and high speed traffic using fast (4-20 ns) edge rates. Low speed devices do not
append a preamble onto their upstream traffic.
While propagating low speed traffic upstream, hubs must be able to respond to EOPs that are either two
low speed bit times or two full speed bit times wide. The former will occur under normal operation in
which the low speed device generates the EOP. The latter occurs if a downstream hub encounters an end
of frame babble or LOA condition and generates an EOP upstream in response (refer to Section 11.4.5).
11.2.5.1 Low Speed Keep-Alive
All hub ports to which low speed devices are connected must generate a low speed keep-alive strobe,
generated at the beginning of the frame, which consists of two low speed bit times of SE0 followed by at
least 0.5 bit times of J state. Low speed devices use the strobe to prevent themselves from going into
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suspend in the absence of low speed bus traffic. The hub repeater generates the keep-alive from its
internal SOF counter, and the keep-alive strobe must start no sooner than the second EOF point and must
complete no later than the EOP of the token packet.
The low speed keep alive strobe must be generated once per frame, and it must track the SOF token such
that the following rules are adhered to.
1. When going into suspend, the keep-alive must not go away before the last SOF.
2. A hub is allowed to synthesize no more than three keep alive strobes after receipt of the last SOF.
3. Keep alive must occur no later than one frame after resumption of SOF.
11.2.6 Hub State Operation
The hub state operation is shown in Figure 11-6. Upon coming out of reset or power-up, a hub starts in
the WFSOP state. The hub waits for a start of a packet (SOP) to be detected on its root port or any of its
enabled downstream ports. If an SOP is detected, the hub establishes connectivity originating from the
port on which the SOP occurred and transitions to its WFEOP state. It remains in this state until an end
of packet (EOP) is encountered or until the end of frame (EOF) occurs. Under normal circumstances, and
when not near the end of a frame, a hub repeater will transition back and forth between WFSOP and
WFEOP.
A hub in the idle (WFSOP) state responds to the end of frame (EOF1) point by transitioning to the
WFSOF state. If a hub is in the WFSOP state at EOF1, it transitions to the WFSOF state. Transitions
from WFSOP and WFEOP to WFSOF are not errors, but simply indicate that the hub is nearing the end
of its frame and cannot establish connectivity until the start of the next frame.
WFEOF2 is a special state which is entered only when a babble or loss of bus activity (LOA) is detected
near the end of a frame and upstream connectivity is established. If a hub repeater is still in the WFEOP
state (i.e., it has not received an EOP) when the EOF1 point is encountered, it transitions to the WFEOF2
state. It will remain there until its EOF2 point or an upstream EOP occurs, at which time it transitions to
WFSOF and awaits the next Downstream SOP (DSOP), which will normally be the SOP associated with
the SOF packet, and indicates the start of the next frame. When a DSOP occurs, the hub returns to the
WFEOP state and waits for the end of the packet. If the end of a frame occurs and a hub still is
maintaining downstream connectivity, the hub does not transition out of the WFEOP state; instead it
waits for the next downstream EOP (usually the EOP of the SOF token), which will cause the hub to
transition to the WFSOP state.
If a hub is still in its WFEOF2 state when EOF2 occurs, the port that established upstream connectivity
must be disabled, regardless of the bus state. If, when EOF2 occurs, a hub is in the WFSOF state, and is
driving upstream connectivity, and its bus is in the idle state, then the previously connected port’s state
must remain unchanged. If the hub is in the WFSOF state at EOF2 and driving upstream connectivity,
but the bus is not in the idle state, the port must be disabled.
A hub will transition to the WFSOF state upon coming out of resume. Doing so guarantees that traffic
cannot propagate upstream (and possibly lock up the bus) until the hub’s frame timer has detected at least
one SOF. The details of the interaction between the frame timer and the hub repeater state machine are
described in Section 11.5.1.1.
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Reset
WFSOP ELSE
SOP
WFSOF !SOP
WFEOP ELSE
EOF2 # EOP
SOP
EOF1
WFEOF2 ELSE
EOP # EOF2
EOF1
Resume
Figure 11-6. Hub Repeater States
The hub repeater maintains state across each packet that is detected and repeated by the hub. The
repeater state machine does not need to track more than a single packet and need not, for example, track
across multiple packets in a transaction. Figure 11-7 shows how hub states change in the course of a
normal packet transmission.
...
WFSOP WFEOP WFSOP
IDLE JK SE0
Hub enables output buffers Hub disables outputs
IDLE
JK
Figure 11-7. Hub States Across a Packet
11.2.6.1 Hub Repeater States
For upstream connections, a hub repeater transitions between four states: wait for start of packet
(WFSOP), wait for end of packet (WFEOP), wait for EOF2 point (WFEOF2), and wait for start of frame
(WFSOF). The EOF1 and EOF2 points are described in Section 11.4.5.1. The four states are described
below. For downstream connections, the hub transitions between WFSOP, WFEOP, and WFSOF.
11.2.6.1.1 Wait for Start of Packet
The Wait for Start of Packet (WFSOP) is the state a hub occupies when there is no packet currently being
propagated to or through the hub. Hubs transition to their WFSOP state upon coming out of reset. In the
WFSOP state, all of a hub’s ports are in the high impedance state, and all of its enabled ports are in the
receive mode with their output buffers in the Hi-Z state. If the root port or any enabled downstream port
detects an SOP, the hub establishes connectivity and transitions to the Wait for End of Packet state.
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11.2.6.1.2 Wait for End of Packet
During the Wait for End of Packet (WFEOP) state, the hub has established its connectivity and is
receiving packet traffic on one of its ports. The hub transparently propagates the traffic in either the
upstream or downstream direction. Connectivity is maintained until the hub transitions out of this state.
A hub transitions out of the WFEOP state when it detects an EOP or if it encounters an end of frame
(EOF1) point (refer to Section 11.4.4). Detection of EOP causes the hub to transition back to WFSOP
and is the normal sequence. If EOF1 is detected, the hub transitions to the WFEOF2 state.
11.2.6.1.3 Wait for EOF2 Point
The WFEOF2 state is entered only when the hub detects its EOF1 point and is still waiting for an EOP
from a downstream port. This condition is potentially indicative of babble or loss of bus activity. A hub
repeater remains in the WFEOF2 state until an EOP is detected or until its EOF2 point occurs.
11.2.6.1.4 Wait for Start of Frame
A hub repeater enters the Wait for Start of Frame (WFSOF) state either when EOF1 is detected and the
hub is in the WFSOP state (normal end of frame behavior) or when the hub is in the WFEOF2 state and
an EOP or EOF2 point is detected (babble/LOA) behavior.
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11.3 Hub I/O Buffer Requirements
All hub ports must be able to detect and generate the J, K, and SE0 bus signaling states. This requires
that hub ports be able to independently drive and monitor their D+ and D- outputs. Each hub port must
have single ended receivers on the D+ and D- lines as well as a differential receiver. Details on voltage
levels and drive requirements appear in Chapter 7. Figure 11-8 shows I/O circuitry for a typical hub port.
D+
D-
RxD+
RxD-
Differe ntial Re ceiver
Single-e n ded Receivers
OE
TxD+
Output Buffers
Vcc
Rpu
Only on
root port
Only on
downstream
ports
Rpd
TxD-
RxD
Figure 11-8. Hub Port I/O Driver and Receiver
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Table 11-2 defines the hub I/O section’s input and output signals.
Table 11-2. Hub I/O Section Signals
Signal Name Direction Description
D+, D- I/O External USB data lines
RxD O Received differential data
RxD+ O Received single-ended value on D+ line
RxD- O Received single-ended value on D- line
TxD+ I Transmitted data value
TxD- I Transmitted data value
OE I Output enable/disable on output buffers
D+ and D- are the I/O lines that connect to the USB physical medium. When placed in the Hi-Z state,
they are pulled to near the ground or Vcc rails by resistors on the hub and device. RxD is the differential
received data. RxD+ and RxD- are the received single ended data. TxD+ and TxD- are used to send
differential data and single ended reset and EOP signaling. OE disables the output drivers.
11.3.1 Pull-up and Pull-down Resistors
Hubs, and the devices to which they connect, use a combination of pull-up and pull-down resistors to
control D+ and D- in the absence of their being actively driven. These resistors establish voltage levels
used to signal connect and disconnect and also maintain the data lines at their idle values when not being
actively driven. Each hub downstream port requires a pull down (R pd) on each data line; the hub root
port requires a pull-up (Rpu) on its D+ line. Values for Rpu and Rpd appear in Chapter 7.
11.3.2 Edge Rate Control
Downstream hub ports must support both low speed and full speed edge rates. Full speed signaling
specifies a rise/fall time of 4-20 ns. Low speed rise/fall times must be within a 75-300 ns range. Edge
rate on a downstream port must be selectable, based upon whether a downstream device was detected as
being full speed or low speed. The hub root port always uses full speed signaling, and its output buffers
always operate with full speed edge rates.
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11.4 Hub Fault Recovery Mechanisms
Hubs are the key USB component for establishing connectivity between the host and other devices. It is
vital that any connectivity faults, especially those that might result in a deadlock, be detected and
prevented from occurring. Hubs need to handle connectivity faults only when they are in the repeater
mode. Hubs must also be able to detect and recover from lost or corrupted packets which are addressed to
the hub controller. Since the hub controller is, in fact, another USB device, it must adhere to the same
time-out rules as other USB devices, as described in Chapter 8.
11.4.1 Hub Controller Fault Recovery
The hub controller must be able to respond to and recover from corrupted and missing packet
transmissions. These include lost or corrupted token, data, and handshake packets. The following table
describes the possible field level errors which the hub controller can detect and its responses.
Table 11-3. Packet Error Types
Field Error
Action
PID PID check, bit stuff
Ignore packet
Address Bit stuff, address CRC
Ignore token
Data Bit stuff, data CRC
Discard data
11.4.2 False EOP
Hub handling of false EOP differs depending on whether the hub is operating as a repeater or is being
accessed as a device. A hub operating as a repeater transparently propagates signaling, and cannot
differentiate between a “good” EOP and a “false” EOP. If any EOP occurs, the hub tears down
connectivity and waits for the next SOP. If the packet transmitter continues sending, the hub re-
establishes connectivity on the next J to K transition. From a hub’s point of view, a false EOP makes a
single packet look like two separate packets. The hub does not participate in false EOP error detection or
recovery process when operating in the repeater mode.
When a hub is accessed as an USB device, the hub controller detects and recovers from false EOP the
same as any other USB device, as described in Section 8.7.3.
11.4.3 Repeater Fault Recovery
Hubs must be able to detect and recover from conditions that leave them waiting indefinitely for an end
of packet or that leaves the bus in a state other than the idle state at the end of a frame, i.e., an SOP
without an EOP. There are two such hub fault conditions: loss of activity and babble. Loss of activity
(LOA) is characterized by detection of a start of packet (SOP) followed by lack of bus activity (bus is
stuck at the idle or K state) and no end of packet (EOP) by frame’s end. Babble is characterized by SOP
followed by the presence of bus activity continuing past the end of a frame. Hubs have no notion of
allocated bandwidth and must rely upon the frame timer to detect LOA and babble conditions. The
recovery mechanism requires that hubs track the host’s frame timing and recover before the beginning of
the next frame.
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Hub fault recovery operates only in the upstream direction. The host is responsible for detecting and
recovering from its own downstream directed errors. Hub repeater fault recovery must meet the
following requirements:
• Devices driving illegal states at the end of a frame must be isolated from the bus by disabling the
downstream hub port to which the device is connected.
• Hubs must return the bus to the idle state before the start of the next frame if the connectivity was
previously established in an upstream direction.
Under non-fault conditions, these requirements are met by virtue of a hub receiving an EOP with every
packet and having no bus traffic occur past the end of a frame. Before describing how hubs implement
fault recovery, the hub frame will be described.
11.4.4 Hub Frame Timer
Each hub has a frame timer whose timing is derived from the host-generated SOF token and tracks the
host SOF packet in both phase and period. The frame timer is reset each time an SOF is detected and is
responsible for generating End of Frame (EOF) points. The hub frame timer must track the host SOF and
be capable of remaining synchronized to the host SOF for the loss of up to two consecutive SOF tokens.
All hubs must have an EOF timer, and it is used to identify two distinct points in time: a point (EOF1) at
which time the hub must terminate its own upstream connectivity, and a point (EOF2) by which time the
bus must see upstream traffic terminated. The delay between EOF1 and EOF2 corresponds to the timing
skews between the host and the hub plus time required for EOP to occur and is illustrated in Figure 11-9.
HOST - HUB
TIMING SKEW
SOF Command
SYNC SOF
PID
1st EOF
Point
EOF1 EOF2
2nd EOF
Point
EOP
Figure 11-9. Host-Hub EOF Skew
The frame timer must to synchronize to the host’s frame timing for worst case tolerances and offsets
between the host and hub. The offsets have to accommodate the hub oscillator tolerance and purposely
introduced deviations from 1.000 ms frame timing which occur when the host locks to an external source.
The above parameters require a minimum frame length (FLmin) of 11,985 and a maximum frame length
(FLmax) of 12,015.
11.4.4.1 Frame Timer Synchronization
The frame timer is clocked from the hub’s clock source and is synchronized via SOF packets to the host’s
frame time. When a hub is first connected to the host the hub’s frame timer is unsynchronized with
respect to the host. When the first SOF token is detected the frame timer resets and starts counting once
per 12 MHz clock cycle; it continues to count up until the next SOF token is detected. At this time the
frame timer is reset and the previous count value is stored as the previous frame length (PFL) count. This
value is updated for each frame in which an SOF is detected and represents, in hub clock counts, the
host’s frame time. The frame timer is considered locked to the host when the difference between the PFL
count and the current count at the end of frame is less than 2 and the PFL lies between FLmin and FLmax.
Should an SOF token fail to be received, the hub uses the previous frame length count as a best
approximation to the current frame length. The frame timer is guaranteed to remain synchronized to the
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host for the loss of up to two consecutive SOF tokens. Hubs must be able to synchronize to the host
within 3 frames after detecting the first SOF packet (assuming no missing SOF packets). When a hub
comes out of resume its frame timer is not synchronized with that of the host. During this time, when the
hub is in the active state, it must be prevented from establishing upstream connectivity until the hub
frame timer is synchronized to the host.
The hub’s error recovery, as described in Section 11.4.5, is operational whenever the hub frame timer is
locked. If the frame timer is not locked the timer uses FLmin as the default frame length.
11.4.5 Hub Behavior Near EOF
Hub behavior near the end of frame is diagrammed in Figure 11-10. There are two end of frame timing
markers, EOF1 and EOF2, corresponding to the first and second EOF points. All hubs receiving
upstream traffic, upon detection of EOF1, transmit an EOP upstream for two full speed bit times, drive
the bus to the idle state for one bit time, and then float the bus. Starting with EOF1, hubs are not
permitted to re-establish upstream connectivity until the end of the next downstream packet which is
usually the next SOF token.
HOST - HUB
TIMING SKEW
SOF Comman d
SYNC SOF
PID
Permit downstream
connectivity to be
established
Issue an EOP
upstream Sample all ports , di sable
if not in IDLE and WFSOF
Watch for EOP from downstream
if in WF EO F2 state
Upstream connectivit y cannot be re-established
Detection of EOP places hub
into WFSOFstate
EOP
WFEOF2WFEOP WFSOF WFSOPHub
State
1st EOF
Point
2nd EOF
Point
WFEOP
Figure 11-10. Hub Behavior Near End of Frame
Hub behavior at the end of frame is summarized below:
1. Hub recovery behavior is applicable only for upstream traffic. If a hub encounters the end of frame
and downstream connectivity is established, it maintains connectivity and waits for the next
downstream EOP, at which time it tears down connectivity and returns to the WFSOP state.
2. At the EOF1 point, all hubs will transmit an upstream EOP, followed by an idle state, and then float
the bus, unless connectivity is already established in the downstream direction. The EOP must not
truncate or lengthen any upstream directed EOP already in progress.
3. Hubs will not allow further connectivity to be established in the upstream direction after EOF1.
4. Hubs that were in the WFEOP state at EOF1 must watch for EOPs on the downstream port on which
connectivity was established. They must monitor the bus from EOF1 to EOF2.
5. If an EOP from downstream is detected by a hub in the WFEOF2 state in the EOF1 to EOF2
window, the hub will transition to the WFSOF state and should see an idle on its port.
6. At EOF2, all ports will be sampled for their state, and a port will be disabled if it is not in the proper
state (refer to Table 11-4). At EOF2, hubs still in WFEOF2 transition to the WFSOF state.
Connectivity is still not allowed from downstream until after a packet is received from the host.
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Table 11-4. Hub Behavior at EOF2
Not Idle State Idle State
WFSOF Disable port Do not disable port
WFEOF2 Disable port Disable port
11.4.5.1 Skew Requirements
The host and hubs, while all synchronized to the host’s SOF, are subject to certain skews which dictate
the length of time between the EOF points, host behavior near EOF, and the next SOF. Figure 11-11
illustrates critical end of frame timing points.
30
40
50 20 10 0
Bit times
EOF2 rangeEOF1 range
SOF
Latest
host
packet EOF2EOF1
Figure 11-11. EOF Timing Points
11.4.5.2 Host-Hub Skew
The timing skew between the host’s SOF point and the hub’s SOF timer is minimized by the requirement
that the hub track the host. Sources of skew include the fact that hubs may miss SOFs, and that the host
frame counter can be adjusted to track an external master clock. The 12 MHz clock is the only clock
actually specified, so this is the best granularity available by specification. Assuming a fixed host SOF
timing and that two consecutive SOFs can be missed, the maximum cumulative host-hub skew without
host timing wander is ±3 clocks. Assuming that the host clock may be adjusted by up to one bit time per
frame, then the host can walk away from the hub by 1 + 2 + 3 = 6 clocks. The maximum host-hub skew
is the sum of these two components or ±9 clocks.
The second EOF point must be sufficiently separated from the SOF point to permit hubs to recover and
be ready to receive the SOF token from the host. A hub must finish sending its EOP before a hub to
which it is attached reaches its second EOF point. This means that all hub EOF2 points must occur at
least one bit time before the host issues SOF. All hub EOF2 points must lie within a ±9 bit time window;
therefore, EOP must lie outside this window and complete at least 2 x 9 + 1 = 19 bit times before host
SOF.
The next step is calculating how long it takes to generate EOP and how far back from SOF it must occur.
Transmitting EOP requires four bit times. Therefore, a hub must start sending its EOP no later than
19 + 4 = 23 bit times before SOP. For a hub to be sure that it starts no later than the 23rd bit time, it must
start 9 bit times before that or at bit time 32, which is the value of the first EOF point. The earliest that a
hub might start sending EOP is 9 bit times before the first EOF point or at bit time 41.
A hub must not see a packet from the host start after the hub reaches its first EOF point. This could be as
early as 41 bit times before SOF. Hub propagation delay must also be figured into the delay budget. The
per-hub delay is approximately one bit time; so for a worst case topology of six hubs away from the host,
there will be an additional 6 bit times of delay. Therefore, the host’s EOF point for transmit is
41 + 6 = 47 bit times from SOF, relative to the host’s SOF timer. If the host is still transmitting at bit 47
and not able to complete before SOF, it must force an error via a bit stuffing violation (at least eight 1’s),
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followed by an EOP. If the host is still receiving a packet or an EOP at bit 41, it should treat the packet
as being in error. Table 11-5 summarizes hub and host EOF timing points.
Table 11-5. Hub and Host EOF Timing Points
Description
Number of
Bits From
Start of SOF Notes
EOF1 32 End of frame point #1
EOF2 10 End of frame point #2
Host invalidates full speed
Tx packet 47 Latest that host may start a full speed
packet
Host invalidates low
speed Tx packet 184 Latest that host may start a low speed
packet (rounded up to the nearest low
speed bit time)
Host invalidates Rx
packet 41 Host treats any packet still being
received at bit time 41 as bad
11.5 Suspend and Resume
Hubs must support suspend and resume both as a USB device and also in terms of propagating the
suspend and resume signaling. Hubs support both global and selective suspend and resume. Selective
suspend and resume are implemented via per port enable/disable. Global suspend is implemented by the
host through the hub’s root port. Global resume may be initiated either from the host or from a hub’s
downstream port.
11.5.1 Global Suspend and Resume
Global suspend is initiated by the host shutting off downstream traffic to the entire bus. A hub enters the
suspend state if it detects a continuous idle state on its root port for at least 3.0 ms. When placed into the
suspend state, a hub puts its repeater into the WFSOP state, floats all of its output drivers, maintains static
values of all its control and status bits, and preserves the current state information for each of its
downstream ports. A suspended hub has its clocks turned off, so it has no concept of time and can only
respond to bus transitions.
Hub resume may be initiated by any bus transition on a hub’s root port or on a downstream port in the
enabled state. Hub resume may also be initiated by the connect/disconnect of a device on a downstream
port in the disconnected, disabled, or suspended states. If a transition occurs on an enabled downstream
port then the hub immediately reflects an idle to a resume bus transition to that port, all other enabled
downstream ports, and its root port. Figure 11-12 shows the timing relationships during a resume
sequence in which a device initiates a wake up to a hub.
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Device
Function
Hub Port
Other DS
Hub Ports
Hub
Root Port
Host Port
Resume stateIDLEstate
Resume stateIDLEstate
Resume stateIDLEstate
Resume state
IDLEstate
Resume stateIDLEstate
Hub signaling DS only
Host ends resume
signaling (EOP)
Host begins resume signaling
Hub begins resume signaling
Remote Wake-up
All devices in
suspend state
Last bus activity
Bus Driving
Bus Idle or
Driven at other end
Device
Hub
Host
T=0
T=t1
T=t2
T=t3T=t4
t1 < 50 µµsec
50 µµsec < t2 < 10 msec
t3 > 20 msec
t4 > 3 msec after EOP
Figure 11-12. Resume Signaling
There are four time parameters that hubs must conform to, as shown in Figure 11-12. T=0 represents the
time when the resume signaling arrives at the port. t1 is the time by which a hub must respond by driving
resume onto the upstream port and all enabled downstream ports. t2 is the time at which the hub must
stop driving an upstream initiated resume to its root port and reflect the bus state now being driven to its
root port onto its downstream ports. The interval, t3 is the time that a hub must generate downstream K
signaling to the device that initiated the resume. t4 is the time that the hub must wait after detecting the
downstream low speed EOP until it can set its interrupt bit, indicating that a resume has occurred after a
selective suspend.
When a device drives an idle to resume transition upstream to the hub the hub responds by driving the
resume (K) state onto the bus to the root port and to all enabled downstream ports including the port
which initiated the resume. (It is acceptable to drive both ends of a bus segment to the same state.) After
driving the resume state, the hub begins the process of returning to a fully awake state (e.g., restart
clocks). When the hub is awake, it will reverse the connectivity so that the K state on the hub’s root port
is sustaining the K on its downstream ports. (This scenario assumes that the hub immediately above the
one in question has received resume signaling and is now driving it downstream.) A hub may not reverse
the connectivity any faster than 50 µs, nor slower than 10 ms after receiving a resume from a
downstream. The t2 parameter also implies that a hub must be fully awake no later than 10 ms after
receiving a resume request.
The resume signal propagates upstream until it reaches the host. The host reflects the resume signaling
downstream for at least 20 ms, which guarantees that all devices will have time to wake up and detect the
downstream resume signaling. The host terminates the resume sequence by driving an EOP for two low
speed bit times. The EOP is interpreted as a valid end of packet, causes all hubs to tear down their
connectivity and informs all devices on the bus that the resume sequence has completed. The device that
initiated the resume must wait until it detects EOP to determine that the resume sequence has completed.
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Hubs must be able to propagate downstream traffic immediately after the end of resume to prevent
downstream devices from re-entering the suspend state. The hub controller must be able to receive
packet traffic no later than 10 ms after the end of resume. Note: a remote wakeup device may not start a
resume sequence until 5 ms after the last bus activity. This allows the hub to go into the suspend state so
that it will resume all ports and not just the full speed ones. Table 11-6 summarizes the behavior of hubs
in response to host initiated, downstream signaling.
Table 11-6. Suspended Hub Behavior During Global Resume
Port Status and Signaling Type Downstream Port Response
Port enabled, resume (K) received Signal resume downstream
Port disabled, resume (K) received Do nothing
Port suspended, resume (K) received Signal resume downstream
11.5.1.1 Resume and Hub Frame Timer
When a hub transitions from the suspended to the awake state, its frame timer is not operational and the
hub’s EOF timing recovery circuitry will not work until the first SOF has been received. To prevent a
babbling downstream device on a recently resumed bus segment from locking up the bus it is necessary
to prevent hubs from propagating upstream traffic while the hub is in the awake state until the hub’s
frame timer has started. This is achieved by making the hub repeater state machine transition to its
WFSOF state upon coming out of resume. While the hub repeater is in the WFSOF state all upstream
traffic is ignored. The repeater state machine remains in the WFSOF state until an SOF is detected, at
which time it transitions to the WFEOP state. Downstream traffic is unaffected by the status of the frame
timer. To prevent needless time-outs it is recommended that the host not send any packets addressed to
devices on a just resumed bus segment until the host has issued at least one SOF token to the recently
resumed hub or hubs. This condition is guaranteed by virtue of the fact that a hub does not report a
resume detect to the host until 3.0 ms after the resume sequence completes.
11.5.2 Selective Suspend and Resume
Selective suspend and resume provide a means for placing a single device or a bus segment into a low
power state. Selective suspend relies on the ability of a hub to selectively suspend individual ports via a
SetPortFeature(PORT_SUSPEND) request, which places a port on a hub (referred to as the disabling hub)
into the suspend state (see Figure 11-4 and Figure 11-5 for state diagrams). In the suspend state a port is
prevented from propagating any bus activity (except the port reset request) downstream, and the port can
only reflect upstream bus state changes via the hub’s status bits; i.e., an awake hub cannot propagate
upstream traffic from a suspended port to its root port. The hub must also insure that the port accessed
via the port suspend request is not suspended in the middle of a packet transaction. In response to a port
suspend, all devices downstream of the targeted port go into suspend after failing to see bus activity for
3.0 ms while the bus is in the idle state The port suspend request is only understood by an awake hub. If
the host wishes to send the request to a suspended hub, it must first wake the hub and then issue the
desired request.
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11.5.2.1 Selective Resume to an Awake Hub
Figure 11-13 shows the signaling between a device and an awake hub (Hub Y). If a hub port is
selectively suspended and the hub is in the awake state, then the hub must prevent any bus activity on the
suspended port from being reflected to other enabled ports on the hub, as these ports may be transporting
bus traffic. In addition, the resume on Port B must not propagate to other suspended ports (such as
Port C). The solid line represents the extent of the resume signaling, which stops at Port B of Hub Y.
Resume signaling may occur either from a host request to directly re-enable the hub port (downstream
resume) or from a device on the suspended bus segment (upstream resume).
Dev B
Hub X
Hub Y
Awake
Ports B and C suspended
IDLE -> Resume or
SE0 -> I DLE or
IDLE -> SE 0
Resume -> Resume
A
B
C
A
B
C
Port A active
Awake
Figure 11-13. Selective Resume Signaling on an Awake Hub
Upstream resume may be initiated by an idle to resume (device issues resume), an idle to SE0 (device
disconnect) or an SE0 to idle (device attach) bus transition on a suspended port. Hub Y, Port B must
reflect a resume state within 50 µs of receipt of resume to inform the remote wakeup device (Device B)
that its resume has been detected. If the transition on Port B was not idle to resume (J to K), Port B does
not reflect any signaling downstream but continues to maintain its output buffers in the Hi-Z state. This
latter case covers connect and disconnect events.
The receipt of resume signaling by Port B of Hub Y causes the following sequence of events to occur.
1. Hub Y, Port B reflects the resume signaling downstream within 50 µs of resume detection.
2. Hub Y maintains resume signaling downstream for at least 20 ms.
3. At the end of 20 ms, the hub terminates the resume with a low speed EOP (Downstream traffic starts
flowing through the port at this time.)
4. 3.0 ms after the end of EOP, an interrupt in the hub controller is set.
The hub is responsible for maintaining all the resume timing parameters. The host need not keep track of
any time and can poll the hub via a GetPortStatus request to determine that a resume event has occurred.
Since the hub process selective resumes autonomously, the host will not receive direct signaling in the
event of a selective resume that does not originate from the host. It is necessary for the host to poll
downstream hubs (as part of its status polling) to determine that a selective resume event has completed.
The 3.0 ms delay between the end of resume signaling and the setting of the interrupt bit guarantees that
no packet traffic with a newly resumed device will occur until newly resumed devices have had time to
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synchronize their frame timers with SOF; however, downstream devices will see traffic and SOF tokens
not specifically addressed to them. When a resume is initiated by a downstream device, the host polls for
the end of selective resume as part of its status polling The hub must also insure that the port is not re-
enabled in the middle of a packet transaction.
Downstream selective resume may also be initiated via a ClearPortFeature(PORT_SUSPEND) request;
This request causes the disabled port to drive resume signaling onto the bus for at least 20 ms, followed
by a low speed EOP. As with a device initiated resume, the interrupt bit in the hub controller, signaling
the end of resume, must not be asserted until 3.0 ms after the end of the low speed EOP. The host must
poll the hub interrupt to determine when the resume has completed. Table 11-7 summarizes the behavior
of an awake hub in the presence of resume signaling originating from downstream.
Table 11-7. Awake Hub Behavior During Resume
Port Status and
Signaling Type Signaled Port
Response
Adjacent
Enabled Port
Response
Adjacent
Disabled Port
Response
Adjacent
Suspended
Port
Response
Port suspended,
resume (K) received Reflect K
downstream on
signaled port.
Initiate port wake-
up. Set status bits
and interrupt.
Do nothing Do nothing Do nothing
Port enabled,
disabled or
suspended and
disconnect received
Set port disconnect,
disable and change
status bits. Set
interrupt.
Do nothing Do nothing Do nothing
Port disabled and
connect received Set port connect,
and change status
bits. Set interrupt.
Do nothing Do nothing Do nothing
11.5.2.2 Selective Resume to a Suspended Hub
It is possible for the host to suspend a hub port and then suspend the entire hub. In this case, the hub’s
connectivity changes once the hub enters the suspended state. Figure 11-14 illustrates device initiated
connectivity for suspended ports on a suspended hub. Device B may issue connect, disconnect, or
resume signaling to Hub Y, Port B. In response, Hub Y must convert that signaling into a (J to K)
transition, regardless of what signaling device B sends upstream. The hub drives the resume state to the
root port and to other enabled downstream ports (Port A). Hub Y must drive Port B to the resume state if
Device B drove the bus to the resume state. If Port B detects a connect or disconnect event, the hub does
not drive a resume downstream to the signaling port, but maintains the port’s output buffers in the Hi-Z
state.
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Dev B
Hub Y
Suspended
Port, B and C
suspended
Hub X
Awake
Port B suspended
IDLE -> K
IDLE -> Resum e or
SE0 -> IDLE or
IDLE -> SE0
IDLE -> Resum e
A
B
C
A
B
C
Resume -> Resume
Port A active
Figure 11-14. Device-Initiated Resume for Suspended Hub
Table 11-8. Suspended Hub Behavior During Resume
Port Status and
Signaling Type Signaled Port
Response
Adjacent
Enabled Port
Response
Adjacent
Disabled Port
Response
Adjacent
Suspended
Port
Response
Port enabled,
resume (K)
received
Reflect resume
upstream and
downstream. Do
not set status bits
Signal resume
downstream
Do nothing
Do nothing
Port disabled,
resume (K)
received
Do nothing Do nothing
Do nothing
Do nothing
Port suspended,
resume (K)
received
Reflect K upstream
on root port and
downstream on
signaled port
Signal resume
downstream
Do nothing
Do nothing
Port enabled,
disabled or
suspended and
disconnect
received
Reflect resume
upstream. Start hub
wake-up. Update
port connect and
change status bits,
set interrupt
Signal resume
downstream
Do nothing
Do nothing
Port disabled and
connect received Reflect resume
upstream. Start hub
wake-up. Set port
connect, and
change status bits,
set interrupt
Signal resume
downstream
Do nothing
Do nothing
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Host initiated signaling for suspended hubs is shown in Figure 11-15. Waking up a device at the bottom
of a string of suspended hubs is a multi step procedure which is described below.
1. The host takes Port B in Hub X out of suspend by issuing a port resume request to the hub. In
response to the resume request, Hub X initiates the resume signaling by driving at least 20 ms of K
signaling followed by a low speed EOP out its Port B. Hub Y sees the resume signaling and starts its
wake up process. The EOP indicates that the resume sequence is completed. 3.0 ms after the EOP is
issued by Hub X, the resume complete status bit in Hub X’s controller is set indicating that the
resume sequence is completed. At the end of this stage Hub X, port B is in the enabled state and
Hub Y is awake.
2. The next step is taking Port B on Hub Y out of the suspended state to the enabled state. The
procedure is identical to that described in the previous step except that the request is issued to Hub
Y, Port B instead of Hub X. At the end of this step, Port B on Hub Y is enabled and device B has
received resume signaling and is awake. At this time the host may communicate with Device B.
This resume policy permits the bus to be sequentially suspended while permitting any device on the
suspended segment to awaken the bus. It also permits nested hubs to be awakened one tier at a time.
De v B
Hu b X
Hu b Y
Awake
Awake
Port B
enabled
A
B
C
A
B
C
Host sends ResumePort
to Hub Y, Port B
Por t B comes
out of suspend
De v B
Hu b X
Hu b Y
Suspended
Por t B
suspended
Awake
A
B
C
A
B
C
Host sends ResumePort
to Hub X, Port B
Por t B comes
out of suspend
De v B
Hu b X
Hu b Y
Awake
Awake
Port B
enabled
A
B
C
A
B
C
Host can communicate
with Device B
Por t B
enabled
Ports C
remains
suspended
Por t A
awake Port C
remains
suspended
Por t C
remains
suspended
Figure 11-15. Host-initiated Resume Through Suspended Hubs
11.6 USB Hub Reset Behavior
A USB hub must be able to generate a per port reset via a host request and accept reset signaling on their
root port. The following sections describe hub reset behavior and its interactions with resume, attach
detect, and power-on.
11.6.1 Hub Receiving Reset on Root Port
Reset signaling to a hub is defined only in the downstream direction which is at the hubs’ root port. An
awake hub may start its reset sequence if it detects 2.5 µs or more of continuous SE0 signaling and must
complete its reset sequence no later than 5.5 µs of continuous SE0. The 2.5 µs lower limit is set by a
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need to prevent low speed EOP strobes (which are up to 1.3 µs in length) from being interpreted as reset.
A suspended hub must interpret the start of reset edge as a resume signaling event and begin its wake-up
sequence. The hub must be awake and have completed reset no later than 10 ms the completion of reset
signaling
After completion of reset a hub controller is in the following state:
• Hub controller default address is 0
• Hub control bits set to default values
• Hub repeater is in the WFSOP state
• All downstream ports in Powered Off state (power-switched hubs)
• All downstream ports in Disconnected state (non power-switched hubs)
If a bus contains hubs with power-switched ports, the host reset is not guaranteed to propagate all the way
downstream. The host has to guarantee that each tier is reset when it goes through the enumeration
process, and the enumeration reset is done on a tier by tier basis. (However, the powered off devices are
effectively reset, if they are off long enough, and self-powered devices/hubs below them reset themselves
and their downstream ports.)
11.6.2 Per Port Reset
A hub can exercise per-port resets via the SetPortFeature(PORT_RESET) request. This request specifies
a downstream port number. In response to a SetPortFeature(PORT_RESET) request, the hub drives an
SE0 onto its downstream port for at least 10 ms, returns the bus to the idle (J) state, and the places the
port into the enabled state. SetPortFeature(RESET) is an atomic operation; the 10 ms delay between start
and end of reset is controlled by the hub. The hub must be able to return to the host the status of the reset
request; i.e., whether the reset has completed, so that the host does not have to keep track of elapsed time
during the reset operation. The port reset request can be issued to a port in any state; however, no
downstream signaling is generated if the reset is issued to a port in the powered off or disconnected
states.
Device attach detection requires that the port in question be power-switched on (if power switching is an
option). When a device is attached, a hub can detect an attach via an SE0 to DIFF1 or DIFF0 bus
transition. This requires that disabled ports not be driven by the hub while attach detection is being
performed. This should not be a problem, as the port will have been disabled and its output drivers
floated by detection of the previous detach event. The host can determine the device’s speed by
examining whether D+ or D- is pulled high.
Before a port to which a device has been connected can be enabled we must be assured that the device
has been reset. Since it is not possible to rely on loss of Vbus, caused by a disconnect event, to reset the
device, a port must be reset before being enabled. This is performed via an atomic
SetPortFeature(PORT_RESET) request which issues SE0 reset signaling and then enables the port. USB
devices, including hubs, must be able to respond to a host access no later than 10 ms after reset is de-
asserted.
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11.6.3 Power Bringup and Reset Delays
Since USB components may be hot plugged, and hubs may implement power switching, it is necessary to
comprehend the delays between power switching and/or device attach and when the device’s internal
power has stabilized.
Figure 11-16 shows the case where a device is connected to a hub whose port is power switched on.
There are two delays that need to be taken into consideration. ∆t1 is the amount of time required for the
hub port switch to operate. ∆t2 is the time required for the device’s internal power rail to stabilize. If a
device were plugged into an non-switched or already switched on port, only ∆t2 would need to be
considered. ∆t1 is a function of the type of hub port switch, and this parameter may be read via a hub
controller command. ∆t2 must be less than 100 ms for all hub and device implementations. It is
necessary to specify a worst-case upper limit on ∆t2, since it is device specific and cannot be reported
until after the power-on and reset sequences are completed.
Hub port
power-on
∆ t1
Hub port
power OK Device
power OK
Attach
Detect Assert
reset
∆ t2
Figure 11-16. Power-on timing
As Figure 11-16 shows, it is possible to detect a device attach before its internal power has stabilized.
One must guarantee a minimum of 10 ms during which a device’s internal power is stable and reset is
asserted. Therefore, reset cannot be asserted immediately after device attach but must instead wait for
∆t2 to elapse. If the hub implements power switching, ∆t1 + ∆t2 must elapse before reset is applied.
USB devices must power on in such a manner that they do not drive D+ or D- (except with the pull-up
resistor) during the reset process. This is required so the upstream hub can drive reset downstream and be
assured that the downstream device will see the reset signaling.
11.7 Hub Power Distribution Requirements
Hubs can supply a specified amount of power to downstream components and are responsible for
reporting their power distribution capabilities to the host during enumeration. USB requirements
stipulate that generalized legal bus topologies be supported while at the same time preventing power-up
of illegal topologies. An illegal power topology is one that violates the power contract established during
enumeration.
Hubs may be either locally powered or bus powered, or a combination of the two. For example, a hub
may derive power for its SIE and root port pull-up resistors from the bus while obtaining power for its
downstream ports from a local power supply. A hub can only supply power in a downstream direction,
and must never drive power upstream. A complete discussion of hub power distribution appears in
Section 7.2.
Bus powered hubs must have port power switching for its downstream ports and are required to power off
all downstream ports when the hub comes out of power-up, or when it receives a reset on its root port.
Ports may also be switched on and off under host software control. An implementation may provide
power switching on a per port basis or have a single switch for all the ports. Port reset requests do not
affect the status of the power switching for a port. A hub port must be powered on in order to perform
connect detection from the downstream direction.
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11.7.1 Overcurrent Indication
For reasons of safety, all locally powered hubs must implement current limiting on their downstream
ports. Under no conditions may more than 25 VA be drawn from any USB hub port. (The actual
overcurrent trip point may be lower than this figure). If an overcurrent condition occurs, even if it is only
momentary, it must be reported to the hub controller. This is done via an overcurrent state that is
reflected through hub requests. The overcurrent detect state is entered on overcurrent detect and cleared
by a host request or upon reset. Detection of overcurrent must disable all affected ports. If the
overcurrent condition has caused a permanent disconnect of power (such as a blown fuse), the hub must
report it upon coming out of reset or power-up.
Overcurrent protection may be implemented over all downstream ports in aggregate, or on a per port
basis. The ports may optionally be split into two or more subgroups, each with its own overcurrent
protection circuit.
11.8 Hub Endpoint Organization
The Hub Class defines one additional endpoint beyond Endpoint 0, which is required for all devices: the
Status Change endpoint. The host system receives port and hub status change notifications through the
Status Change endpoint. The Status Change endpoint supports interrupt transfers. If the hub has not
detected changes on any of its ports, nor any hub status changes, the hub returns a NAK to requests on the
Status Change endpoint. When the hub detects any status change, the hub responds with data describing
the entity that changed. Host software driving the hub is responsible for examining the data transferred to
determine which entity changed. Hubs are logically organized as shown in Figure 11-17.
Port 1 Port N
Port 3Port 2
Status Change
Endpoint
ENDPOINT 0:
Configuration
Information
UPSTREAM CONNECTION
Figure 11-17. Example Hub Organization
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11.8.1 Hub Information Architecture and Operation
Hub Descriptors and Hub/Port Status and Control are accessible through the default pipe. When a hub
detects a change on a port or when the hub changes its own state, the Status Change endpoint transfers
data to the host in the form specified in Section 11.8.3.
USB hubs detect changes in port states. Devices attached to the ports on a hub can cause various
hardware events. In addition, host system software can cause changes to a hub’s state by sending
commands to the hub. Since there are two sources of changes to the hub, USB hubs report change
information for each of the hardware-caused events. The hub continues to report a status change when
polled until that particular event has been successfully acknowledged by the host. Using this reporting
mechanism, system software determines what changes occurred since the last event reported by the hub.
This approach makes it possible to minimize the device state information that system software must
carry.
Status Information
(static)
Change Information
(due to hardware
events)
Control Information
(change device state)
Host Software (e.g., Hub
Driver)
Change Device
State
Software Device
Control
All Status
Changes
Device Control
Hardware Events
Hardware Events
Figure 11-18. Relationship of Status, Status Change, and Control Information to Device States
Host software uses the interrupt pipe associated with the Status Change endpoint to detect changes in hub
and port status.
11.8.2 Port Change Information Processing
Hubs report a port's status through port commands on a per-port basis. Host software acknowledges a
port change by clearing the change state corresponding to the status change reported by the hub. The
acknowledgment clears the change state for that port so future data transfers to the Status Change
endpoint do not report the previous event. This allows the process to repeat for further changes (see
Figure 11-19.)
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System Software reads the port status for affected ports
Any new changes
detected?
Return to
Beginning
Yes
No
Yes
System Software clears corresponding change state (Acknowledge)
System Software processes port change information
System Software requests Interrupt Pipe notification for Status Change endpoint
Was a change
detected?
NAK
Interrupt Pipe notification retired
Begin
No
Re-initialize Interrupt Pipe for Status Change Endpoint
Interrupt Pipe returns Port Change
Bitmap
Figure 11-19. Port Status Handling Method
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11.8.3 Hub and Port Status Change Bitmap
The Hub and Port Status Change Bitmap, shown in Figure 11-20, indicates whether the hub or a port has
experienced a status change. This bitmap also indicates which port(s) have had a change in status. The
hub returns this value on the Status Change endpoint. Hubs report this value in byte-increments. That is,
if a hub has six ports, it returns a byte quantity and reports a zero in the invalid port number field
locations. System software is aware of the number of ports on a hub (this is reported in the hub
descriptor) and decodes the Hub and Port Status Change Bitmap accordingly. The hub reports any
changes in hub status on bit 0 of the Hub and Port Status Change Bitmap.
The Hub and Port Status Change Bitmap size varies from a minimum size of one byte. Hubs only report
as many bits as there are ports on the hub, subject to the byte-granularity requirement (i.e., round up to
the nearest byte).
012N
Port 1 change detected
Hub change detected
Port N change detected
Port 2 change detected
Figure 11-20. Hub and Port Status Change Bitmap
Any time the Status Change endpoint is polled by the host controller and any of the Status Changed bits
are non-zero, the Hub and Port Status Change Bitmap is returned. Hubs sample the change at the End of
Frame (EOF2) in preparation for a potential data transfer in the subsequent USB frame. If a change was
detected, then data will be transferred through the Status Change endpoint in the subsequent USB frame.
Figure 11-21 shows the sampling mechanism for hub and port change bits.
Per-Port Logic
Logical
OR
Change
Detect Logic Change
Information
EOF2
Hub and Port Status Change Bitmap
Port N
N
Example
Figure 11-21. Example Hub and Port Change Bit Sampling
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11.9 Hub Configuration
Hubs are configured through the standard USB device configuration commands. An unconfigured hub
behaves like all other unconfigured devices with respect to power requirements and addressability.
Unconfigured hubs do not turn power onto the downstream ports. Configuring a hub enables the Status
Change endpoint. System software issues commands to the hub to switch port power on and off at
appropriate times.
System software examines hub descriptor information before configuration to determine the hub’s
characteristics. By examining the hub’s characteristics, system software ensures that illegal power
topologies are not allowed by not powering on the hub’s ports if doing so would violate the USB power
topology.
11.10 Hub Port Power Control
Hubs allow port power to be controlled by the host system. As described previously, hubs support per-
port or gang-mode power switching on the downstream ports. Switching port power is done via hub
commands, defined below. Hubs with per-port power switching may also allow gang-mode power
switching by specifying a certain value in a request field in the port power control request (refer to the
SetPortFeature(PORT_POWER) request definition in Section 11.12.2.9).
Hubs may wish to mask the gang-mode power control for certain ports. This allows a hub to
independently control the power switching for certain ports, regardless of the general port switching
characteristics. For example, consider a hub with gang-mode port power switching that has a
permanently attached device on a port (an “embedded port”). If the Port Power Control Mask field for
the embedded port indicates that gang-mode power switching is masked, any hub commands that control
the ports in gang-mode will not affect the embedded port.
11.11 Descriptors
Hub descriptors are derived from the general USB device framework. Hub descriptors define a hub
device and the ports on that hub. The host accesses hub descriptors through the hub’s default pipe.
The USB Specification (refer to Chapter 9) defines the following descriptors:
• Device
• Configuration
• Interface
• Endpoint
• String (optional)
The hub class defines additional descriptors (refer to Section 11.11.2). In addition, vendor-specific
descriptors are allowed in the USB device framework. Hubs support standard USB device commands as
defined in Chapter 9.
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11.11.1 Standard Descriptors
The hub class pre-defines certain fields in standard USB descriptors. Other fields are either
implementation-dependent or not applicable to this class.
Note: For the descriptors and fields shown below, the bits in a field are organized in a little-endian
fashion; that is, bit location 0 is the least significant bit and bit location 8 is the most significant bit of a
byte value.
Device Descriptor
bDeviceClass = HubClass
bDeviceSubClass = HubSubClass
wMaxPacketSize0 = 8 bytes
Interface Descriptor
bNumEndpoints = 1
bInterface = 1
Configuration Descriptor
MaxPower = The maximum amount of bus power this hub will consume in this
configuration.
Endpoint Descriptor (for Status Change Endpoint)
bEndpointAddress = Implementation dependent
wMaxPacketSize = Implementation dependent
bmAttributes = Direction = In, Transfer Type = Interrupt (0b00000111 )
bInterval = 0xFF (Maximum allowable interval)
The hub class driver retrieves a device configuration from host system software using the GetDescriptor
device request. The first endpoint descriptor returned by GetDescriptor request is, by specification, the
Status Change endpoint descriptor. Hubs may define additional endpoints beyond the minimum required
by this class definition. However, hubs conforming to this class standard always return the Status Change
endpoint as the first endpoint descriptor in the standard interface.
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11.11.2 Class-specific Descriptors
11.11.2.1 Hub Descriptor
Table 11-9. Hub Descriptor
Offset Field Size Description
0
bDescLength
1 Number of bytes in this descriptor, including this byte.
1
bDescriptorType
1 Descriptor Type
2
bNbrPorts
1 Number of downstream ports that this hub supports.
3
wHubCharacteristics
2 D1..D0: Power Switching Mode
00 - Ganged power switching (all ports’ power at
once)
01 - Individual port power switching
1X - No power switching (ports always powered
on when hub is on and off when hub is off).
D2: Identifies a Compound Device
0 - Hub is not part of a compound device
1 - Hub is part of a compound device
D4..D3: Over-current Protection Mode
00 -Global Over-current Protection. The hub
reports over-current as a summation of all
ports’ current draw, without a breakdown of
individual port over-current status.
01 -Individual Port Over-current Protection. The
hub reports over-current on a per-port basis.
Each port has an over-current indicator.
1X -No Over-Current Protection. This option is
only allowed for bus-powered hubs that do not
implement over-current protection.
D15..D5: Reserved
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Offset Field Size Description
5
bPwrOn2PwrGood
1 Time (in 2 ms intervals) from the time power on
sequence begins on a port until power is good on that
port. System software uses this value to determine
how long to wait before accessing a powered-on port.
6
bHubContrCurrent
1 Maximum current requirements of the hub controller
electronics in mA.
7
DeviceRemovable
Variable
depend-
ing on
number
of ports
on hub
Indicates if a port has a removable device attached. If a
non-removable device is attached to a port, that port will
never receive an insertion change notification. This
field is reported on byte-granularity. Within a byte, if no
port exists for a given location, the field representing the
port characteristics returns “0”.
Bit definition:
0 - Device is removable
1 - Device is not removable (permanently
attached)
This is a bitmap corresponding to the individual ports on
the hub:
Bit 0: Reserved for future use
Bit 1: Port 1
Bit 2: Port 2
Etc.
Bit
n
: Port
n
(implementation dependent, up to a
maximum of 255 ports).
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Offset Field Size Description
Variable
PortPwrCtrlMask
Variable
depend-
ing on
number
of ports
on hub
Indicates if a port is not affected by a gang-mode power
control request. Ports that have this field set always
require a manual SetPortFeature(PORT_POWER)
request to control the port’s power state.
Bit definition:
0 - Port does not mask the gang-mode power
control capability.
1 - Port is not affected by gang-mode power
commands. Manual commands must be sent
to this port to turn power on and off.
This is a bitmap corresponding to the individual ports on
the hub:
Bit 0: Reserved for future use.
Bit 1: Port 1
Bit 2: Port 2
Etc.
Bit
n
: Port
n
(implementation dependent, up to a
maximum of 255 ports).
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11.12 Requests
11.12.1 Standard Requests
Table 11-10. Hub Responses to Standard Device Requests
bRequest Hub Response
CLEAR_FEATURE Standard
GET_CONFIGURATION Standard
GET_DESCRIPTOR Standard
GET_INTERFACE Optional. Hubs only required to support one interface
GET_STATUS Standard
SET_ADDRESS Standard
SET_CONFIGURATION Standard
SET_DESCRIPTOR Optional
SET_FEATURE Standard
SET_INTERFACE Optional. Hubs only required to support one interface
SYNCH_FRAME Optional. Hubs are not required to have isochronous
endpoints.
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11.12.2 Class-specific Requests
The hub class defines requests to which all hubs must respond.
Table 11-11. Hub Class Requests
Request bmRequestType bRequest wValue
wIndex
wLength
Data
ClearHubFeature 00100000B CLEAR_ FEATURE Feature
Selector
Zero
Zero
None
ClearPortFeature 00100011B CLEAR_ FEATURE Feature
Selector
Port
Zero
None
GetBusState 10100011B GET_ STATE Zero
Port
One
Per Port
Bus State
GetHubDescriptor 10100000B GET_DESCRIPTOR Descriptor
Type and
Descriptor
Index
Zero or
Languag
e ID
Descript
or
Length
Descriptor
GetHubStatus 10100000B GET_ STATUS Zero
Zero
Four
Hub Status
and
Change
Indicators
GetPortStatus 10100011B GET_ STATUS Zero
Port
Four
Port Status
and
Change
Indicators
SetHubDescriptor 00100000B SET_DESCRIPTOR Descriptor
Type and
Descriptor
Index
Zero or
Languag
e ID
Descript
or
Length
Descriptor
SetHubFeature 00100000B SET_ FEATURE Feature
Selector
Zero
Zero
None
SetPortFeature 00100011B SET_ FEATURE Feature
Selector
Port
Zero
None
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Table 11-12. Hub Class Request Codes
bRequest Value
GET_ STATUS 0
CLEAR_ FEATURE 1
GET_STATE 2
SET_ FEATURE 3
reserved for future use 4-5
GET_DESCRIPTOR 6
SET_DESCRIPTOR 7
The following are the valid feature selectors for the hub class. See GetHubStatus and GetPortStatus for a
description of the features.
Table 11-13. Hub Class Feature Selectors
Recipient Value
C_HUB_LOCAL_POWER Hub 0
C_HUB_OVER_CURRENT Hub 1
PORT_CONNECTION Port 0
PORT_ENABLE Port 1
PORT_SUSPEND Port 2
PORT_OVER_CURRENT Port 3
PORT_RESET Port 4
PORT_POWER Port 8
PORT_LOW_SPEED Port 9
C_PORT_CONNECTION Port 16
C_PORT_ENABLE Port 17
C_PORT_SUSPEND Port 18
C_PORT_OVER_CURRENT Port 19
C_PORT_RESET Port 20
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11.12.2.1 Clear Hub Feature
This request resets a value reported in the hub status.
bmRequestType bRequest wValue wIndex wLength Data
00100000B CLEAR_ FEATURE Feature
Selector Zero Zero None
Clearing a feature disables that feature; refer to Table 11-13 for the feature selector definitions. If the
feature selector is associated with a change indicator, clearing that indicator acknowledges the change.
Both C_HUB_LOCAL_POWER and C_HUB_OVER_CURRENT may be acknowledged using this
request.
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11.12.2.2 Clear Port Feature
This request resets a value reported in the port status.
bmRequestType bRequest wValue wIndex wLength Data
00100011B CLEAR_ FEATURE Feature
Selector Port Zero None
The port number must be a valid port number for that hub, greater than zero.
Clearing a feature disables that feature; refer to Table 11-13 for the feature selector definitions. If the
feature selector is associated with a change indicator, clearing that indicator acknowledges the change.
Changes in connection, enable, suspend, reset, and over-current status are acknowledged using this
request.
Clearing the PORT_SUSPEND feature causes a host-initiated resume on the specified port. Clearing the
PORT_ENABLE feature causes the port to be disabled. Clearing the PORT_POWER feature causes the
port to be powered off, subject to the constraints due to the hub’s method of power switching. If a hub
uses gang power switching, all ports must be requested to power off before any of the ports actually
power off.
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11.12.2.3 Get Bus State
This is an optional per-port diagnostic request which reads the bus state value, as sampled at the last
EOF2.
bmRequestType bRequest wValue wIndex wLength Data
10100011B GET_ STATE Zero Port One Per Port Bus
State
The port number must be a valid port number for that hub, greater than zero.
Hubs may implement an optional diagnostic aid to facilitate system debug. Hubs implement this aid
through this optional request. This diagnostic feature provides a glimpse of the USB bus state as sampled
at the last EOF2 sample point.
Hubs that implement this diagnostic feature should store the bus state at each EOF2 state, in preparation
for a potential request in the following USB frame.
The data returned is bit-mapped in the following manner. The value of the D- signal is returned in the
field in bit 0. The value of the D+ signal is returned in the field in bit 1. Bits 2-7 are reserved for future
use and are reset to zero.
Hubs that do not support this request respond with a stall.
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11.12.2.4 Get Hub Descriptor
This request returns the hub descriptor.
bmRequestType bRequest wValue wIndex wLength Data
10100000B GET_DESCRIPTOR Descriptor
Type and
Descriptor
Index
Zero Descriptor
Length Descriptor
The GetDescriptor request for the hub class descriptor follows the same usage model as that of the
standard GetDescriptor request (refer to Chapter 9). The standard hub descriptor is denoted by descriptor
type zero. All hubs are required to implement one hub descriptor, with descriptor index zero.
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11.12.2.5 Get Hub Status
This request returns the current hub status and the states that have changed since the previous
acknowledgment.
bmRequestType bRequest wValue wIndex wLength Data
10100000B GET_ STATUS Zero Zero Four Hub Status
and Change
Indicators
The first word of data contains wHubStatus (refer to Table 11-14). The second word of data contains
wHubChange (refer to Table 11-15).
The fields returned are organized in such a way to allow system software to determine which states have
changed. The bit locations in the wHubStatus and wHubChange fields correspond in a one-to-one
fashion where applicable.
Local power and overcurrent changes are acknowledged using the ClearHubFeature request.
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Table 11-14. Hub Status Field, wHubStatus
BIT DESCRIPTION
0Local Power Status: This is the state of the local power supply.
This field only applies to self-powered hubs whose USB Interface Engine (SIE) is bus-powered or hubs that
support either self-powered or bus-powered configurations. This field is returned as a result of a change to the
hub’s power source. This field reports whether local power is good. This field allows system software to
determine the reason for the removal of power to devices attached to this hub or to react to changes to the local
power supply state.
If the hub does not support this feature, this field is RESERVED and follows the definition of the RESERVED
bits below.
This field reports the power status for the SIE and the remainder of the hub.
0 = Local power supply good
1 = Local power supply lost (inactive)
1Over-Current Indicator: This field only applies to hubs that report over-current conditions on a global hub
basis (as reported in the Hub Descriptor ).
If the hub does not report over-current on a global hub basis, this field is RESERVED and follows the definition
of the RESERVED bits below.
This field indicates that the sum of all the ports’ current has exceeded the specified maximum and power to all
the ports has been shut off. For more details on Over-Current protection, see Section 7.2.1.2.1.
This field indicates an over-current condition due to the sum of all ports’ current consumption.
0 = All power operations normal
1 = An over-current condition exists on a hub-wide basis
2-15 Reserved
These bits return 0 when read.
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Table 11-15. Hub Change Field, wHubChange
BIT DESCRIPTION
0Local Power Status Change: (C_HUB_LOCAL_POWER) This corresponds to Local Power Status, Bit 0
above. This field only applies to locally-powered (i.e., self-powered) hubs whose USB Interface Engine (SIE) is
bus-powered, or hubs that support either self-powered or bus-powered configurations. This field is returned as
a result of a change to the hub’s power source.
If the hub does not support this feature, then this field is RESERVED and follows the definition of the
RESERVED bits below.
This field reports whether a change has occurred to the local power status.
0 = No change has occurred on Local Power Status
1 = Local Power Status has changed
1Over-Current Indicator Change: (C_HUB_OVER_CURRENT) This corresponds to Over-Current
Indicator, Bit 1 above. This field only applies to hubs that report over-current conditions on a global hub basis
(as reported in the Hub Descriptor).
If the hub does not report over-current on a global hub basis, this field is RESERVED and follows the definition
of the RESERVED bits below.
This field reports whether a change has occurred to the Over-Current Indicator. This field is only set if an over-
current condition has occurred (i.e., acknowledgment of this change by system software will not cause another
change to be reported).
0 = No change has occurred on the Over-Current Indicator
1 = Over-Current Indicator has changed (i.e., over-current condition has occurred).
2-15 Reserved
These bits return 0 when read.
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11.12.2.6 Get Port Status
This request returns the current port status and the states that have changed since the previous
acknowledgment.
bmRequestType bRequest wValue wIndex wLength Data
10100011B GET_STATUS Zero Port Four Port Status
and Change
Indicators
The port number must be a valid port number for that hub, greater than zero.
The first word of data contains wPortStatus (refer to Table 11-16). The second word of data contains
wPortChange (refer to Table 11-17).
The fields returned are organized in such a way to allow system software to determine which states have
changed. The bit locations in the wPortStatus and wPortChange fields correspond in a one-to-one
fashion where applicable.
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Table 11-16. Port Status Field, wPortStatus
BIT DESCRIPTION
0Current Connect Status: (PORT_CONNECTION) This field reflects whether or not a device is currently
connected to this port. This value reflects the current state of the port, and may not correspond directly to the event
that caused the Insertion Status Change (Bit 0 in below) to be set.
0 = No device is present on this port
1 = A device is present on this port
NOTE: This field is always 1 for ports that have non-removable devices attached.
1Port Enabled/Disabled: (PORT_ENABLE) Ports can be enabled by host software only. Ports can be disabled
by either a fault condition (disconnect event or other fault condition, including an over-current indication) or by host
software. 0 = Port is disabled
1 = Port is enabled
2Suspend: (PORT_SUSPEND) This field indicates whether or not the device on this port is suspended. Setting
this field causes the device to suspend by not propagating bus traffic downstream. Resetting this field causes the
device to resume. Bus traffic cannot be resumed in the middle of a bus transaction If the device itself is signaling a
resume, this field will be cleared by the hub.
0 = Not suspended
1 = Suspended
3Over-Current Indicator: (PORT_OVER_CURRENT) This field only applies to hubs that report over-current
conditions on a per-port basis (as reported in the Hub Descriptor).
If the hub does not report over-current on a per-port hub basis, this field is RESERVED and follows the definition of
the RESERVED bits below.
This field indicates that the device attached to this port has drawn current that exceeds the specified maximum and
this port’s power has been shut off. Port power shutdown is also reflected in the Port Power field above. For more
details, see Section 7.2.1.2.1.
This field indicates an over-current condition due to the device attached to this port.
0 = All power operations normal for this port.
1 = An over-current condition exists on this port. Power has been shut off to this port.
4Reset: (PORT_RESET) This field is set when the host wishes to reset the attached device. It remains set until
the reset signaling is turned off by the hub and the reset status change field is set.
0 = Reset signaling not asserted
1 = Reset signaling asserted
5-7 Reserved
These bits return a “0” when read.
8Port Power: (PORT_POWER) This field reflects a port’s power state. Since hubs can implement different
methods of port power switching, the meaning of this field varies depending on the type of power switching used.
The device descriptor reports the type of power switching implemented by the hub. Hubs do not provide any power
to their ports until they are in the configured state.
0 = This port is powered OFF
1 = This port is powered ON
NOTE: Hubs that do not support power switching always return a 1 in this field.
9Low Speed Device Attached: (PORT_LOW_SPEED) This is only relevant if a device is attached.
0 = Full Speed device attached to this port
1 = Low speed device attached to this port
10-15 Reserved
These bits return 0 when read.
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Table 11-17. Port Change Field, wPortChange
BIT DESCRIPTION
0Connect Status Change: (C_PORT_CONNECTION) Indicates a change has occurred in the port’s Current
Connect Status. The hub device sets this field for any changes to the port device connect status, even if system
software has not cleared a connect status change.1
0 = No change has occurred on Current Connect Status
1 = Current Connect Status has changed
NOTE: For ports that have non-removable devices attached, this field is set only after a
RESET condition to indicate to system software that a device is present on this port.
1Port Enable/Disable Change: (C_PORT_ENABLE) This field is only activated when a change in the port’s
enable/disable status was detected due to hardware changes. This field is not set if system software caused a port
enable/disable change.
0 = No change has occurred on Port Enabled/Disabled status
1 = Port Enabled/Disabled status has changed
2Suspend Change: (C_PORT_SUSPEND) This field indicates a change in the host-visible power state of the
attached device. It indicates the device has transitioned out of the suspend state. Going into the suspend state will
not set this field. The Suspend Change field is only set when the entire resume process has completed. That is,
the hub has ceased signaling resume on this port and 3 ms have passed to allow the device to resynch to SOF.
0 = No change
1 = Resume complete
3Over-Current Indicator Change: (C_PORT_OVER_CURRENT) This field only applies to hubs that report
over-current conditions on a per-port basis (as reported in the Hub Descriptor).
If the hub does not report over-current on a per-port hub basis, then this field is RESERVED and follows the
definition of the RESERVED bits below.
This field reports whether a change has occurred to the port Over-Current Indicator.
0 = No change has occurred on Over-Current Indicator
1 = Over-Current Indicator has changed
4Reset Change: (C_PORT_RESET) This field is set when reset processing on this port is complete. As a reset
of completing reset processing, the enabled status of the port is also set and the suspend change field reset.
0 = No change
1 = Reset Complete
5-15 Reserved
These bits return 0 when read.
1 If, for example, the insertion status changes twice before system software has cleared the changed condition,
hub hardware will be “setting” an already-set bit (i.e., the bit will remain set). However, the hub will transfer
the change bit only once when the host controller requests a data transfer to the Status Change endpoint.
System software is responsible for determining state change history in such a case.
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11.12.2.7 Set Descriptor
This request overwrites the hub descriptor.
bmRequestType bRequest wValue wIndex wLength Data
00100000B SET_DESCRIPTOR Descriptor
Type and
Descriptor
Index
Zero Descriptor
Length Descriptor
The SetDescriptor request for the hub class descriptor follows the same usage model as that of the
standard SetDescriptor request (refer to Chapter 9). The standard hub descriptor is denoted by descriptor
type zero. All hubs are required to implement one hub descriptor, with descriptor index zero.
This request is optional. This request writes data to a class-specific descriptor. The host provides the
data that is to be transferred to the hub during the data transfer phase of the control transaction. This
request writes the entire hub descriptor at once.
Hubs must buffer all the bytes received from this request to ensure that the entire descriptor has been
successfully transmitted from the host. Upon successful completion of the bus transfer, the hub updates
the contents of the specified descriptor.
Hubs that do not support this request respond with a stall.
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11.12.2.8 Set Hub Feature
This request sets a value reported in the hub status.
bmRequestType bRequest wValue wIndex wLength Data
10100000B SET_ FEATURE Feature
Selector Zero Zero None
Setting a feature enables that feature; refer to Table 11-13 for the feature selector definitions. Change
indicators may not be acknowledged using this request.
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11.12.2.9 Set Port Feature
This request sets a value reported in the port status.
bmRequestType bRequest wValue wIndex wLength Data
10100011B SET_ FEATURE Feature
Selector Port Zero None
The port number must be a valid port number for that hub, greater than zero.
Setting a feature enables that feature; see for the feature selector definitions. Change indicators may not
be acknowledged using this request.
Setting the PORT_SUSPEND feature causes bus traffic to cease on that port and, consequently, the
device to suspend. Setting the reset feature (PORT_RESET) causes the hub to signal reset on that port.
When the reset signaling is complete, the hub sets the C_PORT_RESET change indicator and
immediately enables the port. Refer to Section 11.6.2 for a complete discussion of host initiated reset
behavior.
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