Jan Axelson, USB Complete

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Chapter 4

106

Most device classes specify the class at the interface level rather than at the

device level. Assigning functions to interfaces enables a single configuration to

support multiple functions. When two or more interfaces in a configuration are

associated with the same function, the interface association descriptor tells the

host which interfaces are associated. For example, a video-camera function may

use one interface to control the camera and another to carry video data.

The Interface Association Descriptor ECN says that the descriptor must be sup-

ported by future implementations of devices that use multiple interfaces to

manage a single device function. Devices that comply with the video-class and

audio 2.0 specifications must use interface association descriptors. Class specifi-

cations that predate the IAD don’t require it. For example, the audio 1.0 class

specification defines a class-specific descriptor to associate audio interfaces in a

function. Hosts that don’t support the IAD ignore it. Windows began support-

ing the descriptor with Windows XP SP2. In USB 3.0 devices, every function

with multiple interfaces must use an IAD.

To enable hosts to identify devices that use the Interface Association descriptor,

the device descriptor should contain the following values:

bDeviceClass = EFh (miscellaneous device class)

bDeviceSubClass = 02h (common class)

bDeviceProtocol = 01h (interface association descriptor)

These codes together form the Multi-interface Function Device Class Codes.

Table 4-7: The interface association descriptor can link multiple interfaces to a

single function.

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0 bLength 1 Descriptor size in bytes (08h)

1 bDescriptorType 1 The constant Interface Association (0Bh)

2 bFirstInterface 1 Number identifying the first interface associated

with the function

3 bInterfaceCount 1 The number of contiguous interfaces associated

with the function

4 bFunctionClass 1 Class code

5 bFunctionSubClass 1 Subclass code

6 bFunctionProtocol 1 Protocol code

8 iFunction 1 Index of string descriptor for the function

Enumeration: How the Host Learns about Devices

107

A host retrieves an interface association descriptor as one of the subordinate

descriptors sent in response to a request for a configuration descriptor. The IAD

precedes the interface descriptors that the IAD specifies.

bFirstInterface identifies the interface number of the first of multiple interfaces

associated with a function. The interface number is the value of bInterface-

Number in the interface descriptor. The interface numbers of associated inter-

faces must be contiguous.

bInterfaceCount equals the number of contiguous interfaces associated with

the function.

bFunctionClass is a class code for the function shared by the associated inter-

faces. For classes that don’t specify a value to use, the preferred value is the

bInterfaceClass value from the descriptor of the first associated interface. Values

from 01h–FEh are reserved for USB-defined classes. FFh indicates a ven-

dor-defined class. Zero is not allowed.

bFunctionSubClass is a subclass code for the function shared by the associated

interfaces. For classes that don’t specify a value to use, the preferred value is the

bInterfaceSubClass value from the descriptor of the first associated interface.

bInterfaceProtocol is a protocol code for the function shared by the associated

interfaces. For classes that don’t specify a value to use, the preferred value is the

bInterfaceProtocol value from the descriptor of the first associated interface.

iInterface is an index to a string that describes the function. This value is zero if

there is no string descriptor.

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The interface descriptor provides information about a function or feature that a

device implements. The descriptor contains class, subclass, and protocol infor-

mation and the number of endpoints the interface uses (Table 4-8).

A configuration can have multiple interfaces that are active at the same time.

The interfaces may be associated with a single function or they may be unre-

lated. Each interface has its own interface descriptor and subordinate descrip-

tors. Each of these interfaces can also have one or more alternate interface

settings. The settings are mutually exclusive; only one is active at a time. Each

setting has an interface descriptor and subordinate descriptors as needed.

Devices that use isochronous transfers have alternate interface settings because

the default interface can request no isochronous bandwidth.

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A host retrieves interface descriptors as subordinate descriptors sent in response

to a request for a configuration descriptor.

bInterfaceNumber identifies the interface. In a composite device, a configura-

tion has multiple interfaces that are active at the same time. Each interface must

have a descriptor with a unique value in this field. The default interface is 00h.

bAlternateSetting identifies the default interface setting or an alternate setting.

For each bInterfaceNumber, the device provides an interface descriptor with

bAlternateSetting = 00h. This is the default setting. A descriptor for an alter-

nate setting has the same value in bInterfaceNumber, a unique value in bAlter-

nateSetting, and different values as needed in the descriptor’s final five bytes.

For each bInterfaceNumber, only one bAlternateSetting is active at a time. The

alternate settings enable the host to request an interface with different band-

width or other requirements and capabilities. The Get Interface request

retrieves the currently active bAlternateSetting. The Set Interface request selects

the bAlternateSetting that a specific bInterfaceNumber should use.

bNumEndpoints equals the number of endpoints the interface supports in

addition to endpoint zero. For a device that supports only endpoint zero, this

field is zero.

bInterfaceClass is similar to bDeviceClass in the device descriptor, but for

devices with a class specified by the interface. Table 4-9 shows defined codes.

Table 4-8: The interface descriptor specifies the number of subordinate

endpoints and may specify a USB class.

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0 bLength 1 Descriptor size in bytes (09h)

1 bDescriptorType 1 The constant Interface (04h)

2 bInterfaceNumber 1 Number identifying this interface

3 bAlternateSetting 1 A number that identifies a descriptor with

alternate settings for this bInterfaceNumber.

4 bNumEndpoints 1 Number of endpoints supported not counting

endpoint zero

5 bInterfaceClass 1 Class code

6 bInterfaceSubclass 1 Subclass code

7 bInterfaceProtocol 1 Protocol code

8 iInterface 1 Index of string descriptor for the interface

Enumeration: How the Host Learns about Devices

109

Table 4-9: The bInterfaceClass field in the interface descriptor can name a class

the interface belongs to.

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00 Reserved

01 Audio

02 Communications device class: communication interface

03 Human interface device

05 Physical

06 Image

07 Printer

08 Mass storage

09 Hub

0A Communications device class: data interface

0B Smart Card

0D Content Security

0E Video

0F Personal healthcare device (can instead be declared at the device level)

DC Diagnostic device (can instead be declared at the device level)

bInterfaceSubclass= 01h, bInterfaceProtocol = 01h. USB2 compliance device

E0 Wireless controller

bInterfaceSubclass = 01h

bInterfaceProtocol = 01h: Bluetooth programming interface

(can also be declared at the device level)

bInterfaceProtocol = 02h: UWB Radio control interface (Wireless USB)

bInterface bInterfaceProtocol = 03h: remote NDIS

bInterfaceSubclass = 02h. Host and device wire adapters (Wireless USB)

EF Miscellaneous

bInterfaceSubclass = 01h

bInterfaceProtocol = 01h: active sync

bInterfaceProtocol = 02h: Palm sync

bInterfaceSubclass = 03h. Cable based association framework (Wireless USB)

FE Application specific

bInterfaceSubclass = 01h. Device firmware upgrade

bInterfaceSubclass = 02h. IrDA bridge

bInterfaceSubclass = 03h. Test and measurement

FF Vendor specific (can instead be declared at the device level)

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Values 01h–FEh are reserved for USB-defined classes. FFh indicates a ven-

dor-defined class. Zero is reserved.

bInterfaceSubClass is similar to bDeviceSubClass in the device descriptor, but

for devices with a class defined by the interface. If bInterfaceClass equals 00h,

bInterfaceSubclass must equal 00h. If bInterfaceClass is in the range 01h–FEh,

bInterfaceSubclass equals 00h or a code defined for the interface’s class. FFh

indicates a vendor-defined subclass.

bInterfaceProtocol is similar to bDeviceProtocol in the device descriptor, but

for devices whose class is defined by the interface. The field can specify a proto-

col for the selected bInterfaceClass and bInterfaceSubClass. If bInterfaceClass is

in the range 01h–FEh, bInterfaceProtocol must equal 00h or a code defined for

the interface’s class. FFh indicates a vendor-defined protocol.

iInterface is an index to a string that describes the interface. This value is zero if

there is no string descriptor.

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Each endpoint specified in an interface descriptor has an endpoint descriptor

(Table 4-10). Endpoint zero never has a descriptor because every device must

support endpoint zero, the device descriptor contains the maximum packet size,

and the USB specification defines everything else about the endpoint. A host

retrieves endpoint descriptors as subordinate descriptors sent in response to a

request for a configuration descriptor.

Devices in the audio 1.0 class extend the endpoint descriptor with two addi-

tional bytes of audio-specific information. This is the only allowed extension

that changes the length of a standard descriptor type. Where needed elsewhere,

class and other specifications should define separate, subordinate descriptors

that return extended information. For example, USB 3.0 defines the endpoint

companion descriptor to return SuperSpeed-specific endpoint information.

bEndpointAddress specifies the endpoint number and direction. Bits 3..0 are

the endpoint number. Low-speed devices can have a maximum of 3 endpoint

numbers (usually in the range 0–2), while full- and high-speed devices can have

16 (0–15). Bit 7 is the direction, with OUT = 0 and IN = 1. Bits 6..4 are

unused and must be zero.

bmAttributes sets bits 1..0 to specify the type of transfer the endpoint sup-

ports: 00=control, 01=isochronous, 10=bulk, 11=interrupt. Bits 7..6 are

reserved and must be zero. The functions of the remaining bits vary with the

endpoint type and speed.

Enumeration: How the Host Learns about Devices

111

For isochronous endpoints, bits 5..2 can indicate a synchronization type and

usage type of data or feedback.

For SuperSpeed interrupt endpoints, bits 5..4 indicate a usage type of Notifica-

tion or Periodic. Interrupt endpoints have two primary uses with differing

needs from the host. Some endpoints require quick response or frequent data

transfers in consecutive intervals. For example, users don’t want a noticeable

delay before seeing the effect of pressing a key or moving a mouse. These end-

points should specify the Periodic usage. Other endpoints provide infrequent

notifications or data where timing is less critical. An example is hub notifica-

tions that inform the host of device attachment, removal, or other events. The

endpoints should specify the Notification usage. The host can use the Usage

type in deciding whether to place a port in a low-power state that requires more

time to exit. Any undefined bits are reserved.

wMaxPacketSize specifies the maximum number of data bytes the endpoint

can transfer in a transaction. The allowed values vary with the device speed and

type of transfer.

For USB 2.0, bits 10..0 are the maximum packet size with a range of 0–1024.

For USB 1.x, the range is 0–1023. In USB 2.0, bits 12..11 indicate how many

additional transactions per microframe a high-speed interrupt or isochronous

endpoint supports: 00 = no additional transactions (total of 1 / microframe), 01

= one additional (total of 2 / microframe), 10 = 2 additional (total of 3 /

microframe), 11 = reserved. In USB 1.x, these bits were reserved and zero. Bits

15..13 are reserved and zero.

For SuperSpeed bulk endpoints, the value is 1024. For SuperSpeed interrupt

and isochronous endpoints, the allowed values depend on the value of bMax-

Burst in the SuperSpeed endpoint companion descriptor. If bMaxBurst = 0,

Table 4-10: The endpoint descriptor provides information about an endpoint

address.

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0 bLength 1 Descriptor size in bytes (07h)

1 bDescriptorType 1 The constant Endpoint (05h)

2 bEndpointAddress 1 Endpoint number and direction

3 bmAttributes 1 Transfer type and supplementary information

4 wMaxPacketSize 2 Maximum packet size supported

6 bInterval 1 Service interval or NAK rate

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wMaxPacketSize can be in the range 0–1024 for isochronous endpoints and 1–

1024 for interrupt endpoints. If bMaxBurst > 0, wMaxPacketSize = 1024.

bInterval specifies the service interval for interrupt and isochronous endpoints.

The service interval is a period within which the host must reserve time for an

endpoint’s transactions. The period is an integral number of frames (low and

full speed), microframes (high speed), or bus intervals (SuperSpeed). The

allowed range and usage of bInterval varies with the device’s speed, the transfer

type, and the USB version.

For low-speed interrupt endpoints, bInterval is the maximum latency in ms in

the range 10–255. For all full-speed interrupt endpoints and for full-speed iso-

chronous endpoints on 1.x devices, the interval equals bInterval in ms. For

interrupt endpoints, the value may range from 1–255. For isochronous end-

points in USB 1.x devices, the value must be 1. For isochronous endpoints in

full-speed USB 2.0 devices, values 1–16 are allowed, and the interval is

bInterval-1

in ms, allowing a range from 1 ms to 32.768 seconds.

For high-speed and SuperSpeed endpoints, the value is in units of 125

µs. The

value for interrupt and isochronous endpoints may range from 1–16, and the

interval is calculated as 2

bInterval-1

, allowing a range from 125 µs to 4.096 sec-

onds.

For high-speed bulk and control OUT endpoints, the field can contain a maxi-

mum NAK rate used for compliance purposes only. Devices typically set the

field to zero. For other bulk transfers and control transfers, the value is reserved.

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Every SuperSpeed endpoint has a companion descriptor to support SuperSpeed

capabilities. A host retrieves endpoint companion descriptors as subordinate

descriptors sent in response to a request for a configuration descriptor when the

configuration has one or more endpoints.

bMaxBurst specifies the maximum number of packets the endpoint can send

or receive in a burst minus one. A value of zero means one packet per burst.

The maximum value is 15, indicating 16 packets per burst. A Data Packet in a

burst can transmit without waiting for an acknowledgement of the previous

Data Packet in the burst.

bmAttributes provides information specific to bulk and isochronous end-

points. For bulk endpoints, bits 4..0 are a MaxStreams value that indicates the

maximum number of streams the endpoint supports. Zero means the endpoint

Enumeration: How the Host Learns about Devices

113

doesn’t define streams. For values 1–16, the number of streams equals

MaxStreams

for a maximum value of 65,536.

For isochronous endpoints, bits 1..0 are a Mult value that indicates, along with

bMaxBurst, the maximum number of packets in a service interval. The maxi-

mum number of packets equals (bMaxBurst + 1) × (Mult + 1). Valid values for

Mult are 0–2. The maximum allowed number of packets thus equals (15 + 1) ×

(2 + 1), or 48.

wBytesPerInterval is the maximum number of bytes a periodic interrupt or

isochronous endpoint expects to transfer per service interval.

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A string descriptor (Table 4-12) contains descriptive text. Other descriptors can

contain indexes to strings that describe the manufacturer, product, serial num-

ber, configuration, and interface. Class- and vendor-specific descriptors can

contain indexes to additional string descriptors. Support for string descriptors is

optional, though a class may require them. A host retrieves a string descriptor

by sending a Get Descriptor request with the high byte of the Setup transac-

tion’s wValue field equal to 03h.

When the host requests a string descriptor, the low byte of the wValue field is

an index value. An index value of zero has the special function of requesting

language IDs, while other index values request strings.

Table 4-11: A SuperSpeed endpoint has a companion descriptor to provide a

maximum burst value.

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0 bLength 1 Descriptor size in bytes (06h)

1bDescriptorType1The constant

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(30h)

2 bMaxBurst 1 The maximum number of packets the endpoint

can send or receive as part of a burst - 1.

3 bmAttributes 1 For bulk endpoints, the maximum number of

streams. For isochronous endpoints, the maximum

number of packets in a service interval.

4 wBytesPerInterval 2 For periodic interrupt and isochronous endpoints,

the maximum number of bytes the endpoint

expects to transfer per service interval.

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wLANGID[0...n] is valid for string descriptor zero only. This field contains

one or more 16-bit language ID codes that indicate the languages the strings are

available in. U.S. English (0409h) is likely to be the only code supported by an

operating system. The wLANGID value must be valid for any string to be

valid. Devices that return no string descriptors must not return an array of lan-

guage IDs. The USB-IF’s website has a list of defined USB language IDs.

bString is valid for string descriptors one and higher and contains a string in

Unicode UTF-16LE format. In this format, most characters are encoded as

16-bit code units with the low byte of the code unit transmitted first. For U.S.

English, the low byte of the code unit is the character’s ASCII code. For exam-

ple, the character A transmits as the byte 41h followed by 00h. Some rarely used

characters are encoded as surrogate pairs consisting of two 16-bit code units.

The strings are not null-terminated.

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Some devices use additional descriptors to store information that is specific to a

technology or a device function. To provide a standard way to provide this

information, the Wireless USB specification introduced two new descriptor

types. The USB 2.0 Link Power Management Addendum and USB 3.0 specifi-

cation also incorporate these types.

The binary device object store (BOS) descriptor (Table 4-13) functions as a

base descriptor for one or more related device capability descriptors. A device

capability descriptor (Table 4-14) provides information about a specific capabil-

ity or technology. These are the defined device capability descriptors:

• Wireless USB provides information about wireless features.

• USB 2.0 Extension indicates that a device supports the Link Power Man-

agement protocol when operating at low, full, or high speed. All Super-

Table 4-12: A string descriptor identifies a supported language or stores a string

of text.

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0 bLength 1 Descriptor size in bytes (variable)

1 bDescriptorType 1 The constant String (03h)

2bSTRING or

wLANGID

varies For string descriptor zero, an array of one or more

Language Identifier codes. For other string

descriptors, a Unicode UTF-16LE string.

Enumeration: How the Host Learns about Devices

115

Speed devices must provide this descriptor and must support Link Power

Management when operating at high speed. USB 2.0 devices that support

Link Power Management also provide this descriptor.

• SuperSpeed USB specifies which speeds the device supports, the lowest

speed that provides full functionality, and power-management capabilities.

All SuperSpeed devices must provide this descriptor.

• Container ID provides a 128-bit universally unique identifier (UUID) that

identifies the device instance. The descriptor is mandatory for USB 3.0

hubs and optional for other SuperSpeed devices.

A host retrieves a BOS descriptor and all of its subordinate device capability

descriptors by sending a Get Descriptor request with the high byte of the Setup

transaction’s wValue field set to 0Fh and the wLength field equal to the descrip-

tor’s wTotalLength value. There is no request for reading only a device capabil-

ity descriptor.

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Other standard descriptor types are OTG and debug.

Devices that support On-The-Go’s Host Negotiation Protocol (HNP) or Ses-

sion Request Protocol (SRP) have an OTG descriptor that indicates the sup-

ported protocols. Chapter 20 has more about this descriptor.

Intel’s proposed specification USB2 Debug Device: A Functional Device Specifi-

cation defines a debug descriptor. A debug device connects to the optional

debug port defined in the EHCI specification for high-speed host controllers.

The debug port and device are intended to replace the RS-232 port that PCs

have long used for debugging purposes.

Table 4-13: A binary device object store (BOS) descriptor provides a way to

support descriptors that store additional information about a device.

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0 bLength 1 Descriptor size in bytes (05h).

1bDescriptorType1BOS (0Fh)

2 wTotalLength 2 The number of bytes in this descriptor and all of its

subordinate descriptors

4 bNumDeviceCaps 1 The number of device capability descriptors

subordinate to this BOS descriptor.