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Interface Descriptor
An interface descriptor describes a specific interface provided by the associated configuration (see Table 5). It is always
returned as part of a configuration descriptor. An interface descriptor never includes endpoint zero in the number of
endpoints.
Table 5. Interface Descriptors
Offset
Field
Size
Value
Number
Constant
Number
Default
09
Description
Size of this descriptor in bytes.
0
1
2
bLength
1
1
1
bDescriptorType
bInterfaceNumber
04
INTERFACE Descriptor Type.
00
Number of interface. Zero-based value identifying the index in the
array of concurrent interfaces supported by this configuration.
3
4
5
bAlternateSetting
bNumEndpoints
bInterfaceClass
1
1
1
Number
Number
Class
00
04
00
Value used to select alternate setting for the interface identified in
the prior field.
Number of endpoints used by this interface (excluding endpoint
zero). If this value is 0, this interface only uses endpoint zero.
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
7
bInterfaceSubClass
bInterfaceProtocol
1
1
SubClass
Protocol
00
00
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.
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
00
Index of string descriptor describing this interface.
CONEXANT [ CONEXANT SYSTEMS, INC ]
