CM108B Datasheet, PDF (16/24 Pages) C-Media Electronics – A Highly Integrated USB Audio Single Chip

CM108B
USB Audio Single Chip
7.3 HID feature
USB protocols can configure devices at startup or when they are plugged in at run time. These devices are
categorized into various device classes. Each device class defines the common behavior and protocols for devices
that serve similar functions. The HID (Human Interface Device) class is one of the device classes.
The HID class consists primarily of devices that are used to control the operation of computer systems. Typical
examples of HID class devices include:
The CM108B’s HID feature allows users to set volume up, volume down, playback mute and recording mute button
pins, and reports the changes to the host to synchronize host side settings. In addition, all CM108B internal registers
can be accessed via HID function call.
7.3.1 HID descriptors
HID Interface Descriptor
Offset
Field
Size
0
bLength
1
1
bDescriptorType
1
2
bInterfaceNumber
1
3
bAlternateSetting
1
4
bNumEndpoints
1
5
bInterfaceClass
1
6
bInterfaceSubClass
1
7
bInterfaceProtocol
1
8
iInterface
1
Value (Hex)
Description
09
Size of this descriptor: 9 bytes
04
Interface descriptor type
03
Interface number: 3
00
Alternate: 0
01
Number of endpoints used by this interface: 1
03
Interface class: HID
00
Subclass: no
00
Must be set to 0
00
String descriptor index that characterizes this interface
HID Descriptor
Offset
Field
Size
0
bLength
1
1
bDescriptorType
1
2
bcdHID
2
4
bCountryCode
1
5
bNumDescriptors
1
6
bDescriptorType
1
7
wDescriptorLength
2
Interrupt IN Endpoint Descriptor
Offset
Field
Size
0
bLength
1
1
bDescriptorType
1
2
bEndpointAddress
1
3
bmAttributes
1
4
wMaxPacketSize
2
6
bInterval
1
Value (Hex)
09
21
0100
00
01
22
003C
Description
Total: 9 bytes
HID descriptor type
HID class version 1.0
Report descriptor
Total size of the optional descriptor: 60 bytes
Value (Hex)
07
05
87
03
0004
2
Description
Total: 7 bytes
Endpoint descriptor type
In Endpoint Number = 3
Interrupt endpoint type
Maximum packet size: 4 bytes
2ms
Datasheet Revision: 1.11
Page 16 / 24
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