CM118B Datasheet, PDF (17/25 Pages) C-Media Electronics – Highly Integrated & Configurable USB Audio Single Chip

CM118B
Highly Integrated & Configurable USB Audio Single Chip
4.5 HID Feature and Descriptor
HID feature is provided by CM118B, so user settings (volume up / volume down / playback mute button) were
all reported to the host to synchronize host side setting. In addition, CM118B internal registers can be accessed via
HID function call.
USB protocols can configure devices at start up or when they are plugged in at run time. These devices are
broken down into various device classes. Each device class defines the common behavior and protocols for devices
that has similar functions. The HID (Human Interface Device) class is one of the device classes.
The HID class consists primarily of devices that are used by humans to control the operation of computer
systems. Typical examples of HID class devices include keyboards and pointing devices (i.e. mouse, trackballs, and
joysticks)
HID Interface Descriptor
Offset
Field
Size Value(Hex)
Description
0
bLength
1
09
Sizeofthisdescriptor:9byte
1
bDescriptorType
1
04
INTERFACE descriptor type
2
bInterfaceNumber
1
03
Number of interface: 3
3
bAlternateSetting
1
00
Alternate 0
4
bNumEndpoints
1
01
Number of endpoints used by this
interface:1
5
bInterfaceClass
1
03
HID Interface Class
6
bInterfaceSubClass
1
00
No Subclass
7
bInterfaceProtocol
1
00
Must be set to 0
8
iInterface
1
00
Index of a string descriptor that describes this
interface
HID Descriptor
4.
Offset
Field
Size
0
bLength
1
1
bDescriptorType
1
2
bcdHID
2
4
bCountryCode
1
5
bNumDescriptors
1
6
bDescriptorType
1
7
wDescriptorLength
2
Value(Hex)
09
21
0100
00
01
22
003C
Description
Total 9 bytes
HID descriptor type
HID class version 1.00
-
-
Report descriptor
Numeric expression equal to the total size of the
optional descriptor: 60 Bytes
Datasheet Revision: 1.00
Page 17 / 25
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