Audio controls via Human Interface Device (HID)

The design supports simple audio controls such as play/pause, volume up/down etc via the USB Human Interface Device Class Specification.

This functionality is enabled by setting the HID_CONTROLS define to 1. Setting to 0 disables this feature.

When turned on the following items are enabled:

  1. HID descriptors are enabled in the Configuration Descriptor informing the host that the device has a HID interface.

  2. A Get Report Descriptor request is enabled in endpoint0.

  3. Endpoint data handling is enabled in the buffer thread.

The Get Descriptor Request enabled in endpoint 0 returns the report descriptor for the HID device. This details the format of the HID reports returned from the device to the host. It maps a bit in the report to a function such as play/pause.

The USB Audio Framework implements a report descriptor that should fit most basic audio device controls. If further controls are necessary the HID Report Descriptor in hid_report_descriptor.h should be modified. The default report size is 1 byte with the format as follows:

Table 42 Default HID Report Format

Bit

Function

0

Play/Pause

1

Scan Next Track

2

Scan Prev Track

3

Volume Up

4

Volume Down

5

Mute

6-7

Unused

On each HID report request from the host the function UserHIDGetData() is called from XUA_Buffer_Ep(). This function is passed an array hidData[] by reference. The programmer should report the state of the buttons into this array. For example, if a volume up command is desired, bit 3 should be set to 1, else 0.

Since the UserHIDGetData() function is called from the XUA_Buffer_Ep() thread, care should be taken not to add to much execution time to this function since this could cause issues with servicing other endpoints.