USB Audio Design Guide
Due to bandwidth limitations of full-speed USB the following sample-frequency
restrictions are also applied:
·
Sample rate is limited to a maximum of 48kHz if both input and output are
enabled.
·
Sample rate is limited to a maximum of 96kHz if only input
or
output is enabled.
4.3
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 HID interface
2. A Get Report Descriptor request is enabled in
endpoint0
.
3. Endpoint data handling is enabled in the
buffer
core
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
descriptors.h
should be modified. The default report size is 1 byte with the
format as follows:
Bit
Function
0
Play/Pause
1
Scan Next Track
2
Scan Prev Track
3
Volume Up
4
Volume Down
5
Mute
6-7
Unused
Figure 28:
Default HID
Report
Format
On each HID report request from the host the function
Vendor_ReadHidButtons(unsigned char hidData[])
is called from
buffer()
. This function is passed an array
hidData[]
by reference.
The programmer should report the state of his buttons into this array.
For
example, if a volume up command is desired, bit 3 should be set to 1, else 0.
XM0088546.1
Summary of Contents for xCORE-200 Multi-channel Audio board
Page 6: ...1 Overview XM0088546 1...
