AN295
0x01,
0x02,
// bSourceID (input terminal 1)
// bControlSize (2 bytes)
bSourceID
associates the feature unit with the input terminal.
bControlSize
tells the host how many controls the feature unit includes. The number is actually an array size
that describes the number of two-byte elements that follows. In this project, the Feature Unit has a Master Control
at index 0, control for the left channel on index 1, and control for the right channel at index 2 in the array that
immediately follows.
0x0100,
0x0000,
0x0000,
// Master controls
// Channel 0 controls (LEFT)
// Channel 1 controls (RIGHT)
Each two-byte value is a bit map where each bit is defined in the Audio Class Specification. Bits correspond to
Feature Unit functionality such as volume control, bass, treble, and mid levels, etc. This project supports the “mute”
control on the master channel. The master controls apply to every channel of the associated terminal.
4.1.7. Output Terminal Audio Class Descriptor
The next Descriptor tells the host about the capabilities of the Interface’s output terminal, which will contains the
audio stream. The firmware defines the output terminal’s Descriptor as follows:
0x09,
0x24,
0x03,
0x03,
0x0101,
0x00,
0x02,
0x00
0x03,
0x0101,
//
//
//
//
//
//
//
//
bLength (9)
bDescriptorType (CS_INTERFACE)
bDescriptorSubtype (OUTPUT_TERMINAL)
bTerminalID (3)
wTerminalType (USB streaming)
bAssocTerminal (none)
bSourceID (feature unit 2)
iTerminal (none)
// bTerminalID (3)
// wTerminalType (USB streaming)
The value bTerminalID gives
the output terminal a unique ID, chosen by the programmer.
wTerminalType
defines the terminal’s stream according to definitions found in the document called “USB Audio
Terminal Types”.
0x00,
0x02,
// bAssocTerminal (none)
// bSourceID (feature unit 2)
This terminal is not associated with any other terminals.
This value establishes a connection between the output terminal and the feature unit defined in a previous
descriptor.
4.1.8. Audio Interface Descriptor
The next two descriptors define the audio streaming interface for the Tone Generator, which will contain the
isochronous endpoint. USB Specification requires that interfaces with an isochronous endpoint must also define an
alternative interface without that isochronous endpoint. The specification imposes this requirement so that a device
can successfully enumerate even if the host cannot allocate the data bandwidth required by the isochronous
endpoint.
The firmware defines the first standard Interface Descriptor for the audio stream as follows:
0x09,
0x04,
0x01,
0x00,
//
//
//
//
bLength (9)
bDescriptorType (INTERFACE)
bInterfaceNumber (1)
bAlternateSetting (0)
10
Rev. 0.1
SILABS [ SILICON LABORATORIES ]
