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3.5.4 Audio Request: Setting The Sample Rate

When the host requests a change of sample rate, it sends a command to Endpoint

Since the

DescriptorRequests()

function does not deal with audio requests it re-

turns 1. After some parsing the request is handled by either the

AudioRequests_1()

AudioRequests_2()

function (based on whether the device is running in Audio

Class 1.0 or 2.0 mode).

3.5.5 Audio Request: Volume Control

When the host requests a volume change, it sends an audio interface request to

Endpoint 0. An array is maintained in the Endpoint 0 core that is updated with

such a request.

When changing the volume, Endpoint 0 applies the master volume and channel

volume, producing a single volume value for each channel. These are stored in the

array.

The volume will either be handled by the decoupler or the mixer component (if the

mixer component is used). Handling the volume in the mixer gives the decoupler

more performance to handle more channels.

If the eﬀect of the volume control array on the audio input and output is imple-

mented by the decoupler, the decoupler core reads the volume values from this

array. Note that this array is shared between Endpoint 0 and the decoupler core.

This is done in a safe manner, since only Endpoint 0 can write to the array, word

update is atomic between cores and the decoupler core only reads from the array

(ordering between writes and reads is unimportant in this case). Inline assembly is

used by the decoupler core to access the array, avoiding the parallel usage checks

in XC.

If volume control is implemented in the mixer, Endpoint 0 sends a mixer command

to the mixer to change the volume. Mixer commands are described in §3.9.

3.6 Audio Endpoints (Endpoint Buﬀer and Decoupler)

3.6.1 Endpoint Buﬀer

All endpoints other that Endpoint 0 are handled in one core. This core is imple-

mented in the ﬁle usb_buffer.xc. This loop is responsive to the XUD library.

This core is also responsible for feedback calculation based on SOF notiﬁcation

and reads from the port counter of a port connected to the master clock.

3.6.2 Decoupler

The decoupler supplies the USB buﬀering core with buﬀers to transmit/receive

audio data to/from the host. It marshals these buﬀers into FIFOs. The data from

the FIFOs are then sent over XC channels to other parts of the system as they need

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