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USB Audio Design Guide 17/61

The ﬁrst operation the interrupt handler does is to send back a word acknowledging

the request (if there was a change of sample frequency a control token would

instead be sent—the audio system uses a testct() to inspect for this case).

Sample transfer may now take place. First the audio subsystem transfers samples

destined for the host, then the decouple core sends samples from the host to

device. These transfers always take place in channel count sized chunks (i.e.

NUM_USB_CHAN_OUT

and

NUM_USB_CHAN_IN

). That is, if the device has 10 output

channels and 8 input channels, 10 samples are sent from the decouple core and 8

received every interrupt.

The complete communication scheme is shown in the table below (for non sample

frequency change case):

Decouple Audio System Note

outuint() Audio system requests sample exchange

inuint() Interrupt ﬁres and inuint performed

outuint() Decouple sends ack

testct() Checks for CT indicating SF change

inuint() Word indication ACK input (No SF change)

inuint() Sample transfer (Device to Host)

inuint()

...

outuint() Sample transfer (Host to Device)

outuint()

...

Figure 8:

Decouple/Audio

System

Channel Com-

munication

3.6.4.1 Aysnc Feedback

The device uses a feedback endpoint to report the rate at which audio is output/in-

put to/from external audio interfaces/devices. This feedback is in accordance with

the USB Audio Class 2.0 speciﬁcation.

After each received USB SOF token, the buﬀering core takes a timestamp from

a port clocked oﬀ the master clock. By subtracting the timestamp taken at the

previous SOF, the number of master clock ticks since the last SOF is calculated.

From this the number of samples (as a ﬁxed point number) between SOFs can be

calculated. This count is aggregated over 128 SOFs and used as a basis for the

feedback value.

The sending of feedback to the host is also handled in the USB buﬀering core.

REV 6.1

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