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Divide Output pattern Outputs per sample

2 0xAAAAAAAA 2

4 0xCCCCCCCC 4

8 0xF0F0F0F0 8

Figure 13:

Output

patterns

In any case, the bit clock outputs 32 clock cycles per sample. In the special

case where the divide is 1 (i.e. the bit clock frequency equals the master clock

frequency), the

p_bclk

port is set to a special mode where it simply outputs its

clock input (i.e.

p_mclk

). See

configure_port_clock_output()

xs1.h

for details.

p_lrclk

is clocked by

p_bclk

. The port outputs the pattern

0x7fffffff

followed

0x80000000

repeatedly. This gives a signal that has a transition one bitclock

before the data (as required by the I2S standard) and alternates between high and

low for the left and right channels of audio.

3.8.2 Changing Audio Sample Frequency

When the host changes sample frequency, a new frequency is sent to the audio

driver core by Endpoint 0. First, a change of sample frequency is reported by

sending the new frequency over an XC channel. The audio core detects this using

the

select

function on a channel (a

default

case such that processing can continue

if no signal is present on the channel).

Upon receiving the change of sample frequency request, the audio core stops

the I2S interface and calls the CODEC/port conﬁguration functions. Once this is

complete, the I2S interface is restarted at the new frequency.

3.9 Digital Mixer

The mixer core takes outgoing audio from the decoupler and incoming audio from

the audio driver. It then applies the volume to each channel and passes incoming

audio on to the decoupler and outgoing audio to the audio driver. The volume

update is achieved using the built-in 32bit to 64bit signed multiply-accumulate

function (macs). The mixer is implemented in the ﬁle mixer.xc.

The mixer takes two cores and can perform eight mixes with up to 18 inputs at

sample rates up to 96kHz and two mixes with up to 18 inputs at higher sample

rates. The component automatically moves down to two mixes when switching to

a higher rate.

The mixer can take inputs from either:

· The USB outputs from the host—these samples come from the decoupler.

The inputs from the audio interface on the device—these samples come from

the audio driver.

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