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3.12.1 Integration

The ADAT receive function communicates with the clockGen component which

passes audio data onto the audio driver and handles locking to the ADAT clock

source if required.

3.13 Clock Recovery

An application can either provide ﬁxed master clock sources via selectable oscil-

lators, clock generation IC, etc., to provide the audio master or use an external

PLL/Clock Multiplier to generate a master clock based on reference from the XS1.

Using an external PLL/Clock Multiplier allows the design to lock to an external

clock source from a digital steam (e.g. S/PDIF or ADAT input).

The clock recovery core (clockGen) is responsible for generating the reference

frequency to the Fractional-N Clock Generator. This, in turn, generates the master

clock used over the whole design.

When running in Internal Clock mode this core simply generates this clock using a

local timer, based on the XS1 reference clock.

When running in an external clock mode (i.e. S/PDIF Clock” or “ADAT Clock” mode)

digital samples are received from the S/PDIF and/or ADAT receive core. The

external frequency is calculated through counting samples in a given period. The

reference clock to the Fractional-N Clock Multiplier is then generated based on this

external stream. If this stream becomes invalid, the timer event will ﬁre to ensure

that valid master clock generation continues regardless of cable unplugs etc.

This core gets clock selection Get/Set commands from Endpoint 0 via the

c_clk_ctl

channel. This core also records the validity of external clocks, which is also queried

through the same channel from Endpoint 0.

This core also can cause the decouple core to request an interrupt packet on change

of clock validity. This functionality is based on the Audio Class 2.0 status/interrupt

endpoint feature.

3.14 MIDI

The MIDI driver implements a 31250 baud UART input and output. On receiving

32-bit USB MIDI events from the

buffer

core, it parses these and translates them

to 8-bit MIDI messages which are sent over UART. Similarly, incoming 8-bit MIDI

messages are aggregated into 32-bit USB-MIDI events and passed on to the

buffer

core. The MIDI core is implemented in the ﬁle usb_midi.xc.

3.15 Audio Controls via Human Interface Device (HID)

The design supports simple audio controls such as play/pause, volume etc via the

USB Human Interface Device Class Speciﬁcation.

REV 6.1

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