ADAT receive¶

lib_xua supports the development of devices with ADAT receive functionality through the use of lib_adat. The XMOS ADAT receiver runs on a single thread.

The ADAT receive component receives up to eight channels of audio at a sample rate of 44.1kHz or 48kHz. The API for calling the receiver functions is described in the ADAT receive example in lib_adat .

The component outputs 32 bits words split into nine word frames. The frames are laid out in the following manner:

Control byte

Channel 0 sample

Channel 1 sample

Channel 2 sample

Channel 3 sample

Channel 4 sample

Channel 5 sample

Channel 6 sample

Channel 7 sample

An example of how to read the output of the ADAT component is shown below:

control = inuint(oChan);

for(int i = 0; i < 8; i++)
{
    sample[i] = inuint(oChan);
}

Samples are 24-bit values contained in the lower 24 bits of the word.

The control word comprises four control bits in bits [11..8] and the value 0b00000001 in bits [7..0]. This control word enables synchronization at a higher level, in that, on the channel, a single odd word is always read followed by eight words of data.

Usage and integration¶

Since the ADAT is a digital stream the device’s master clock must be synchronised to it. The integration of ADAT receive is much the same as S/PDIF receive in that the ADAT receive function communicates with the Clock Gen thread. This Clock Gen thread then passes audio data onto the Audio Hub thread. It also handles locking to the ADAT clock source.

There are some small differences with the S/PDIF integration accounting for the fact that ADAT typically has 8 channels compared to S/PDIF’s two.

The Clock Gen thread also handles SMUX II (e.g. 4 channels at 96kHz) and SMUX IV (e.g. 2 channels at 192 kHz), populating the sample FIFO as appropriate. SMUX modes are communicated to the Clock Gen thread from Endpoint 0 via the c_clk_ctl channel. SMUX modes are exposed to the USB host using Alternative Interfaces, with appropriate channel counts, for the streaming input Endpoint.