Audio Hub and I²S

The AudioHub task performs many functions. It receives and transmits samples from/to the Decoupler or Mixer thread over a channel.

It also drives several in and out I²S/TDM channels to/from a CODEC, DAC, ADC etc. From now on these external devices will be termed “audio hardware”.

If the firmware is configured with the xcore as I²S master the required clock lines will also be driven from this task. It also has the task of forwarding on and receiving samples to/from other audio related tasks/threads such as S/PDIF tasks, ADAT etc.

In master mode, the xcore generates the I²S “Continuous Serial Clock” (SCK), or “Bit-Clock” (BCLK) and the “Word Select” (WS) or “Left-Right Clock” (LRCLK) signals. Any CODEC or DAC/ADC combination that supports I²S and can be used.

The LR-clock, bit-clock and data are all derived from the incoming master clock (typically the output of the external oscillator or PLL). This is not part of the I²S standard but is commonly included for synchronizing the internal operation of the analog/digital converters.

The Audio Hub task is implemented in the file xua_audiohub.xc.

Table 30 shows the signals used to communicate audio between the XMOS device and the external audio hardware.

Table 30 I²S Signals

Signal	Description
LRCLK	The word clock, transition at the start of a sample
BCLK	The bit clock, clocks data in and out
SDIN	Sample data in (from CODEC/ADC to the XMOS device)
SDOUT	Sample data out (from the XMOS device to CODEC/DAC)
MCLK	The master clock running the CODEC/DAC/ADC

The bit clock controls the rate at which data is transmitted to and from the external audio hardware.

In the case where the xcore is the master, it divides the MCLK to generate the required signals for both BCLK and LRCLK, with BCLK then being used to clock data in (SDIN) and data out (SDOUT) of the external audio hardware.

Table 31 shows some example clock frequencies and divides for different sample rates:

Table 31 Clock Divide examples

Sample Rate (kHz)	MCLK (MHz)	BCLK (MHz)	Divide
44.1	11.2896	2.819	4
88.2	11.2896	5.638	2
176.4	11.2896	11.2896	1
48	24.576	3.072	8
96	24.576	6.144	4
192	24.576	12.288	2

For xcore-200 devices the master clock must be supplied by an external source e.g. clock generator, fixed oscillators, PLL etc. xcore.ai devices may use the integrated secondary PLL.

Two master clock frequencies are required to support 44.1 kHz and 48 kHz audio frequencies (e.g. 11.2896/22.5792MHz and 12.288/24.576MHz respectively). This master clock input is then provided to the external audio hardware and the xcore device.

Port configuration (xcore master)

The default software configuration is of xcore being the I²S master. That is, the xcore device provides the BCLK and LRCLK signals to the external audio hardware

xcore ports and clocks provide many valuable features for implementing I²S. This section describes how these are configured and used to drive the I²S interface.

Fig. 4 Ports and clocks (xcore master)

The code to configure the ports and clocks is in the ConfigAudioPorts() function. Developers should not need to modify this.

The xcore inputs MCLK and divides it down to generate BCLK and LRCLK.

To achieve this MCLK is input into the device using the 1-bit port p_mclk. This is attached to the clock block clk_audio_mclk, which is in turn used to clock the BCLK port, p_bclk. BCLK is used to clock the LRCLK (p_lrclk) and data signals SDIN (p_sdin) and SDOUT (p_sdout).

Again, a clock block is used (clk_audio_bclk) which has p_bclk as its input and is used to clock the ports p_lrclk, p_sdin and p_sdout. Fig. 3 shows the connectivity of ports and clock blocks.

p_sdin and p_sdout are configured as buffered ports with a transfer width of 32, so all 32 bits are input in one input statement. This allows the software to input, process and output 32-bit words, whilst the ports serialize and deserialize to the single I/O pin connected to each port.

Unlike previous xcore devices, xcore-200 (XS2 architecture) and xcore.ai (XS3 architecture) series devices have the ability to divide an external clock in a clock-block.

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() in xs1.h for details.

p_lrclk is clocked by p_bclk. In I²S mode the port outputs the pattern 0x7fffffff followed by 0x80000000 repeatedly. This gives a signal that has a transition one bit-clock before the data (as required by the I²S standard) and alternates between high and low for the left and right channels of audio.

Changing audio sample frequency

When the host changes sample frequency, a new frequency is sent to the audio driver thread by Endpoint 0 (via the buffering threads and mixer).

First, a change of sample frequency is reported by sending the new frequency over an XC channel. The audio thread detects this by checking for the presence of a control token on the channel channel

Upon receiving the change of sample frequency request, the audio thread stops the I²S/TDM interface and calls the CODEC/port configuration functions.

Once this is complete, the I²S/TDM interface (i.e. the main loop in AudioHub) is restarted at the new frequency.