ST STM32G4 Series Reference Manual page 1755

RM0440
Slave mode
The SAI expects to receive timing signals from an external device.
•
If the SAI subblock is configured in asynchronous mode, then SCK_x and FS_x pins
are configured as inputs.
•
If the SAI subblock is configured to operate synchronously with the second audio
subblock, the corresponding SCK_x and FS_x pins are left free to be used as general
purpose I/Os.
In slave mode, MCLK_x pin is not used and can be assigned to another function.
It is recommended to enable the slave device before enabling the master.
Configuring and enabling SAI modes
Each audio subblock can be independently defined as a transmitter or receiver through the
MODE bit in the SAI_xCR1 register of the corresponding audio block. As a result, SAI_SD_x
pin will be respectively configured as an output or an input.
Two master audio blocks in the same SAI can be configured with two different MCLK and
SCK clock frequencies. In this case they have to be configured in asynchronous mode.
Each of the audio blocks in the SAI are enabled by SAIEN bit in the SAI_xCR1 register. As
soon as this bit is active, the transmitter or the receiver is sensitive to the activity on the
clock line, data line and synchronization line in slave mode.
In master TX mode, enabling the audio block immediately generates the bit clock for the
external slaves even if there is no data in the FIFO, However FS signal generation is
conditioned by the presence of data in the FIFO. After the FIFO receives the first data to
transmit, this data is output to external slaves. If there is no data to transmit in the FIFO, 0
values are then sent in the audio frame with an underrun flag generation.
In slave mode, the audio frame starts when the audio block is enabled and when a start of
frame is detected.
In Slave TX mode, no underrun event is possible on the first frame after the audio block is
enabled, because the mandatory operating sequence in this case is:
1. Write into the SAI_xDR (by software or by DMA).
2. Wait until the FIFO threshold
3. Enable the audio block in slave transmitter mode.
39.3.4 SAI synchronization mode
SAI sub-clock A and B can be synchronized.
Internal synchronization
An audio subblock can be configured to operate synchronously with the second audio
subblock in the same SAI. In this case, the bit clock and the frame synchronization signals
are shared to reduce the number of external pins used for the communication. The audio
block configured in synchronous mode sees its own SCK_x, FS_x, and MCLK_x pins
released back as GPIOs while the audio block configured in asynchronous mode is the one
for which FS_x and SCK_x ad MCLK_x I/O pins are relevant (if the audio block is
considered as master).
) flag is different from 0b000 (FIFO empty).
(FLH
RM0440 Rev 1
Serial audio interface (SAI)
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