Sai Synchronization Mode - ST STM32L4x6 Reference Manual

Serial audio interface (SAI)
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.
39.3.3

SAI synchronization mode

There are two levels of synchronization, either at audio sub-block level or at SAI level.
Internal synchronization
An audio sub-block can be configured to operate synchronously with the second audio sub-
block in the same
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).
Typically, the audio block in synchronous mode can be used to configure the SAI in full
duplex mode. One of the two audio blocks can be configured as a master and the other as
slave, or both as slaves with one asynchronous block (corresponding SYNCEN[1:0] bits set
to 00 in SAI_xCR1) and one synchronous block (corresponding SYNCEN[1:0] bits set to 01
in the SAI_xCR1).
Note: Due to internal resynchronization stages, PCLK APB frequency must be higher than twice
the bit rate clock frequency.
External synchronization
The audio sub-blocks can also be configured to operate synchronously with another SAI.
This can be done as follow:
1. The SAI, which is configured as the source from which the other SAI is synchronized,
has to define which of its audio sub-block is supposed to provide the FS and SCK
signals to other SAI. This is done by programming SYNCOUT[1:0] bits.
2. The SAI which shall receive the synchronization signals has to select which SAI will
provide the synchronization by setting the proper value on SYNCIN[1:0] bits. For each
of the two SAI audio sub-blocks, the user must then specify if it operates synchronously
with the other SAI via the SYNCEN bit.
Note: SYNCIN[1:0] and SYNCOUT[1:0] bits are located into the SAI_GCR register, and SYNCEN
bits into SAI_xCR1 register.
If both audio sub-blocks in a given SAI need to be synchronized with another SAI, it is
possible to choose one of the following configurations:
•
Configure each audio block to be synchronous with another SAI block through the
SYNCEN[1:0] bits.
•
Configure one audio block to be synchronous with another SAI through the
SYNCEN[1:0] bits. The other audio block is then configured as synchronous with the
second SAI audio block through SYNCEN[1:0] bits.
1322/1693
) flag is different from 000b (FIFO empty).
(FLH
SAI. In this case, the bit clock and the frame synchronization signals are
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