RM0440 Rev 4 1797/2126
RM0440 Serial audio interface (SAI)
1858
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.
40.3.5 Audio data size
The audio frame can target different data sizes by configuring bit DS[2:0] in the SAI_xCR1
register. The data sizes may be 8, 10, 16, 20, 24 or 32 bits. During the transfer, either the
MSB or the LSB of the data are sent first, depending on the configuration of bit LSBFIRST in
the SAI_xCR1 register.
40.3.6 Frame synchronization
The FS signal acts as the Frame synchronization signal in the audio frame (start of frame).
The shape of this signal is completely configurable in order to target the different audio
protocols with their own specificities concerning this Frame synchronization behavior. This
reconfigurability is done using register SAI_xFRCR. Figure 609 illustrates this flexibility.
Figure 609. Audio frame
In AC’97 mode or in SPDIF mode (bit PRTCFG[1:0] = 10 or PRTCFG[1:0] = 01 in the
SAI_xCR1 register), the frame synchronization shape is forced to match the AC’97 protocol.
The SAI_xFRCR register value is ignored.
Each audio block is independent and consequently each one requires a specific
configuration.
Frame length
• Master mode
The audio frame length can be configured to up to 256 bit clock cycles, by setting
FRL[7:0] field in the SAI_xFRCR register.
If the frame length is greater than the number of declared slots for the frame, the
remaining bits to transmit will be extended to 0 or the SD line will be released to HI-z
MSv30037V2
The falling edge can occur into this area
SCK
Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 …... Slot 0
FS
SD
(FSOFF = 0)
FS Length: up to 256 bits
FS active: up to 128 bits
Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 …... Slot 0
SD
(FSOFF = 1)
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