ST STM32F413 Reference Manual page 980

Serial audio interface (SAI)
Bit 4 Reserved, always read as 0.
Bits 3:2 PRTCFG[1:0]: Protocol configuration. These bits are set and cleared by software.
00: Free protocol
01: Not used
10: AC'97 protocol
11: Not used
Free protocol selection allows to use the powerful configuration of the audio block to address a
specific audio protocol (like I2S, LSB/MSB justified, TDM, PCM/DSP...) setting most of the
configuration register bits as well as frame configuration register.
These bits have to be configured when the audio block is disabled.
Bits 1:0 MODE[1:0]: Audio block mode. These bits are set and cleared by software.
00: Master transmitter
01: Master receiver
10: Slave transmitter
11: Slave receiver
These bits have to be configured when the audio block is disabled.
Note: In Master transmitter mode the audio block will start to generate the FS and clocks
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