DocID018909 Rev 11
925/1731
RM0090
Serial audio interface (SAI)
957
flow.
29.7.3 Frame
synchronization active level length
Bit FSALL[6:0] in the SAI_xFRCR register configures the length of the active level of the
Frame synchronization signal. The length can be set from 1 to 128 bit clock SCK.
The active length may be half of the frame length in I2S, LSB or MSB-justified modes for
instance, or one-bit wide for PCM/DSP or TDM mode, or even 16-bit length in AC’97.
29.7.4 Frame
synchronization offset
Depending on the audio protocol targeted in the application, the Frame synchronization
signal can be asserted when transmitting the last bit or the first bit of the audio frame (as for
instance, respectively, in I2S standard protocol and in MSB-justified protocol). Bit FSOFF in
the SAI_xFRCR register makes the choice.
29.7.5 FS
signal
role
The FS signal may have a different meaning depending on the FS function. Bit FSDEF in
the SAI_xFRCR register selects which meaning it will have. It may be either:
•
0: a start of frame, like for instance the PCM/DSP, TDM, AC’97, audio protocols,
•
1: a start of frame and a channel side identification within the audio frame like for the
I2S, the MSB or LSB-justified protocols.
When the FS signal is considered as a start of frame and channel side identification within
the frame, the number of declared slots must be considered to be half of the number for the
left channel and half of the number for the right channel. If the number of bit clock on half
audio frame is greater than the number of slots dedicated to a channel side, if TRIS = 0, 0 is
sent for transmission for the remaining bit clock in the SAI_xCR2 register, otherwise if TRIS
= 1, the SD line is released to HI-Z. In reception, the remaining bit clock are not considered
until the channel side changes.