Serial audio interface (SAI) RM0090
946/1749 RM0090 Rev 18
Figure 295. FIFO underrun event
29.13.2 Anticipated frame synchronisation detection (AFSDET)
This flag AFSDET is used only in Slave mode. In master mode, it is never asserted. It
informs about the detection of a frame synchronisation (FS) earlier than expected since the
frame length, the frame polarity, the frame offset are defined and known.
Early detection sets flag AFSDET in the SAI_xSR register.
This detection has no effect on the current audio frame which is not sensitive to the
anticipated FS. This means that “parasitic” events on signal FS are flagged without any
perturbation of the current audio frame.
If bit AFSDETIE is set in the SAI_xIM register, an interrupt is generated. To clear the flag
AFSDET, bit CAFSDET in the SAI_xCLRFR register has to be set.
To resynchronize with the master after Anticipated frame detection error, four steps should
be respected:
1. SAI block should be disabled by resetting SAIEN bit in SAI_xCR1 register, to be sure
that the SAI is disabled SAIEN bit is should be equal to 0 (reading back this bit).
2. FIFO should be flushed via FFLUS bit in SAI_xCR2 register.
3. Re-enabling the SAI peripheral (SAIEN bit set to 1) then the SAI.
4. SAI block will wait for the assertion on FS to restart the synchronization with master.
Note: This flag is not asserted in AC’97 since the SAI audio block acts as a link controller and
generates the FS signal even when declared as slave.
29.13.3 Late frame synchronization detection
Flag LFSDET in the SAI_xSR register can be set only when the SAI audio block is defined
as slave. The frame length, the frame polarity and the frame offset configuration are known
in register SAI_xFRCR.
If the external master does not send the FS signal at the expecting time (generating the
signal too late), the flag LFSDET in the SAI_xSR register will be set and an interrupt is
generated if bit LFSDETIE in the SAI_xIM register is set.
The flag is cleared when bit CLFSDET is set in the SAI_xCLRFR register.
MS192347V2
sck
data
Slot 0 ON MUTE
Slot size = data size
SD (output)
Example: FIFO underrun on Slot 1
Audio frame Audio frame
MUTE Slot 1 ON ... ON Slot 0 ON
OVRUND=1
MUTE
FIFO empty
OVRUND
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