
DocID024597 Rev 5
RM0351
Serial audio interface (SAI)
1490
The mute mode bit is strobed only at the end of the frame. If it is set at this time, the mute
mode is active at the beginning of the new audio frame and for a complete frame, until the
next end of frame. The bit is then strobed to determine if the next frame will still be a mute
frame.
If the number of slots set through NBSLOT[3:0] bits in the SAI_xSLOTR register is lower
than or equal to 2, it is possible to specify if the value sent in mute mode is 0 or if it is the last
value of each slot. The selection is done via MUTEVAL bit in the SAI_xCR2 register.
If the number of slots set in NBSLOT[3:0] bits in the SAI_xSLOTR register is greater than 2,
MUTEVAL bit in the SAI_xCR2 is meaningless as 0 values are sent on each bit on each
slot.
The FIFO pointers are still incremented in mute mode. This means that data present in the
FIFO and for which the mute mode is requested are discarded.
Audio sub-block in reception mode
In reception mode, it is possible to detect a mute mode sent from the external transmitter
when all the declared and valid slots of the audio frame receive 0 for a given consecutive
number of audio frames (MUTECNT[5:0] bits in the SAI_xCR2 register).
When the number of MUTE frames is detected, the MUTEDET flag in the SAI_xSR register
is set and an interrupt can be generated if MUTEDETIE bit is set in SAI_xCR2.
The mute frame counter is cleared when the audio sub-block is disabled or when a valid slot
receives at least one data in an audio frame. The interrupt is generated just once, when the
counter reaches the value specified in MUTECNT[5:0] bits. The interrupt event is then
reinitialized when the counter is cleared.
Note:
The mute mode is not available for SPDIF audio blocks.
Mono/stereo mode
In transmitter mode, the mono mode can be addressed, without any data preprocessing in
memory, assuming the number of slots is equal to 2 (NBSLOT[3:0] = 0001 in SAI_xSLOTR).
In this case, the access time to and from the FIFO will be reduced by 2 since the data for
slot 0 is duplicated into data slot 1.
To enable the mono mode,
1.
Set MONO bit to 1 in the SAI_xCR1 register.
2. Set NBSLOT to 1 and SLOTEN to 3 in SAI_xSLOTR.
In reception mode, the MONO bit can be set and is meaningful only if the number of slots is
equal to 2 as in transmitter mode. When it is set, only slot 0 data will be stored in the FIFO.
The data belonging to slot 1 will be discarded since, in this case, it is supposed to be the
same as the previous slot. If the data flow in reception mode is a real stereo audio flow with
a distinct and different left and right data, the MONO bit is meaningless. The conversion
from the output stereo file to the equivalent mono file is done by software.
Companding mode
Telecommunication applications can require to process the data to be transmitted or
received using a data companding algorithm.
Depending on the COMP[1:0] bits in the SAI_xCR2 register (used only when TDM mode is
selected), the application software can choose to process or not the data before sending it
on SD serial output line (compression) or to expand the data after the reception on SD serial