ST STM32F405 Reference Manual page 940

Serial audio interface (SAI)
29.12 Specific features
The SAI has some specific functions which can be useful depending on the audio protocol
selected. These functions are accessible through specific bits in the SAI_xCR2 register.
29.12.1 Mute mode
Mute mode may be used when the audio block is a transmitter or receiver.
Transmitter
In transmitter mode, Mute mode can be selected at anytime. Mute mode is active for entire
audio frames. The bit MUTE in the SAI_xCR2 register requests Mute mode when it is set
during an on-going frame.
The mute mode bit is strobed only at the end of the frame. If set at this time, the mute mode
is active at the beginning of the new audio frame, for a complete frame, until the next end of
frame, it then strobes the bit to determine if the next frame will still be a mute frame.
If the number of slots set in bit NBSLOT[3:0] in the SAI_xSLOTR register is lower than or
equal to two, it is possible to specify if the value sent during the Mute mode is 0 or if it is the
last value of each slot. The selection is done via bit MUTEVAL in the SAI_xCR2 register.
If the number of slots set in bit NBSLOT[3:0] in the SAI_xSLOTR register is greater than
two, MUTEVAL bit in the SAI_xCR2 has no meaning as 0 values are sent on each bit on
each slot.
During Mute mode, the FIFO pointers are still incremented, meaning that data which was
present in the FIFO and for which the Mute mode is requested is discarded.
Receiver
In receiver 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 (bit MUTECNT[5:0] in the SAI_xCR2 register).
When the number of MUTE frames is detected, flag MUTEDET in the SAI_xSR register is
set and an interrupt can be generated if bit MUTEDETIE is set in the SAI_xCR2.
The mute frame counter is cleared when the audio 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 specified value in bit MUTECNT[5:0]. Then the interrupt event is re-
armed when the counter is cleared.
29.12.2 MONO/STEREO function
In transmission mode, it is possible to address the Mono mode without any data pre-
processing in memory when the number of slot is equal to 2 (NBSLOT[3:0] = 0001 in the
SAI_xSLOTR). In such a case, the access to and from the FIFO will be reduced by two
since in transmission, the data for slot 0 is duplicated into data slot 1.
To select the Mono feature, set bit MONO in the SAI_xCR1 register.
In reception mode, bit MONO can be set and has a meaning only if the number of slots is
equal to 2 like for the transmission mode. When it is set, only the data of slot 0 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 flux in reception is a real stereo audio flow
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