ST STM32L4+ Series Reference Manual page 1904

Serial audio interface (SAI)
It is possible to choose the position of the first data bit to transfer within the slots. This offset
is configured by FBOFF[4:0] bits in the SAI_xSLOTR register. 0 values will be injected in
transmitter mode from the beginning of the slot until this offset position is reached. In
reception, the bit in the offset phase is ignored. This feature targets the LSB justified
protocol (if the offset is equal to the slot size minus the data size).
It is mandatory to respect the following conditions to avoid bad SAI behavior:
FBOFF ≤(SLOTSZ - DS),
DS ≤SLOTSZ,
NBSLOT x SLOTSZ ≤FRL (frame length),
The number of slots must be even when bit FSDEF in the SAI_xFRCR register is set.
In AC'97 and SPDIF protocol (bit PRTCFG[1:0] = 10 or PRTCFG[1:0] = 01), the slot size is
automatically set as defined in
1904/2301
Figure 550. Slot size configuration with FBOFF = 0 in SAI_xSLOTR
Audio block is transmitter
Slot size = data size
slotx data size
data size
slotx data size 00..00
16-bit
slotx data size
32-bit
Audio block is transmitter
Slot size = data size
slotx data size
data size
FBOFF
data size 00
00
slotx
16-bit
FBOFF = SLOT SZ -DS
slotx
00..00 data size
32-bit
Section 53.4.11: AC'97 link
Audio block is receiver
Slot size = data size
slotx data size
data size
slotx data size
00..00
slotx data size
X: don't care
Figure 551. First bit offset
Audio block is receiver
Slot size = data size
slotx data size
data size
FBOFF
data size
XX
slotx
FBOFF = SLOT SZ -DS
slotx
XX .. XX
X: don't care
RM0432 Rev 6
X..X
16-bit
XX..XX
32-bit
XX
16-bit
data size
32-bit
controller.
RM0432
MS30033V1
MS30034V1