DocID018909 Rev 11
899/1731
RM0090
Serial peripheral interface (SPI)
918
Figure 277. LSB justified 16-bit extended to 32-bit packet frame with CPOL = 0
When 16-bit data frame extended to 32-bit channel frame is selected during the I
2
S
configuration phase, Only one access to SPI_DR is required. The 16 remaining bits are
forced by hardware to 0x0000 to extend the data to 32-bit format. In this case it corresponds
to the half-word MSB.
If the data to transmit or the received data are 0x76A3 (0x0000 76A3 extended to 32-bit),
the operation shown in
is required.
Figure 278. Example of LSB justified 16-bit extended to 32-bit packet frame
In transmission mode, when TXE is asserted, the application has to write the data to be
transmitted (in this case 0x76A3). The 0x000 field is transmitted first (extension on 32-bit).
TXE is asserted again as soon as the effective data (0x76A3) is sent on SD.
In reception mode, RXNE is asserted as soon as the significant half-word is received (and
not the 0x0000 field).
In this way, more time is provided between two write or read operations to prevent underrun
or overrun conditions.
CK
WS
SD
Channel left 32-bit
Channel right
MSB
LSB
16-bit remaining
0 forced
16-bit data
Transmission
Reception
0X76A3
Only one access to SPI_DR