RM0390 Rev 4 867/1328
RM0390 Serial peripheral interface/ inter-IC sound (SPI/I2S)
896
The received CRC is stored in the Rx buffer like any other data frame.
A CRC-format transaction takes one more data frame to communicate at the end of data
sequence.
When the last CRC data is received, an automatic check is performed comparing the
received value and the value in the SPIx_RXCRC register. Software has to check the
CRCERR flag in the SPIx_SR register to determine if the data transfers were corrupted or
not. Software clears the CRCERR flag by writing '0' to it.
After the CRC reception, the CRC value is stored in the Rx buffer and must be read in the
SPIx_DR register in order to clear the RXNE flag.
CRC transfer managed by DMA
When SPI communication is enabled with CRC communication and DMA mode, the
transmission and reception of the CRC at the end of communication is automatic (with the
exception of reading CRC data in receive-only mode). The CRCNEXT bit does not have to
be handled by the software. The counter for the SPI transmission DMA channel has to be
set to the number of data frames to transmit excluding the CRC frame. On the receiver side,
the received CRC value is handled automatically by DMA at the end of the transaction but
user must take care to flush out the CRC frame received from SPI_DR as it is always loaded
into it.
At the end of the data and CRC transfers, the CRCERR flag in the SPIx_SR register is set if
corruption occurred during the transfer.
Resetting the SPIx_TXCRC and SPIx_RXCRC values
The SPIx_TXCRC and SPIx_RXCRC values are cleared automatically when CRC
calculation is enabled.
When the SPI is configured in slave mode with the CRC feature enabled, a CRC calculation
is performed even if a high level is applied on the NSS pin. This may happen for example in
case of a multislave environment where the communication master addresses slaves
alternately.
Between a slave disabling (high level on NSS) and a new slave enabling (low level on NSS),
the CRC value should be cleared on both master and slave sides to resynchronize the
master and slave respective CRC calculation.
To clear the CRC, follow the below sequence:
1. Disable the SPI
2. Clear the CRCEN bit
3. Enable the CRCEN bit
4. Enable the SPI
Note: When the SPI is in slave mode, the CRC calculator is sensitive to the SCK slave input clock
as soon as the CRCEN bit is set, and this is the case whatever the value of the SPE bit. In
order to avoid any wrong CRC calculation, the software must enable the CRC calculation
only when the clock is stable (in steady state). When the SPI interface is configured as a
slave, the NSS internal signal needs to be kept low between the data phase and the CRC
phase once the CRCNEXT signal is released.
At TI mode, despite the fact that the clock phase and clock polarity setting is fixed and
independent on the SPIx_CR1 register, the corresponding setting CPOL=0 CPHA=1 has to
be kept at the SPIx_CR1 register anyway if CRC is applied. In addition, the CRC calculation
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