49.7.7.6 Single Data Mode
In Single Data mode, only one data is written every time SPI_TDR is accessed, and only one data is read every time SPI_RDR is accessed.
When SPI_FMR.TXRDYM = 0, the Transmit FIFO operates in Single Data mode.
When SPI_FMR.RXRDYM = 0, the Receive FIFO operates in Single Data mode.
If Host mode is used (SPI_MR.MSTR=1), the Receive FIFO must operate in Single Data mode.
If Variable Peripheral Select mode is used (SPI_MR.PS=1), the Transmit FIFO must operate in Single Data mode.
See sections SPI Transmit Data Register and SPI Receive Data Register.