The SPI Baud rate clock is generated by dividing the peripheral clock by a value between 1 and 255.
If SPI_CSRx.SCBR is programmed to 1, the operating baud rate is peripheral clock (refer to the section “Electrical Characteristics” for the SPCK maximum frequency). Triggering a transfer while SPI_CSRx.SCBR is at 0 can lead to unpredictable results.
At reset, SPI_CSRx.SCBR=0 and the user has to program it to a valid value before performing the first transfer.
The divisor can be defined independently for each chip select, as it has to be programmed in SPI_CSRx.SCBR. This allows the SPI to automatically adapt the baud rate for each interfaced peripheral without reprogramming.