In SPI mode, the baud rate generator operates in the same way as in
USART Synchronous mode. See “Baud Rate in Synchronous Mode or SPI Mode”. However, there are some restrictions:
In SPI Master mode:
- The external clock SCK must not be selected (USCLKS ≠ 0x3), and US_MR.CLKO must be written to ‘1’, in order to generate correctly the serial clock on the SCK pin.
- To obtain correct behavior of the receiver and the transmitter, the value programmed in US_BRGR.CD must be greater than or equal to 6.
- If the divided peripheral clock is selected, the value programmed in CD must be even to ensure a 50:50 mark/space ratio on the SCK pin. This value can be odd if the peripheral clock is selected.
In SPI Slave mode:
- The external clock (SCK) selection is forced regardless of the value of the US_MR.USCLKS. Likewise, the value written in US_BRGR has no effect, because the clock is provided directly by the signal on the USART SCK pin.
- To obtain correct behavior of the receiver and the transmitter, the external clock (SCK) frequency must be at least 6 times lower than the system clock.