45.6.1.3 Baud Rate in Synchronous Mode or SPI Mode

If the USART is programmed to operate in Synchronous mode, the selected clock is divided by the value of US_BRGR.CD.

In Synchronous mode, if the external clock is selected (USCLKS = 3), the clock is provided directly by the signal on the USART SCK pin. No division is active. The value written in US_BRGR has no effect. The external clock frequency must be at least 3 times lower than the system clock. In Host mode, Synchronous mode (USCLKS = 0 or 1, CLKO set to 1), the receive part limits the SCK maximum frequency to Selected Clock/3 in USART mode, or Selected Clock/6 in SPI mode.

When either the external clock SCK or the internal clock divided (peripheral clock/DIV) is selected, the value of CD must be even if the user has to ensure a 50:50 mark/space ratio on the SCK pin. When the peripheral clock is selected, the baud rate generator ensures a 50:50 duty cycle on the SCK pin, even if the value of CD is odd.