33.6.2.3 Clock Generation
In the SPI host operation (CTRLA.MODE = 0x3), the serial clock (SCK) is generated internally by the SERCOM Baud Rate Generator (BRG).
In the SPI mode, the BRG is set to Synchronous mode. The 8-bit Baud register (BAUD) value is used for generating SCK and clocking the Shift register (see Clock Generation – Baud-Rate Generator from Related Links).
In the SPI client operation (CTRLA.MODE = 0x2), the clock is provided by an external host on the SCK pin. This clock is used to clock the SPI Shift register.