33.6.2.3 Clock Generation
In SPI Host operation (CTRLA.MODE=0x3), the serial clock (SCK) is generated internally by the SERCOM baud-rate generator.
In SPI mode, the baud-rate generator is set to synchronous mode. The 8-bit Baud register (BAUD) value is used for generating SCK and clocking the shift register. Refer to the Clock Generation – Baud-Rate Generator for more details.
In SPI Client operation (CTRLA.MODE is 0x2), the clock is provided by an external Host on the SCK pin. This clock is used to directly clock the SPI shift register.
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