34.6.4.2.2 Clock Generation and Selection

For both Synchronous and Asynchronous modes, the clock used for shifting and sampling data can be generated internally by the SERCOM baud-rate generator or supplied externally through the XCK line.

The Synchronous mode is selected by writing a '1' to the Communication Mode bit in the Control A register (CTRLA.CMODE), the Asynchronous mode is selected by writing a zero to CTRLA.CMODE.

The internal clock source is selected by writing 0x1 to the Operation Mode bit field in the Control A register (CTRLA.MODE), the external clock source is selected by writing 0x0 to CTRLA.MODE. The SERCOM baud-rate generator is configured as in the figure below.

In Asynchronous mode (CTRLA.CMODE=0), the 16-bit Baud register value is used.

In Synchronous mode (CTRLA.CMODE=1), the eight LSBs of the Baud register are used. Refer to Clock Generation – Baud-Rate Generator for details on configuring the baud rate.