29.4.2.1 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.

Synchronous mode is selected by writing ‘1’ to the Communication Mode bit in the Control A register (CTRLA.CMODE), while Asynchronous mode is selected by writing ‘0’ to CTRLA.CMODE.

The internal clock source is selected by writing ‘1’ to the Operation Mode bit field in the Control A register (CTRLA.MODE), while the external clock source is selected by writing ‘0’ to CTRLA.MODE.

The SERCOM baud rate generator is configured as in the figure below.

In Asynchronous mode, the 16-bit Baud register value is used.

In Synchronous mode, the eight LSbs of the Baud register are used. Refer to Clock Generation – Baud-Rate Generator for details on configuring the baud rate.