32.6.2.3 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 ‘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), the external clock source is selected by writing ‘0’ to CTRLA.MODE.

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

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.