Block Diagram

Figure 1. CLKCTRL Block Diagram
The clock system consists of the main clock and clocks derived from the main clock, as well as several asynchronous clocks:

The clock source for the main clock domain is configured by writing to the Clock Select (CLKSEL) bit field in the Main Clock Control A (CLKCTRL.MCLKCTRLA) register. This register has Configuration Change Protection (CCP), and the appropriate key must be written to the CCP register before writing to the CLKSEL bit field. The asynchronous clock sources are configured by the registers in the respective peripheral.