Block Diagram - CLKCTRL

Figure 1. CLKCTRL Block Diagram
The clock system consists of the Main Clock and other asynchronous clocks:

The clock source for the for the Main Clock domain is configured by writing to the Clock Select bits (CLKSEL) in the Main Clock Control A register (CLKCTRL.MCLKCTRLA). The asynchronous clock sources are configured by registers in the respective peripheral.