11.3.4 Clock Sources

All internal clock sources are enabled automatically when they are requested by a peripheral. The crystal oscillator, based on an external crystal, must be enabled by writing a '1' to the ENABLE bit in the 32.768 kHz Crystal Oscillator Control A (CLKCTRL.XOSC32KCTRLA) register before it can serve as a clock source.

The respective Oscillator Status bits in the Main Clock Status (CLKCTRL.MCLKSTATUS) register indicate whether the clock source is running and stable.