Clock Sources

All the internal clock sources are automatically enabled when they are requested by a peripheral. The crystal oscillators, based on an external crystal, must be enabled before it can serve as a clock source.

The XOSC32K oscillator is enabled by writing a ‘1’ to the ENABLE bit in the 32.768 kHz Crystal Oscillator Control A (CLKCTRL.XOSC32KCTRLA) register
The XOSCHF oscillator is enabled by writing a ‘1’ to the ENABLE bit in the High-Frequency Crystal Oscillator Control A (CLKCTRL.XOSCHFCTRLA) register

After Reset, the device starts up running from the internal high-frequency oscillator or the internal 32.768 kHz oscillator.

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