11.3.4 Clock Sources

The clock sources are divided into two main groups: internal oscillators and external clock sources. All the internal clock sources are automatically enabled when they are requested by a peripheral.

The crystal oscillator 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.

After Reset, the device starts 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.