Clock Sources

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