10.4.5 Default Clock Source

The calibrated internal 8MHz oscillator is always selected as main clock when the device is powered up or has been reset. The synchronous system clock is the main clock divided by 8, controlled by the System Clock Prescaler. The Clock Prescaler Select Bits in Clock Prescale Register (CLKPSR.CLKPS) can be written later to change the system clock frequency. See section “System Clock Prescaler”.