12.3.7 Sleep Mode Operation

When a clock source is not used or requested, it stops. It is possible to request a clock source directly by writing a ‘1’ to the Run Standby (RUNSTDBY) bit in the respective oscillator’s Control A (CLKCTRL.oscillatorCTRLA) register. This causes the oscillator to run constantly, except in Power-Down sleep mode. Additionally, when this bit is set to ‘1’, the oscillator start-up time is eliminated when the clock source is requested by a peripheral.

The main clock always runs in Active mode and Idle sleep mode. In Standby sleep mode, the main clock runs only if any peripheral is requesting it or if the RUNSTDBY bit in the respective oscillator’s CLKCTRL.oscillatorCTRLA register is set to ‘1’.

In Power-Down sleep mode, the main clock stops after all Nonvolatile Memory (NVM) operations are completed. Refer to the SLPCTRL - Sleep Controller chapter for more details on sleep mode operation.