12.3.8 Sleep Mode Operation

When a clock source is not used or requested, it will stop. 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 will cause the oscillator to run constantly, except for Power-Down sleep mode. Additionally, when this bit is written to a ‘1’, the oscillator start-up time is eliminated when the clock source is requested by a peripheral.

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

In Power-Down sleep mode, the main clock will stop after all nonvolatile memory (NVM) operations are completed. Refer to the SLPCTRL - Sleep Controller section for more details on sleep mode operation.

In sleep, the Clock Failure Detection (CFD) will only be enabled if the selected source is active. After a Reset, the CFD will not start looking for failure until a time equivalent to the monitored Oscillator Start-up Timer (SUT) has expired.