When a clock source is not used/requested, it will turn off. It is possible to
request a clock source directly by writing a '1' to the Run in Standby
(RUNSTDBY) bit in the respective oscillator's Control A (CLKCTRL.[osc]CTRLA) register. This
will cause the oscillator to run constantly, except for Power-Down sleep mode.
Additionally, when this bit is written to '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 and Idle sleep mode. In Standby sleep
mode, the main clock will only run if any peripheral is requesting it, or the Run in
Standby (RUNSTDBY) bit in the respective oscillator's Control A (CLKCTRL.[osc]CTRLA)
register is written to '1'.
In Power-Down sleep mode, the main clock will stop after all NVM operations are completed.