The ADC is by default disabled In Standby sleep mode.
The ADC can stay fully operational in Standby sleep mode if the Run in Standby bit (RUNSTDBY) in the Control A register (ADC.CTRLA) is written to '1'.
When the device is entering Standby sleep mode when RUNSTDBY is one, the ADC will stay active, hence any ongoing conversions will be completed, and interrupts will be executed as configured.
In Standby sleep mode an ADC conversion must be triggered via the Event System (EVSYS), or the ADC must be in free-running mode with the first conversion triggered by software before entering sleep. The peripheral clock is requested if needed and is turned off after the conversion is completed.
When an input Event trigger occurs, the positive edge will be detected, the Start Conversion bit (STCONV) in the Command register (ADC.COMMAND) set, and the conversion will start. When the conversion is completed, the Result Ready Flag (RESRDY) in the Interrupt Flags register (ADC.INTFLAGS) is set and the STCONV bit in ADC.COMMAND is cleared.
The reference source and supply infrastructure need time to stabilize when activated in Standby sleep mode. Configure a delay for the start of the first conversion by writing a non-zero value to the Initial Delay bits (INITDLY) in the Control D register (ADC.CTRLD).
In Power Down sleep mode, no conversions are possible. Any ongoing conversions are halted and will be resumed when going out of sleep. At end of conversion, the Result Ready Flag (RESRDY) will be set, but the content of the result registers (ADC.RES) is invalid since the ADC was halted in the middle of a conversion.