The Voltage Reference (VREF) peripheral provides voltage reference sources used by several peripherals. The user can select the reference voltages for the Analog Comparators by writing to the appropriate bit field in the Control A (VREF.CTRLA) register.
A voltage reference source is enabled automatically when requested by a peripheral. The user can enable the reference voltage sources (and thus, override the automatic disabling of unused sources) by writing to the respective ALWAYSON bit in the Control B (VREF.CTRLB) register. This will decrease the start-up time at the cost of increased power consumption.