The Voltage Reference (VREF) peripheral provides control registers for the voltage reference sources used by several peripherals. The user can select the reference voltages for the ADC0 by writing to the ADC0 Reference Select (ADC0REFSEL) bit field in the Control A (VREF.CTRLA) register, and for the AC0 by writing to the AC0 Reference Select (DAC0REFSEL) bit field in the Control A (VREF.CTRLA) register.
A voltage reference source is automatically enabled 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 Force Enable (ADC0REFEN, DAC0REFEN) bit in the Control B (VREF.CTRLB) register. This may be done to decrease start-up time, at the cost of increased power consumption.