14.7.1 Voltage Reference Enable Signals and Start-up Time

The voltage reference has a start-up time that may influence the way it should be used. To save power, the reference is not always turned ON. The reference is ON during the following situations:
  1. When the BOD is enabled (by programming the BODLEVEL [2:0] Fuses).
  2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in ACSR (ACSR.ACBG)).
  3. When the ADC is enabled.

Thus, when the BOD is not enabled, after setting ACSR.ACBG or enabling the ADC, the user must always allow the reference to start up before the output from the Analog Comparator or ADC is used. To reduce power consumption in the Power-Down mode, the user can avoid the three conditions above to ensure that the reference is turned OFF before entering Power-Down mode.