Analog reference voltage
type VRefSelect = enumeration(AREF "AREF used as VRef and internal VRef is turned off", AVCC "AVCC with external capacitor at the AREF pin is used as VRef (5V on Arduino)", Internal2 "This is a reserved setting in most AVRs. On ATmega1280, it is 1.1 internal reference voltage", Internal "Internal reference voltage (1.1V (ATmega168/328) or 2.56V on (ATmega8, ATmega1280))");