Configures the reference voltage used for analog input (i.e. the value used as the top of the input range). The options are:
type: which type of reference to use (DEFAULT, INTERNAL, INTERNAL1V1, INTERNAL2V56, or EXTERNAL).
After changing the analog reference, the first few readings from analogRead() may not be accurate.
If you're using an external reference voltage (applied to the AREF pin), you must set the analog reference to EXTERNAL before calling analogRead(). Otherwise, you will short together the active reference voltage (internally generated) and the AREF pin, possibly damaging the microcontroller on your Arduino board.
Alternatively, you can connect the external reference voltage to the AREF pin through a 5K resistor, allowing you to switch between external and internal reference voltages. Note that the resistor will alter the voltage that gets used as the reference because there is an internal 32K resistor on the AREF pin. The two act as a voltage divider, so, for example, 2.5V applied through the resistor will yield 2.5 * 32 / (32 + 5) = ~2.2V at the AREF pin.
Corrections, suggestions, and new documentation should be posted to the Forum.
This reference is licensed under a Creative Commons Attribution-ShareAlike 3.0 License and is based on the Arduino reference. Code samples in the reference are released into the public domain.