analogReference
void analogReference(uint8_t value);
This function sets the reference voltage for an analog pin, or the top voltage you expect to read from it. You have two options:
DEFAULT: The default value for the board is used (5v)
EXTERNAL: The current voltage applied to the AREF pin is used (0v - 5v ONLY)
Usage:
//I want my analog reverence to be 3v, and it is being applied to the AREF pin
analogReference(EXTERNAL);
//some time passes
//now I want to use the default value again
analogReference(DEFAULT);