SparkFun Forums

[DonpK]

I wanted to use the 3.3V output on my Mega 2560 for an external reference voltage. I connected the 3.3V output directly to the AREF pin. I included analogReference(EXTERNAL); statement in my sketch and uploaded it to the Mega 2560. However, I may have done an analogRead() before the analogReference(EXTERNAL);

I have since learned elsewhere, that this can possibly damage the Arduino.

Now when I measure the voltage to ground at the AREF, all I read is the 5V Vcc regardless of whether I have set analogReference() to the 1.1V and 2.56V internal reference voltages or to analogReference(EXTERNAL); Do these measurements confirm that the AREF circuit has been damaged? If so, is any other part of the Mega 2560 likely to be damaged? Can I still use the analogReference(DEFAULT) mode?

[jremington]

So that others don't have to follow the link to be warned, here is what the Arduino AnalogReference page states:

Warning
Don't use anything less than 0V or more than 5V for external reference voltage on the AREF pin! If you're using an external reference on 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.

It is anyone's guess what might be wrong. Continue using that board at your own risk.