arduino reference changed to 2.5V

I am using arduino mega2560. I have experience that default value of reference changed to 2.5V. It displays 1023 count at 2.5V. I am not be able to read voltage above 2.5V. Anyone have any idea why this is happening?

There could be a clue in your program but we have never seen it. http://arduino.cc/forum/index.php/topic,148850.0.html

And maybe you might explictly set the analog reference to some known value and then measure again.

My best guess is that you set the reference voltage to the internal 2.5V reference.

Or dodgy wiring , power supply problems or ....

.. it's the moon!

Does you code ever call analogReference(INTERNAL2V56);?