analogRead() strange behavior

Development Tools IDE 1.x
1

When I run the Potentiometer example (http://www.arduino.cc/en/Tutorial/Potentiometer) on my Mega 2560 board, I keep getting only 9 distinct values. In the table below, the first column is the input voltage measured on the input pin by a multimeter , the second column is the corresponding analogRead() output.

0V-1.36V 0
1.36V-1.72V 15
1.72V-1.80V 31
1.80V-2.04V 527
2.04V-2.11V 559
2.11V-2.17V 591
2.17V-2.19V 799
2.19V-2.35V 1007
2.35V-5.00V 1023

So for example for any voltage in the 1.36V-1.72V range, analogRead() returns 15. This behavior is the same on all analog pins and is not definitely caused by an incorrect wiring. I tried this on a UNO board and it works as expected - 0-5V is correctly linearly mapped to 0-1023.

Any idea what could be the problem?

2

Seems like a defective ADC to me, ... This sketch is about the minimal test

If you remove the potmeter and let the analog line "float", does it give other values than the ones you mentioned?

3

I'm getting 527 (no fluctuation) without the potmeter.

I also tend to think that this is a hardware problem. Do you have any idea what could cause this? I'm not 100% sure but I think that analogRead() worked fine on this board before.

4

Yep sounds like HW problem. As the ADC is multiplexed for all pins they will all fail. It might be broken with some experiment or an accident. I blew up an LCD once by connecting the backlight to 12V iso 5V (was a nice short flash).

The pins might still work as digitalOut pins. pinMOde(A0, OUTPUT); digitalwrite(A0, HIGH/LOW) etc.