[Mega R3] Damaged Analog Ports

Hi, I've got one Arduino Mega here with what looks like damaged analog pins (A5 & A6).

A6 is completly dead and always reading "2", no matter if pulled up or down. A5 can be connected to +5V and grounded and will read correctly, however, when nothing is attached, the pin ACTIVELY goes to 2.73V (not just noise). Pulling up A5 to +5V with a 100 Ohm resistor doesn't work, instead of +5V the pin goes to 3.22V only.

I've confirmed the readings with a multimeter.

[A5 no connection] 561 = 2.74V [active voltage] [A5 pulled with +5V over 100Ohm] 629 = 3.22V

[A6 no connection] 2 = 0.01V [A6 +5V directly] 2 = 0.01V

Did anyone see this kind of damage to an Atmega 2560 before?

Looks like you may have screwed them up.


You've blown the pins.

A6 is shorted to ground (ie, low driver is permanently on); this is the most common failure mode when you blow a pin.

A5 sounds like both low and high drivers are blown (ie, both are permanently on, and fighting eachother). If you measure current, you'll find it to be like 20-50mA higher than it should be, and I'll bet the chip is getting warm too.

Because A5 lost both drivers, and they're now fighting eachother, the board should not even be used for applications where those pins are not in use. Trashcan it, buy a new one, and don't abuse the pins this time.

Usually pins get blown like that one of two ways - applying external voltages higher than Vcc or lower than Ground (you get like half a volt of margin in either direction per datasheet. Remember that when the board is not powered, Vcc is 0v, so you can't apply an external voltage to any pin!), or exceeding max current spec from the pin (and that usually doesn't kill the pins).

DrAzzy: You've blown the pins. [...]

Thanks for your great analysis - especially on the thermal part, the Atmel is indeed getting warm and over USB I notice a slight voltage drop.

For "production" use I've got more Arduino Mega, however I just wanted to find out where I blew the thing. But it's probably been an incorrectly connected 12V pin.

I've "fixed" the pins by cutting the lines and rewiring for debugging and it's working fine, thanks!