I'm using a Mega to drive a 4x20 LCD via I2C. Worked for a few seconds, then nothing. Tried another one, same thing. I've duplicated the symptoms using a 3.3v Pro Mini, eg, toasted one of the SDC/SLC pins. I've got a bunch of older Megas driving these displays no problem. Both the toasted Megas and Pro Minis continue to operate fine, except for the SDC/SLC pins (they reprogram and send out messages on Serial).
Did the Mega go 3.3v when I wasn't looking and how would I tell by looking at the product? Some of the Pro Minis have a 3.3v text on the back, but I don't see anything on these Megas.
Both those pins on the displays are inputs and so should not cause a problem if reversed or the voltage was wrong? Suggests a faulty display or a short circuit somewhere .
According to the specifications for the ATmega2560 of the Mega, the minimum Vcc is 4.5 V. There is an ATmega2560V that has a 1.8 V minimum, but I'm not aware of any Arduino boards that use it (certainly not the official Mega).