Yes, the question is a little too wide.
right now I'm using an ATmega128RFA1 microcontroller whose power supply is rated to be 3.6V max.
I did some test with some boards powering them through USB (5V) just to test if they whould burn, but they didn't.
After that I've been powering them with a LiPo battery (4.2V max) and they're working fine for now.
Should the chip be damaged for using it at 4.2V?
Is that too much of an overvoltage?
Is there any symptom that I should/could check?
Since these are for prototipes, I don't care if they burn, in fact I'd like to know how much voltage they can tolerate without damaging them.