I'm stuck with a rather funny situation, it has never happened to be before. Here I have an ATmega8, a 12MHz XTAL (CKOPT programmed, full swing) and a few other components that blink mostly. Everything is kind of ok, until at a certain moment it dies.
The death is very curious:
*) oscillator is on
*) WDT (previously verified to work reliably) does not reset the MCU
*) pulling RESET down does not reset the MCU
*) at the same time, it answers to avrdude (via ISP) and can be reprogrammed and verified correctly
*) the only way to make it tick again is by cycling the power
*) I had the same problem with the builtin RC oscillator but then I suspected that it's the oscillator that is the problem and added a crystal.. alas
I suspected poor grounds so I added more wires around GND and +5V just to make sure, in case there's a broken trace where I can't see it. Also, it appears that this fault is temperature-related: when I heat the board to some +30-40ÂºC it fails almost instantly. When cooled down, which is easy at this time of the year, it keeps going on as if there's nothing wrong.
Obvious problems that I'm aware of:
*) lack of proper ground plane (it's a single-sided PCB)
*) lack of load caps "” I know, bad, but the oscillator is working, besides, see above about RC
What would you suspect here? I'm rather frustrated.