I distinguish between "hang-up" and "lock-up". Maybe these terms are incorrect, because I'm not an english native speaker.

Hang-up - The micro doesn't exactly do what he is intented to do. But can be restarted by a reset:

Due to wrongly written code, the code makes jumps to addresses, where the micro cannot read a proper instruction. Or the code contains invalid instructions, i.e., bytes that do not correspond to a valid instruction. These failures can happen, if the code isn't written correctly, or the programming failed resulting in code failures, or the code memory was damaged by radiation, heat or improper power-ups and -downs with no valid reset signal (a known problem of flash code micros). Another cause of improper running can be unwanted skips of the program counter due to noise and glitches on the port lines or else pins.

The micro can be restarted by a reset puls, but this does not necessarily mean that it is running properly afterwards...


Lock-up - The micro stops from doing anything, but becomes hot and can only be restarted by a full power-down and power-up:

This severe failure mode is often the result from strong interference hitting the micro and deranging one of the many internal micro routines always running to make the micro properly work. Lock-up can also be the result from a CMOS latch-up on the die, due to heavy glitches on the port lines or else pins.

These are the failure modes that just come into my mind, but there are many others, I'm sure...

Kai