Why do controllers stop working?

Question

Electronics

I have already soldered 3 MEGA 2560 controllers and they all stop working for an unknown reason. This is a board, roughly speaking, a tester for another board that is installed on top of mine. There are 8 optocouplers on the ward, max 232 and wiring for MEGA 2560. After about a day of working with the controller, it stops showing signs of life. When you upload the firmware writes this error.

spoiler

avrdude: Yikes! Invalid device signature.
Double check connections and try again, or use -F to override
this check.
Произошла ошибка при загрузке скетча

All this is powered from 24V with a step down to 8V (8V because the display backlight is connected directly) and 8V goes to the controller through a diode and a 5V zener diode. Everything works and nothing is heated, the consumption is not more than 70mA.
But I have already changed two controllers and that's the question, can the controller die due to the firmware and can it be resurrected?

Answer 1

The problem is most likely here:
> 8V goes to the controller through a diode and a 5V zener diode.
The zener diode operates in a certain range of currents, outside this range the voltage may not be 5V. Since the controller's consumption changes rapidly during operation, you are unlikely to detect with a voltmeter that something is wrong.
Solution: put a normal stabilizer 8V -> 5V.

Answer 2

Signature may be Erased in Serial Programming Mode
If the signature bytes are read before a chiperase command is completed, the signature may be erased causing the device ID and calibration bytes to disappear. This is critical, especially if the part is running on internal RC oscillator. But this is during the firmware when erasing