Watchdog timer
A hardware countdown that reboots the system if the software stops proving it's alive.
Firmware has to write to a register at regular intervals — "petting" or "kicking" the dog — before the counter reaches zero. If the program gets stuck in an infinite loop, a deadlock or an interrupt storm, the kick never arrives and the watchdog pulls the reset line. For a device bolted inside an electricity meter or on a mountaintop, a two-second reboot is often the best error handling available.
The classic mistake is kicking the dog from a timer interrupt: the interrupt keeps ticking long after the main loop has died, and the watchdog becomes decoration. Better designs only kick once every task has checked in, and windowed watchdogs also fire when the kick comes too early — a sign the program skipped something.