A watchdog timer is a computer hardware timing device that triggers a system reset if the main program, due to some fault condition, such as a hang, neglects to regularly service the watchdog (writing a 'service pulse' to it). The intention is to bring the system back from the hung state into normal operation.
Watchdog timers may be more complex, attempting to save debug information onto a persistent medium; i.e. information useful for debugging the problem that caused the fault. In this case a second, simpler, watchdog timer ensures that if the first watchdog timer does not report completion of its information saving task within a certain amount of time, the system will reset with or without the information saved. The most common use of watchdog timers is in embedded systems, where this specialized timer is often a built-in unit of a microcontroller.
Watchdog timers may also trigger control systems to move into a safety state, such as turning off motors, high-voltage electrical outputs, and other potentially dangerous subsystems until the fault is cleared.
