Single and Multistage Watchdog Timers

A watchdog timer is an electronic circuit that initiates corrective action in response to a computer hardware malfunction or program error. It is an essential component of systems that are difficult or impossible to physically access because it provides a way to automatically recover from transient faults. Also, a watchdog timer can respond to faults more quickly than a human operator, making it invaluable in cases where a human operator would be too slow to react to a fault condition. Watchdog timers are widely used in embedded and remote systems, in equipment ranging from microwave ovens to Mars rovers. Every watchdog timer, however simple or sophisticated, must initiate two corrective actions. First, it must set the computer's control outputs to safe levels so that potentially dangerous devices such as motors and heaters will not pose threats to people or equipment. This is a high priority action that must occur as soon as a fault is detected. After setting the outputs to safe levels, the next order of business is to restore normal system operation. This can be as simple as restarting the computer, as if a human operator has pressed the computer's reset pushbutton, or it may involve a sequence of actions that ultimately ends with a computer restart. ■