Watchdog timer

A watchdog timer (sometimes called a computer operating properly or COP timer, or simply a watchdog) is an electronic timer that is used to detect and recover from computer malfunctions. During normal operation, the computer regularly resets the watchdog timer to prevent it from elapsing, or "timing out". If, due to a hardware fault or program error, the computer fails to reset the watchdog, the timer will elapse and generate a timeout signal. The timeout signal is used to initiate corrective action or actions. The corrective actions typically include placing the computer system in a safe state and restoring normal system operation.

Watchdog timers are commonly found in embedded systems and other computer-controlled equipment where humans cannot easily access the equipment or would be unable to react to faults in a timely manner. In such systems, the computer cannot depend on a human to reboot it if it hangs; it must be self-reliant. For example, remote embedded systems such as space probes are not physically accessible to human operators; these could become permanently disabled if they were unable to autonomously recover from faults. A watchdog timer is usually employed in cases like these. Watchdog timers may also be used when running untrusted code in a sandbox, to limit the CPU time available to the code and thus prevent some types of denial-of-service attacks.

The act of restarting a watchdog timer is commonly referred to as "kicking the dog" or other similar terms; this is typically done by writing to a watchdog control port. Alternatively, in microcontrollers that have an integrated watchdog timer, the watchdog is sometimes kicked by executing a special machine language instruction. An example of this is the CLRWDT (clear watchdog timer) instruction found in the instruction set of some PIC microcontrollers.

In computers that are running operating systems, watchdog resets are usually invoked through a device driver. For example, in the Linux operating system, a user space program will kick the watchdog by interacting with the watchdog device driver, typically by writing a zero character to /dev/watchdog. The device driver, which serves to abstract the watchdog hardware from user space programs, is also used to configure the time-out period and start and stop the timer.

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