Watchdog timer


A watchdog timer is an electronic or software 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 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 invoke a reboot 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.
Watchdog timers are also used in operating systems where certain high priority operations are required to complete in a specific time interval. If the timer expires before the operation completes then the OS responds to the timer interrupt by recording error data and terminating the operation. A system may have both types of watchdog timer.

Architecture and operation

Watchdog restart

The act of restarting a watchdog timer, commonly referred to as "kicking" the watchdog, 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 or setting a specific bit in a register. An example of this is the CLRWDT 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.

Single-stage watchdog

Watchdog timers come in many configurations, and many allow their configurations to be altered. Microcontrollers often include an integrated, on-chip watchdog. In other computers the watchdog may reside in a nearby chip that connects directly to the CPU, or it may be located on an external expansion card in the computer's chassis. The watchdog and CPU may share a common clock signal, as shown in the block diagram below, or they may have independent clock signals.

Multistage watchdog

Two or more timers are sometimes cascaded to form a multistage watchdog timer, where each timer is referred to as a timer stage, or simply a stage. For example, the block diagram below shows a three-stage watchdog. In a multistage watchdog, only the first stage is kicked by the processor. Upon first stage timeout, a corrective action is initiated and the next stage in the cascade is started. As each subsequent stage times out, it triggers a corrective action and starts the next stage. Upon final stage timeout, a corrective action is initiated, but no other stage is started because the end of the cascade has been reached. Typically, single-stage watchdog timers are used to simply restart the computer, whereas multistage watchdog timers will sequentially trigger a series of corrective actions, with the final stage triggering a computer restart.

Time intervals

Watchdog timers may have either fixed or programmable time intervals. Some watchdog timers allow the time interval to be programmed by selecting from among a few selectable, discrete values. In others, the interval can be programmed to arbitrary values. Typically, watchdog time intervals range from ten milliseconds to a minute or more. In a multistage watchdog, each timer may have its own, unique time interval.

Corrective actions

A watchdog timer may initiate any of several types of corrective action, including maskable interrupt, non-maskable interrupt, processor reset, fail-safe state activation, power cycling, or combinations of these. Depending on its architecture, the type of corrective action or actions that a watchdog can trigger may be fixed or programmable. Some computers require a pulsed signal to invoke a processor reset. In such cases, the watchdog typically triggers a processor reset by activating an internal or external pulse generator, which in turn creates the required reset pulses.
In embedded systems and control systems, watchdog timers are often used to activate fail-safe circuitry. When activated, the fail-safe circuitry forces all control outputs to safe states to prevent injuries and equipment damage while the fault persists. In a two-stage watchdog, the first timer is often used to activate fail-safe outputs and start the second timer stage; the second stage will reset the computer if the fault cannot be corrected before the timer elapses.
Watchdog timers are sometimes used to trigger the recording of system state information—which may be useful during fault recovery—or debug information onto a persistent medium. In such cases, a second timer—which is started when the first timer elapses—is typically used to reset the computer later, after allowing sufficient time for data recording to complete. This allows time for the information to be saved, but ensures that the computer will be reset even if the recording process fails.
For example, the above diagram shows a likely configuration for a two-stage watchdog timer. During normal operation the computer regularly kicks Stage1 to prevent a timeout. If the computer fails to kick Stage1, Stage1 will eventually timeout. This event will start the Stage2 timer and, simultaneously, notify the computer that a reset is imminent. Until Stage2 times out, the computer may attempt to record state information, debug information, or both. The computer will be reset upon Stage2 timeout.

Fault detection

A computer system is typically designed so that its watchdog timer will be kicked only if the computer deems the system functional. The computer determines whether the system is functional by conducting one or more fault detection tests and it will kick the watchdog only if all tests have passed. In computers that are running an operating system and multiple processes, a single, simple test may be insufficient to guarantee normal operation, as it could fail to detect a subtle fault condition and therefore allow the watchdog to be kicked even though a fault condition exists.
For example, in the case of the Linux operating system, a user-space watchdog daemon may simply kick the watchdog periodically without performing any tests. As long as the daemon runs normally, the system will be protected against serious system crashes such as a kernel panic. To detect less severe faults, the daemon can be configured to perform tests that cover resource availability, evidence of expected process activity, overheating, and network activity, and system-specific test scripts or programs may also be run.
Upon discovery of a failed test, the Linux watchdog daemon may attempt to perform a software-initiated restart, which can be preferable to a hardware reset as the file systems will be safely unmounted and fault information will be logged. However it is essential to have the insurance of the hardware timer as a software restart can fail under a number of fault conditions. In effect, this is a dual-stage watchdog with the software restart comprising the first stage and the hardware reset the second stage.