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Watchdog timer
 
 
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A watchdog timer is a computer
Computer

A computer is a machine that manipulates Data according to a list of Code .The first devices that resemble modern computers date to the mid-20th century , although the computer concept and various machines similar to computers existed earlier....
 hardware timing device that triggers a system reset
Reset (Computing)

In a computer or data transmission system, to reset means to clear any pending errors or events and bring a system to normal condition or initial state usually in a controlled manner....
 if the main program
Computer program

Computer programs are Instruction for a computer. A computer requires programs to function. Moreover, a computer program does not run unless its instructions are executed by a Central processing unit; however, a program may communicate an Algorithm#Formalization of algorithms to people without running....
, due to some fault condition, such as a hang
Hang (computing)

In computing, a hang or freeze occurs when either a single computer program or the whole system becomes unresponsive to computer keyboard and computer mouse input/output....
, neglects to regularly service the watchdog (writing a “service pulse” to it, also referred to as “petting the dog” or "feed the watchdog" or "waking the watchdog"). 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.






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Encyclopedia


A watchdog timer is a computer
Computer

A computer is a machine that manipulates Data according to a list of Code .The first devices that resemble modern computers date to the mid-20th century , although the computer concept and various machines similar to computers existed earlier....
 hardware timing device that triggers a system reset
Reset (Computing)

In a computer or data transmission system, to reset means to clear any pending errors or events and bring a system to normal condition or initial state usually in a controlled manner....
 if the main program
Computer program

Computer programs are Instruction for a computer. A computer requires programs to function. Moreover, a computer program does not run unless its instructions are executed by a Central processing unit; however, a program may communicate an Algorithm#Formalization of algorithms to people without running....
, due to some fault condition, such as a hang
Hang (computing)

In computing, a hang or freeze occurs when either a single computer program or the whole system becomes unresponsive to computer keyboard and computer mouse input/output....
, neglects to regularly service the watchdog (writing a “service pulse” to it, also referred to as “petting the dog” or "feed the watchdog" or "waking the watchdog"). 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 system
Embedded system

An embedded system is a special-purpose computer system designed to perform one or a few dedicated functions, often with real-time computing constraints....
s, where this specialized timer is often a built-in unit of a microcontroller
Microcontroller

A microcontroller is a small computer on a single integrated circuit consisting of a relatively simple CPU combined with support functions such as a crystal oscillator, timers, watchdog, serial and analog I/O etc....
.

Even more complex watchdog timers may be used to run untrusted
Untrusted

The term, "untrusted" is used exclusively in the field of computer science and information security to denote code from an unknown source that may be potentially harmful to a system....
 code in a sandbox
Sandbox (computer security)

In computer security, a sandbox is a security mechanism for safely running programs. It is often used to execute untested code, or untrusted programs from unverified third-parties, suppliers and untrusted users....
.

Watchdog timers may also trigger fail-safe
Fail-safe

Fail-safe or fail-secure describes a device or feature which, in the event of Failure mode, responds in a way that will cause no harm or at least a minimum of harm to other devices or danger to personnel....
 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.

For example, a watchdog timer can be implemented with a x-bit counter
Counter

In digital logic and computing, a counter is a device which stores the number of times a particular event or Process has occurred, often in relationship to a clock signal....
 in a system working with a clock signal
Clock signal

In electronics and especially Synchronous logic digital circuits, a clock signal is a Signalling used to coordinate the actions of two or more Electronic circuit....
 of y MHz, therefore, the system will shut down if the timer is not reset in a period of seconds.

For those embedded systems that can't be constantly watched by a human, watchdog timers may be the solution. For example, most embedded systems need to be self-reliant, and it's not usually possible to wait for someone to reboot them if the software hangs. Some embedded designs, such as space probes, are simply not accessible to human operators. If their software ever hangs, such systems are permanently disabled. In cases similar to these, a watchdog timer can help in solving the problem.

The watchdog timer is a chip external to the processor. However, it could also be included within the same chip as the CPU; this is done in many microcontrollers. In either case, the watchdog timer is tied directly to the processor's reset signal. Expansion card
Expansion card

An expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard to add additional functionality to a computer system....
 based watchdog timers exist and can be fitted to computers without an onboard watchdog.

See also

  • Immunity Aware Programming
    Immunity Aware Programming

    When writing firmware for an embedded system, immunity aware programming is a set of programming techniques used in an attempt to tolerate transient errors in the program counter or other that would otherwise lead to failure....
  • Power distribution unit
    Power distribution unit

    A Power distribution unit is a device that distributes electric power.Large industrial units are used for taking high voltage and current and reducing it to more common and useful levels, for example from 240V 30A single phase to multiple 120V 15A or 120V 20A plugs....
  • System time
  • Dead man's switch
    Dead man's switch

    A dead man's switch , as its name suggests, is a switch that is automatically operated in case the human operator becomes incapacitated.The switch usually stops a machine, and is a form of fail-safe....


External links

  • - Article by Jack Ganssle