Watchdog timer
The term watchdog (English for guard dog, also known as WDC for Watchdog Counter) is generalized used for a component of a system that monitors the operation of other components. Possible malfunction is detected, then either it signals according to the system agreement or initiated a suitable jump instruction that cleans up the problem at hand. The signal or jump instructions are used directly or indirectly as a trigger for other co-operating system components to solve the problem.
Use in electrical engineering / computer science
Specifically watchdogs are used in microcontroller - controlled electric devices to forestall a complete failure of the device through software failure. Prevents the loss of an automatic microcomputer system by the software periodically notifies the watchdog that it is still functioning properly.
Alternatively, the same can be also realized by a counter which is set at regular intervals, by the software on a certain value. This counter is decremented on the hardware side constantly; it reaches 0, it has the software failed to raise it in time and is probably " stuck " or failed. The watchdog will then set back the unit by resetting (reset ) to the defined initial state for the system controlled by the software system can work properly monitored again.
The Intelligent Platform Management IPMI system specified as a standard interface between a computer and a watchdog, so that one does not need any additional hardware for standard server motherboards.
Generally the unit is during a restart ( after the reset ) is not capable of real-time response and thus can not process data, or to respond to requests.
Watchdogs are used in various functional versions:
Hardware watchdog
The hardware watchdog is integrated in the microcontroller, or a built-up on the board microelectronic device or an external microelectronic component with communication to the microcontroller:
Watchdog timeout
When time-out watchdog, the microcontroller must elapse of a predetermined time before the Watchdog Report ( principle Deadman at the track ). In case of error, a reset of the microcontroller and possibly some peripheral components is triggered. If the watchdog is integrated in the microcontroller, in addition a so-called trap ( = interrupt) can be triggered. It is then the task of the trap associated software module to perform an adapted response ( save for example, that there has been a watchdog problem, bring the system to a safe state and then a partial or complete restart to perform ).
If a microcontroller does not have a special watchdog circuit, and the software interrupt a normal timer can be used.
Window watchdog
In the window watchdog to the microcontroller within a predetermined time window must at - Report watchdog - usually external. The reaction in absence of message corresponds to the response in time-out watchdog.
Intelligent watchdog
When intelligent watchdog, the microcontroller has to answer a question put by the external watchdog module question correctly; The method is therefore also referred to as a challenge-response approach.
In case of error, a reset of the microcontroller is not only produced but the watchdog usually brings the whole system - regardless of the apparently uncontrolled current microcontroller - in a safe condition. Example: The 3- level concept with an intelligent watchdog the Electronic accelerator (E - gas) in the car. With skillful choice of questions triggered by the arithmetic operations, the CPU - internal modules (ALU, program memory, RAM and registers) can be monitored.
Software watchdog
The software watchdog is a tested software in the microcontroller. The watchdog software module checks if all important program modules are executed correctly in a reasonable time frame, or if a module requires an unacceptably long time for editing. This must not necessarily caused by a faulty execution, but can also be caused by a deadlock ( deadlock ). The software watchdog can in turn be monitored by a hardware watchdog.
Further use
Meanwhile, the term is also used synonymously in other contexts, such as in the discourse of investigative journalism as 'Watch Dog' in the sense of a fourth power or in the debate on good governance and the corrective function of a vital civil society.