For those embedded systems that can’t be constantly watched by a human, watchdog timers may be the solution.
The watchdog timer is an important device in the embedded system , which is used to develop reliable products. Most of the embedded systems need to be self-reliant in order to restart and restore the system if any software bug disturbs the system. It is not always possible for human operators to wait for rebooting the system for every software problem. The watchdog timer is a piece of hardware that provides ultimate solutions for the real-time industries, which used to detect system abnormalities automatically and to reset the processor.
A watchdog timer is a piece of hardware that can be used to automatically detect software anomalies and reset the processor if any occur. Generally speaking, a watchdog timer is based on a counter that counts down from some initial value to zero. The embedded software selects the counter’s initial value and periodically restarts it. If the counter ever reaches zero before the software restarts it, the software is presumed to be malfunctioning and the processor’s reset signal is asserted. The processor (and the embedded software it’s running) will be restarted as if a human operator had cycled the power.
Figure 1 shows a typical arrangement. As shown, 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 micro-controllers. In either case, the output from the watchdog timer is tied directly to the processor’s reset signal.
kicking the dog
The process of restarting the watchdog timer’s counter is sometimes called “kicking the dog.” The appropriate visual metaphor is that of a man being attacked by a vicious dog. If he keeps kicking the dog, it can’t ever bite him. But he must keep kicking the dog at regular intervals to avoid a bite. Similarly, the software must restart the watchdog timer at a regular rate, or risk being restarted.
Advantage of Watchdog Timers:
Resets automatically without human intervention.
Detects the errors in the program and reboot the system
Saves the time and money
No need to place the employers to monitor the software debugs.
- Increases the system performance.
A number of considerations must go into any design that uses a watchdog as a monitor. Once the timeout period is determined, the system software must be analyzed to determine where to locate the watchdog restart instructions. For an effective design, the number of watchdog restarts should be kept to a minimum, and some consideration should be given to the likelihood of incorrectly executing a restart. As mentioned previously, some system software is too convoluted or data-dependent to ensure that all software flow paths are covered by a watchdog restart. This may dictate that a self-diagnostic software approach might be required. If there is an expected failure mechanism such as a periodic EMI burst or power supply glitch, the watchdog timeout should consider this period.
Pushpraj Kumar 🙂