Setting up a Watchdog

Introduction link

A watchdog is an electronic circuit or software used in digital electronics to ensure that an automated system or computer does not remain stuck at a particular step of its processing. It’s a protection measure generally intended to restart the system if a defined action is not executed within a specified time.

In industrial computing, the watchdog is often implemented through an electronic device, generally a monostable flip-flop. It works on the principle that each step of the processing must execute within a maximum time. At each step, the system arms a timer before execution. If the flip-flop returns to its stable state before the task is completed, the watchdog triggers. It implements a backup system that can either trigger an alarm, restart the automaton, or activate a redundant system. Watchdogs are often integrated into microcontrollers and motherboards dedicated to real-time processing.

When implemented in software, it generally consists of a counter that is regularly reset to zero. If the counter exceeds a given value (timeout), the system performs a reset (restart). The watchdog often consists of a register that is updated via a regular interrupt. It can also consist of an interrupt routine that must perform certain maintenance tasks before returning control to the main program. If a routine enters an infinite loop, the watchdog counter will no longer be reset to zero and a reset is ordered. The watchdog also allows for a restart if no instruction is provided for this purpose. It is then sufficient to write a value exceeding the counter’s capacity directly into the register: the watchdog will initiate the reset.

Configuration link

Configuration of the watchdog:

• Edit the /etc/rc.conf file and modify the following line:

  watchdogd_flags=NO # for normal use: ""
  

to

  watchdogd_flags="" # for normal use: ""
  

• Reboot, a watchdogd process should be present, check the parameters:

  sysctl -a
  

• Modify the sysctl parameters:

  kern.watchdog.period=30
kern.watchdog.auto=1