The RevPi Connect 4 has two independent watchdogs.
A watchdog is a timer that runs for 60 seconds after power up and then restarts the RevPi. Therefore, as long as your system is functioning properly, the watchdog must be restarted again and again. If an error occurs (e.g. if the application process crashes), this restart does not take place and the watchdog restarts the system.
The first is part of the BCM2837 SoC, which is also used on the Raspberry Pi 3. The second is the PCF2129T external watchdog module.
The BCM2837 watchdog is the preferred watchdog because it behaves like other watchdogs under Linux. You can access it either via the device file
/dev/watchdog0 or as a default watchdog via the device file
The PCF2129T can be accessed via the device file
Watchdog under Linux
There are different ways to use a watchdog under Linux. The RevPi image and Raspbian rely on “systemd”. This is therefore also the preferred method of using the watchdog. “systemd” can be used to trigger a hardware watchdog. Applications that are to be monitored by the watchdog are then monitored by “systemd”. This has the advantage that several applications can be monitored. Consequently, the entire system does not necessarily have to be restarted, but only the individual application.