This SBC implements a Watch Dog Timer in a microcontroller on the SBC. An external WDT allows for high assurance operation of the WDT, as it uses external hardware, kernel level software, and userspace level software. If any of the three fail then the WDT will timeout and reboot the whole system. A standard kernel WDT driver is in place. As soon as the kernel starts it will start the WDT and feed it on 30 second timeouts every 15 seconds. If a userspace application opens and uses the watchdog file, the kernel will stop auto-feeding and the user application is now responsible for feeding the WDT. The kernel driver supports the "Magic Close" feature of the WDT. This means that a 'V' character must be fed in to the watchdog file before the file is closed in order to disable the WDT. Additionally, if the kernel is compiled with CONFIG_WATCHDOG_NOWAYOUT then the WDT can never be stopped once it is started at boot.
See the Linux WDT API documentation for more information.