During the BOOT image creation, config_fpd_wdt.cdo added in boot.bif will configure the expiry action of the FPD WDT to Subsystem Restart. PLM configures this action in the error management module. Whenever FPD WDT expires, the PLM gets the notification to perform the configured action.
In Linux system, the watchdog driver is enabled by default. The Linux device tree has the FPD WDT node enabled. So the FPD watchdog is populated as hardware watchdog device. The watchdog-daemon opens this device and configure it for 20 seconds. Then this application is running an infinite loop to kick the watchdog every 5 seconds.
When this application is killed or hanged, the watchdog stops receiving the kick (heartbeats), resulting in watchdog expiry. And hence PLM will perform Subsystem Restart.