How does the system watchdog work?

FAQ #100908

 

What for is a system watchog?
The system watchdog is a security measure which avoids a situation where the system (PCD) is blocked and does not properly execute the program any more. If the OS (Operating System) is "locked" in a loop and does no longer properly execute its tasks (among others, the user program), the PCD is automatically re-started. 

What does the system watchdog do?
The (internal) system watchog is configured directly on the CPU and will cause a re-boot if it is not triggered in a cyclic manner. Every reboot caused by the system watchdog is entered in the PCD history with the message SWTO ERROR (SWTO stands for System Watchdog TimeOut).
The behaviour after the automatic reboot caused by the system watchdog is depending on the software watchdog which can be programmed in the user program:

• If the software watchdog in the user program is enabled, the PCD will automatically go in RUN after a reboot caused by the system watchdog. 
• If no software watchdog is programmed in the user program, the PCD will go to "Halt" after it has rebooted because of the system watchdog.

Does the system watchdog conflict with other watchdogs?
No, the system watchdog does not conflict with e.g. the software watchdog programmed in the user program or with a hardware watchdog.  
 
When has the system watchdog been implemented?
The system watchdog has been implemented in firmware version 1.08.xx. The first production versions for the relevant systems can be found in the table below:

PCD system	first production firmware with system watchdog
PCD3.Mxxx0	1.08.23
PCD2.M480	1.08.21
PCD2.M5xx0	1.08.19

Remark
Note that the system watchog does not detect a bogus user program (e.g. an infinite loop) and therefore the usage of the software watchdog in the user program is still recommended.