Watchdog Timing Diagram
The implementation of two distinct monitors allows the programmer to define two separate watchdog time-out, one for the start-up phase, and one for the normal operation phase.
The timing diagram of the watchdog operation is shown below.
Watchdog timing diagram
- It is assumed that the software application has set the StartupTimeout element during a previous PC operation. Hence the start-up monitor has memorized an ARMED state. When the PC power supply is switched on, the start-up monitor goes in the CNT DOWN state.
- After the PC has booted up, the application is launched and begins its normal operation. At this point, the start-up monitor counting down is stopped when the application sets the StartupTimeout element. The start-up monitor goes in the ARMED state, and is ready to monitor the next start-up sequence.
- The application monitor goes in the CNT DOWN state when the application sets the ApplicationTimeout element.
- The system is now in normal operation and the application periodically sets the ApplicationTimeout element to inform the application monitor that everything is working properly.
- If a system malfunction occurs for example, the application is frozen and the internal counter of the application monitor reaches 0 after the time-out period the application monitor closes the PC reset relay during 1 second, and goes in the OFF state. The relay closure generates a reboot of the PC. The start-up monitor changes from the ARMED state to the CNT DOWN state, and monitors the coming start-up sequence.
To ensure system monitoring without interruption, the software application must perform a initial setting of ApplicationTimeout element before StartupTimeout element.