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

  1. 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.
  2. 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.
  3. The application monitor goes in the CNT DOWN state when the application sets the ApplicationTimeout element.
  4. 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.
  5. 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.