Syscon/104 Watchdog Circuit

The System Controller includes two separate watchdog circuits. These circuits require a periodic "tick" pulse from the system software or they will take action to bring the data acquisition system back under control. The tick pulse is generated by writing to a particular I/O address (0x319).

After the initial tick pulse, the first circuit requires subsequent pulses to arrive at intervals of less than 1.6 seconds or a hard reset will be sent to the processor, causing the computer system to reboot. This circuit can be disabled by writing to another specific I/O address (0x311). In normal operation, tick pulses are issued as part of the data acquisition loop, and the watchdog circuit is disabled when data collection is terminated.

The second circuit is independent of the first except that it references the same tick pulse. This circuit will turn on a fail indicator lamp in the cockpit if a tick pulse is not received within two minutes. There is no mechanism for disabling this circuit. The clock starts on power up, giving the system two minutes to boot up and begin data acquisition. If the system fails to boot within that time, the fail light will be illuminated. If the system reboots due to a timeout of the 1.6 second watchdog time, the pilot will not see any indication unless the system fails to reboot within two minutes.

Since the tick is only issued as part of the data acquisition loop, after data acquisition is terminated, the fail light will also turn on. Thus it is normal for the fail light to come on at the end of a flight. The fail light merely informs the pilot that the data system is not acquiring data. It is left to the pilot to determine whether or not that is appropriate.