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In another space mission that was successful, several watchdog resets occurred during the NASA Pathfinder mission to Mars before the code problem was discovered and fixed with new software that was downloaded in deep space via a radio link. A watchdog timer would have likely detected the fault and shut down the rocket motors in time to save the mission. The processor had a watchdog timer but it was not used. By the time the ground controllers realized the problem and sent a radio signal back to force a hardware reset, the probe had used all of the fuel. In one famous case, a NASA deep space probe’s computer locked up with the thruster rockets firing.
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On the mbed this register is called the Reset Source Identification Register (RSID). Typically after reset, a register can also be read to determine if the watchdog timer generated the reset or if it was a normal reset. Used in this way, the watchdog timer can detect a fault on an unattended embedded device and attempt corrective action with a reset. If a fault occurs and the main program does not get back to reset the timer before it counts down, an interrupt is generated to reset the processor. Each time it goes through the main loop the code resets the watchdog timer (sometimes called “kicking” or “feeding” the dog). The watchdog timer is loaded with an initial value greater than the worst case time delay through the main program loop. The main program typically has a loop that it constantly goes through performing various functions. Many microcontrollers including the mbed processor have watchdog timer hardware.
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Some systems may also refer to it as a computer operating properly (COP) timer. It is often used to automatically reset an embedded device that hangs because of a software or hardware fault. A watchdog timer (WDT) is a hardware timer that automatically generates a system reset if the main program neglects to periodically service it.