What is the function and implementation of a watchdog timer in an embedded controller environment?

The function of a watchdog timer in an embedded controller environment is to provide a safety mechanism that automatically resets the controller if the software malfunctions and fails to execute properly within a specified timeframe. An embedded controller is a specialized computer system designed to control specific functions within a larger system, like in automated machinery. The watchdog timer acts as an independent observer, constantly monitoring the main program's execution. The implementation involves configuring the watchdog timer with a specific timeout period. During normal operation, the main program must periodically 'kick' or 'pet' the watchdog timer, which means writing a specific value to a designated register before the timer reaches zero. This action resets the timer and prevents it from triggering a reset of the controller. If, however, the main program becomes stuck in a loop, crashes, or otherwise fails to execute correctly, it will not be able to kick the watchdog timer within the timeout period. When the timer reaches zero, it triggers a hardware reset of the controller, effectively restarting the program from a known good state. For example, imagine a PLC controlling a robotic arm. If a software bug causes the PLC to freeze, the watchdog timer would detect this inactivity and automatically reset the PLC, preventing the robot arm from continuing its last commanded action indefinitely, which could cause damage or injury. The watchdog timer provides a crucial layer of protection against software failures in embedded systems, ensuring that the system can recover automatically from unexpected errors. Selecting the timeout period is a trade-off: too short, and normal variations might trigger false resets; too long, and a real failure might go uncorrected for an unacceptable time.