Introduction to Watchdog Timers
- A watchdog timer is a hardware or software timer that triggers a system reset if the main program crashes or hangs due to software glitches or unexpected conditions.
- For firmware developers, implementing a watchdog is crucial to ensure system reliability.
Identify Missing Implementation
- Review your codebase to confirm that a software watchdog is not already implemented. Search for key terms like "watchdog" or "WDT" to spot existing attempts or documentation references.
- Check the requirements or design documents to understand what was originally planned for the watchdog implementation.
Hardware Watchdog Configuration
- If the hardware supports, configure the hardware watchdog timer. This typically involves setting up registers via an SDK or direct register manipulation.
- Refer to the microcontroller's datasheet for specifics on how to configure and enable the watchdog.
- Ensure any hardware configuration does not cause an immediate reset by understanding the timeout and refresh requirements.
Software Watchdog Implementation
- Create a dedicated software thread or task that periodically "feeds" or "kicks" the watchdog to prevent a system reset.
- Establish a timeout period in which the watchdog expects to be reset. Ex: `int timeout_ms = 1000;`
- Use a timer or a counter to track the elapsed time. Suppose the watchdog is not reset in the specified time interval, trigger appropriate corrective actions.
volatile int watchdog_counter = 0;
void watchdog_task() {
while (1) {
sleep(1); // simulate 1 second delay
watchdog_counter++;
if (watchdog_counter >= timeout_ms) {
reset_system(); // define this to handle resets
}
}
}
void feed_watchdog() {
watchdog_counter = 0; // reset the counter
}
Integrate the Watchdog
- Identify critical sections in your code that must feed the watchdog. Typically, this is done at points where successful completion is guaranteed.
- Invoke `feed_watchdog()` function at these strategic points.
- Simplify feeding during lengthy computations or network-related activities to ensure the watchdog counter is reset.
Testing and Validation
- Deliberately introduce hangs or infinite loops in your code to verify whether the watchdog timer correctly triggers a reset.
- Check system performance and ensure no unnecessary resets occur during normal operation.
Debugging Tips
- Utilize print statements to log watchdog activity and transitions during your test phases.
- Use a debugger to step through your code or inspect variables when integrating the watchdog to observe behavior and catch any anomalies.
Final Considerations
- Refactor your code to make watchdog handling seamless and non-intrusive. Avoid deeply coupling it with business logic.
- Document the watchdog's behavior, timeout periods, and reset conditions for future maintenance and reference.
