The **Watchdog** is a critical component in embedded systems, designed to monitor and reset the system when software malfunctions. Its primary function is to act as a timer that counts down from a preset value. Under normal operation, the program periodically resets this counter. If the system becomes unresponsive or enters an infinite loop, the watchdog timer will overflow, triggering an interrupt. This interrupt can then initiate a system reset, restoring normal operation. In short, the watchdog ensures that the system remains functional by restarting it if the program fails to reset the timer on time.
Software reliability is a major concern in all computing environments, including embedded systems. Even the most well-designed software can crash due to unexpected errors or interference. In industrial applications, where single-chip microcontrollers are commonly used, external factors like voltage fluctuations or electrical noise can cause system failures. For devices like water meters or electricity meters that operate without human supervision, such failures can be particularly problematic. To ensure automatic recovery from these issues, the use of a **Watchdog Timer (WDT)** is essential.
Many modern MCUs, such as the TI MSP430 series, Philips P87XXX/P89XXX, Microchip PIC, Atmel AT89SXX, and Holtek HTxxx, come with built-in WDTs. However, these internal watchdogs are not always foolproof. A simple experiment was conducted using an MSP430-based circuit to test the effectiveness of the built-in watchdog. The setup included an LED connected to the MCU’s output port, and the code was designed to toggle the LED while resetting the watchdog regularly. The experiment aimed to observe how often the watchdog failed to restart the system when the power was cycled.
During testing, two scenarios were considered: one where K2 was open, allowing the watchdog to function normally, and another where K2 was closed, potentially interfering with the reset signal. The results showed that in the first case, the system successfully restarted 154 out of 155 times, indicating a failure rate of about 0.6%. In the second scenario, the success rate dropped significantly, with only 17 out of 18 restarts being successful, resulting in a failure rate of around 5.5%. Similar results were observed when other MCUs with built-in WDTs were tested, suggesting that this issue is common across various platforms.
Analysis of the problem revealed several possible causes. First, many WDTs share the same clock source as the CPU, making them ineffective during system failures. Second, if the system clock is not properly initialized, the watchdog may not function correctly. Third, some WDTs require software configuration, which can fail if the initialization process is interrupted.
To address these issues, an external watchdog chip, the **TPS3823**, was used instead. This device provides an independent timing source, ensuring reliable operation even when the system is unstable. The new circuit successfully achieved a 100% restart success rate under the same test conditions. This highlights the importance of using an external watchdog in high-reliability applications.
Future built-in watchdogs should include independent clocks and be enabled automatically at startup without requiring software intervention. They should also be disabled only through hardware or fuse settings. For now, external watchdog solutions remain a better choice for systems where reliability is crucial. Additionally, after a reset, the program must determine whether the restart was due to a power-on event or a watchdog trigger, as this affects whether field data should be preserved. These considerations make the watchdog an essential tool in embedded system design.
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