The **Watchdog** is a critical component in embedded systems, designed to monitor and restore the system in case of software failures. Its basic function is to act as a timer that resets the system if the program becomes unresponsive or stuck. Normally, the watchdog timer counts up from zero, and the program must periodically reset it to prevent an overflow. If the system fails to reset the watchdog, the timer will overflow, triggering an interrupt that initiates a system reset. This mechanism ensures that the system can recover automatically from unexpected issues.
Software reliability remains a major concern in embedded systems. Even with careful design, software can crash or run into infinite loops, especially in environments where manual intervention is not possible. In industrial settings, such as water meters or electricity meters, system crashes due to power fluctuations or electrical interference can lead to serious operational problems. To address this, the Watchdog Timer (WDT) plays a vital role in ensuring system stability and recovery.
Many modern microcontrollers, 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 test was conducted using the MSP430 to evaluate the effectiveness of its internal watchdog. The circuit included an LED connected to the I/O port, which would flash if the system was functioning correctly. The program was designed to periodically reset the watchdog. Despite this, the experiment revealed some limitations.
In the test, when K2 was disconnected and K1 was repeatedly pressed, the system failed approximately 0.6% of the time. When K2 was closed, the failure rate increased to about 5.5%. These results indicated that the internal watchdog was not entirely reliable under certain conditions. Further analysis suggested several potential reasons: the watchdog's clock was not independent, making it vulnerable to system failures; the clock could be set by software, which might result in no clock signal during startup; and some watchdogs required software initialization, which might not occur after a reset, leading to disablement.
To improve reliability, an external watchdog chip, TPS3823, was used instead. This chip provides a separate oscillating circuit for the watchdog, ensuring it operates independently of the system clock. The new circuit significantly improved the system’s restart success rate to 100%. This highlights the importance of using an external watchdog in high-reliability applications.
Future built-in watchdogs should have independent and reliable clocks, be enabled automatically at power-up, and require hardware or fuse-based disabling rather than software control. Additionally, after a reset, the system should be able to determine whether the reset was caused by a power-on event or a watchdog trigger, so that field data can be handled appropriately. These considerations are essential for effective watchdog implementation in real-world applications.
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