Watchdog timer in microcontroller embedded system

A watchdog timer (WDT) is a critical component in many microcontroller-based embedded systems. It is essentially a timer that is used to monitor the operation of a microcontroller or a microcontroller based system. Its primary purpose is to reset the microcontroller in case it becomes unresponsive or gets stuck in an infinite loop. This ensures the system's reliability and fault tolerance.

This is a mechanism which helps microcontroller reboots (or restarts) when software hangs due to any external or internal faults or issues. It is basically hardware timer which should be serviced periodically by the software program else it will generate the system reset.

As shown in the figure-1, Microcontroller (ATmega328) is transmitting to the device using UART port. Here microcontroller transmits character and waits for character to be received by the device. If the character gets lost then the code hangs.

watchdog timer basics

The watchdog timer helps to overcome this situation. Here programmer will place "clrwdt" instruction at regular intervals in the source code. This is machine language instruction found in instruction set of PIC microcontroller.

Due to some reasons if this instruction does not get executed in some particular time period (known as time out period), the microcontroller gets reset. There are different registers and instructions used for various microcontrollers in order to enable watchdog timer. The time out period for the timer should be specified in the program which is called initialization of watchdog timer.

When the watchDog timer times out, it will cause "RESET". After this microcontroller executes instruction from location 0x0000.

watchdog timer operation

The figure-2 depicts the operation of the watchdog timer. The timer requires clock for its hardware operation. This clock is internal or external to the timer. The watchdog timer send reset signal to the processor/microcontroller when timeout is reached. Based on clock frequency, time out interval wll vary. For 16 MHz clock, timeout interval is 92 ms.

Watchdog timer applications

Following are the applications of watchdog timer:
• An application in mobile phone is that display is off in case no GUI interaction takes place within a watched time interval. This will save good amout of battery power.
• An application in temperature controller is that if controller takes no action to switch off the current within preset watched time interval, the current is switched off and warning signal is raised as indication of controller failure. Failure to switch off current may burst a boiler in which water is heated.
• If Software hangs due to some bug/issue, it helps to reset the system automatically without any human interactions.

Watchdog timer advantages

The benefits of watchdog timer in a microcontroller embedded system are as follows.
➨Fault Tolerance: It helps ensure that the microcontroller-based system continues to operate reliably, even if there are software or hardware faults that could otherwise lead to system lock-ups.
➨Recovery from Faults: When a fault occurs and the watchdog timer triggers a reset, the system has a chance to recover and start fresh. This can be especially crucial in safety-critical applications.
➨Stability: It prevents a microcontroller from getting stuck in an infinite loop or hanging due to software bugs or unexpected conditions, thereby increasing system stability.
➨Real-Time Monitoring: The watchdog timer is a form of real-time monitoring that provides a safety net for the microcontroller's operation.
➨Security: In some cases, the watchdog timer can be used to detect and respond to security breaches or tampering attempts by resetting the system when unexpected behavior is detected.

Conclusion: In summary, a watchdog timer is an essential component in many microcontroller-based embedded systems. It offers safety mechanism to ensure that the system remains responsive and reliable even in the face of faults or unexpected events. Proper configuration and periodic resetting of the watchdog timer are critical for its effective operation.

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