What is a Watchdog?

A Watchdog (or Watchdog Timer, WDT) is a critical hardware or software mechanism designed to detect and recover from system crashes or freezes in embedded systems (e.g., microcontrollers like STM32, Arduino, or ESP32). It acts as a "safety net" by resetting the system if the software fails to respond within a predefined time window.

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How a Watchdog Works

1. Initialization:

   • The watchdog timer is configured with a timeout period (e.g., 1 second).

   • The system must periodically "feed" (reset) the watchdog before this timer expires.

2. Operation:

   • If the software runs normally, it sends regular "keep-alive" signals to the watchdog.

   • If the system freezes or crashes, the watchdog is not fed, and it triggers a reset to restore functionality.

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Types of Watchdogs

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Key Applications

✔ Embedded Systems (Prevents lockups in IoT devices, industrial controllers).
✔ Safety-Critical Systems (Medical devices, automotive ECUs).
✔ Remote Devices (Recovers unattended systems without manual intervention).

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Example Code (STM32 with HAL)

c

#include "stm32f4xx_hal.h"

IWDG_HandleTypeDef hiwdg;  // Independent Watchdog Timer

void Init_Watchdog() {
  hiwdg.Instance = IWDG;
  hiwdg.Init.Prescaler = IWDG_PRESCALER_32;  // Timeout ~1s
  hiwdg.Init.Reload = 0x0FFF;                // Reset period
  HAL_IWDG_Init(&hiwdg);
}

void Feed_Watchdog() {
  HAL_IWDG_Refresh(&hiwdg);  // Reset the watchdog counter
}

int main() {
  Init_Watchdog();
  while (1) {
    // Normal operation
    Feed_Watchdog();  // Must be called before timeout
    HAL_Delay(500);   // Simulate work
  }
}

Failure Scenario: If Feed_Watchdog() is not called within 1 second, the system reboots.

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Advantages

• Prevents Infinite Loops: Resets the system if code gets stuck.

• Improves Reliability: Critical for unattended devices.

• Low Hardware Cost: Often built into microcontrollers.

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Watchdog vs. Reset Button

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Did you know? NASA’s Mars rovers use watchdogs to recover from radiation-induced glitches!