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Here is the circuit I use which depends on a heartbeat from the arduino and resets the arduino if 8 pulses are missed and also enables the arduino code to know that an external watchdog reset occured

and the Arduino code which can be incorporated into your sketch

//Watchdog #define ResetDetect 8 // watchdog detect pin, HIGH if a watchdog reset has occured #define heartbeat 9 // heartbeat pin int pulseState = LOW; // pulseState used to set the heartbeat pin long lastbeat = 0; // will store last time the heartbeat pin was updated long HeartBeatFreq = 500; // interval at which to blink (milliseconds) boolean ResetHappened; // Set to true if a watchdog reset has occurred

void setup(){ pinMode(ResetDetect, INPUT); // set Watchdog Reset sensing pin as input digitalWrite(ResetDetect, HIGH);// and turn on pullup
pinMode(heartbeat, OUTPUT); // set the heartbeat pin as output:

// Check if Restarting after Watchdog Reset
// NB must come before heartbeat resets external counter
int ResetSet = digitalRead(ResetDetect);
if (ResetSet == HIGH){
  ResetHappened = true;
}
else {
  ResetHappened = false;
}

}

void loop(){

// Heartbeat resets external watchdog when pin goes high if ((long)( millis() - (lastbeat + HeartBeatFreq)) >= 0) {
lastbeat = millis();
// if the LED is off turn it on and vice-versa: if (pulseState == LOW) pulseState = HIGH; else pulseState = LOW; // set the LED with the ledState of the variable: digitalWrite(heartbeat, pulseState); } }