A watchdog timer for ESP8266 (or just the Arduino). Based on the code of https://www.youtube.com/watch?v=D_7ciW_TCac

watchdog.ino

#include <Ticker.h>

Ticker secondTick;

#define debug 1

volatile int watchdogCount = 0;

void ISRwatchdog() {
  watchdogCount++;
  if ( watchdogCount == 5 ) {
     // Only print to serial when debugging
     (debug) && Serial.println("The dog bites!");
     ESP.reset();
  }
}

void setup() {
  Serial.begin(115200);
  Serial.println("Starting");
  secondTick.attach(1, ISRwatchdog);
}

void loop() {
  Serial.print("Watchdog counter = ");
  Serial.println(watchdogCount);
  // Feeding the dog
  watchdogCount = 0;
  delay(1000);
}