/*
   Watchdog Timer Demonstration
   Arduino Uno / Nano (ATmega328P)

   LED blinks normally.
   Every 15 seconds the program intentionally locks up.
   The watchdog resets the MCU after 2 seconds.

   Demonstrates:
   - Enabling the watchdog
   - Resetting ("kicking") the watchdog
   - Detecting watchdog resets
   - Recovering from software hangs
*/

#include <avr/wdt.h>
#include <avr/io.h>

const byte LED_PIN = 13;

unsigned long startTime;
bool watchdogResetOccurred = false;

void checkResetCause()
{
    byte resetFlags = MCUSR;

    watchdogResetOccurred = resetFlags & (1 << WDRF);

    // Clear reset flags
    MCUSR = 0;

    // Disable watchdog after reset
    wdt_disable();
}

void setup()
{
    checkResetCause();

    Serial.begin(115200);

    pinMode(LED_PIN, OUTPUT);

    Serial.println();
    Serial.println("Watchdog Timer Demo");

    if (watchdogResetOccurred)
    {
        Serial.println("*** Previous reset caused by WATCHDOG ***");
    }
    else
    {
        Serial.println("Normal power-on reset");
    }

    // Enable watchdog with 2-second timeout
    wdt_enable(WDTO_2S);

    startTime = millis();
}

void loop()
{
    static unsigned long lastBlink = 0;

    // Feed watchdog
    wdt_reset();

    // Blink LED every second
    if (millis() - lastBlink >= 1000)
    {
        lastBlink = millis();

        digitalWrite(LED_PIN, !digitalRead(LED_PIN));

        Serial.print("Running for ");
        Serial.print(millis() / 1000);
        Serial.println(" seconds");
    }

    // Simulate a software lockup after 15 seconds
    if (millis() - startTime > 15000)
    {
        Serial.println();
        Serial.println("*** Simulating software hang ***");
        Serial.println("*** Watchdog should reset MCU in ~2 seconds ***");

        while (true)
        {
            // Intentionally do nothing
            // No wdt_reset() calls
        }
    }
}