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

  Timer1 generates an interrupt every 100 ms.
  LED blinks once per second using timer ticks.
*/

const byte LED_PIN = 13;

volatile unsigned long timerTicks = 0;
volatile bool oneSecondElapsed = false;

ISR(TIMER1_COMPA_vect)
{
    timerTicks++;

    if (timerTicks % 10 == 0)   // 10 x 100ms = 1 second
    {
        oneSecondElapsed = true;
    }
}

void setupTimer1()
{
    cli();  // Disable interrupts during setup

    TCCR1A = 0;
    TCCR1B = 0;

    TCNT1 = 0;

    /*
       Timer frequency calculation:

       CPU = 16 MHz
       Prescaler = 64

       Timer clock = 16,000,000 / 64 = 250,000 Hz

       For 100 ms:
       250,000 * 0.1 = 25,000 counts

       OCR1A = 24,999
    */

    OCR1A = 24999;

    // CTC mode
    TCCR1B |= (1 << WGM12);

    // Prescaler = 64
    TCCR1B |= (1 << CS11) | (1 << CS10);

    // Enable compare match interrupt
    TIMSK1 |= (1 << OCIE1A);

    sei();  // Re-enable interrupts
}

void setup()
{
    Serial.begin(115200);

    pinMode(LED_PIN, OUTPUT);

    setupTimer1();

    Serial.println("Timer1 Demo Started");
}

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

    // Handle timer event generated by ISR
    if (oneSecondElapsed)
    {
        noInterrupts();
        oneSecondElapsed = false;
        unsigned long count = timerTicks;
        interrupts();

        digitalWrite(LED_PIN, !digitalRead(LED_PIN));

        Serial.print("Seconds elapsed: ");
        Serial.println(count / 10);
    }

    // Simulate other work
    if (millis() - lastPrint > 250)
    {
        lastPrint = millis();
        Serial.println("Main loop is busy...");
    }
}