#define F_CPU 16000000UL

#include <avr/io.h>
#include <avr/interrupt.h> // Required for ISRs and sei()

int main(void)
{
    // --- SETUP ---
    // 1. Set built-in LED (PB5) as an OUTPUT
    DDRB |= (1 << DDB5);

    // 2. Set Timer1 Mode to "Normal" (overflow at 0xFFFF)
    TCCR1A = 0x00; // WGM10 and WGM11 are 0
    TCCR1B = 0x00; // WGM12 and WGM13 are 0
    
    // 3. Set Prescaler to 1024
    // CS12=1, CS11=0, CS10=1
    TCCR1B |= (1 << CS12) | (1 << CS10);

    // 4. Set the Preload Value for 1 second
    // 65535 - 15625 = 49910
    TCNT1 = 49910;

    // 5. Enable the Timer1 Overflow Interrupt
    // TIMSK1 (Timer/Counter1 Interrupt Mask Register)
    // TOIE1 (Timer/Counter1 Overflow Interrupt Enable)
    TIMSK1 |= (1 << TOIE1);

    // 6. Enable Global Interrupts (Master Switch)
    sei();

    // --- LOOP ---
    // The main loop is empty! The CPU is free.
    // It could be reading sensors, updating an LCD, etc.
    while (1) 
    {
        // Do other work...
    }
}

// --- INTERRUPT SERVICE ROUTINE ---
// This function is called AUTOMATICALLY every 1 second
ISR(TIMER1_OVF_vect)
{
    // 1. Toggle the LED
    // Using XOR (^) is a fast way to flip the bit
    PORTB ^= (1 << PB5);
    
    // 2. IMPORTANT: Reload the preload value
    // The timer reset to 0 after overflowing, so we must
    // set it back to 49910 for the *next* 1-second interrupt.
    TCNT1 = 49910;
}