#include <avr/io.h>
#include <avr/interrupt.h>
#include <stdio.h>
#include <util/delay.h>

volatile uint32_t milliseconds = 0;
volatile uint8_t running = 0;  // 1 = running, 0 = stopped

void USART_init(unsigned int ubrr) {
    // Set baud rate
    UBRR0H = (unsigned char)(ubrr >> 8);
    UBRR0L = (unsigned char)ubrr;
    // Enable transmitter
    UCSR0B = (1 << TXEN0);
    // Set frame format: 8-bit data, 1 stop bit
    UCSR0C = (1 << UCSZ01) | (1 << UCSZ00);
}

void USART_send(char data) {
    while (!(UCSR0A & (1 << UDRE0)));  // Wait for empty buffer
    UDR0 = data;
}

void USART_print(const char *str) {
    while (*str) {
        USART_send(*str++);
    }
}

void Timer1_init() {
    TCCR1B |= (1 << WGM12);   // CTC mode
    OCR1A = 15624;            // 1s interval (16MHz clock, prescaler 1024)
    TCCR1B |= (1 << CS12) | (1 << CS10); // Prescaler 1024
    TIMSK1 |= (1 << OCIE1A);  // Enable Timer1 Compare Match Interrupt
    sei();                    // Enable global interrupts
}

ISR(TIMER1_COMPA_vect) {
    if (running) {
        milliseconds += 1000;  // Increment time in ms
    }
}

ISR(INT0_vect) {  // Start/Stop button
    running = !running;
}

ISR(INT1_vect) {  // Reset button
    milliseconds = 0;
    running = 0;
}

void INT0_INT1_init() {
    EICRA |= (1 << ISC01) | (1 << ISC11); // Falling edge for INT0, INT1
    EIMSK |= (1 << INT0) | (1 << INT1);   // Enable external interrupts
}

int main() {
    char buffer[20];
    USART_init(103);  // 9600 baud rate for 16MHz
    Timer1_init();
    INT0_INT1_init();

    while (1) {
        sprintf(buffer, "Time: %lu ms\r\n", milliseconds);
        USART_print(buffer);
        _delay_ms(500);
    }
}