#include <stdint.h>

#define U_DDRB     ((volatile uint8_t*)0x24)
#define U_PORTB    ((volatile uint8_t*)0x25)
#define U_TCCR1A   ((volatile uint8_t*)0x80)
#define U_TCCR1B   ((volatile uint8_t*)0x81)
#define U_OCR1AH   ((volatile uint8_t*)0x89)
#define U_OCR1AL   ((volatile uint8_t*)0x88)
#define U_TIMSK1   ((volatile uint8_t*)0x6F)
#define U_SREG     ((volatile uint8_t*)0x5F)

// Correct ISR vector for TIMER1 COMPA is __vector_17 (per datasheet + 0-indexed vector table)
void __vector_17(void) __attribute__((signal, used));
void __vector_17(void) {
    *U_PORTB ^= (1 << 7); // Toggle PB7
}

int main(void) {
    *U_DDRB |= (1 << 7);    // Set PB7 as output
    *U_PORTB &= ~(1 << 7);  // Ensure LED is initially OFF

    *U_TCCR1A = 0x00;       // Normal port operation
    *U_TCCR1B = (1 << 3);   // WGM12 = 1 (CTC mode)
    *U_TCCR1B |= (1 << 2) | (1 << 0); // Prescaler = 1024

    // Set OCR1A = 15625 for 1-second interval
    *U_OCR1AH = (15625 >> 8);
    *U_OCR1AL = (15625 & 0xFF);

    *U_TIMSK1 |= (1 << 1);   // Enable Timer1 COMPA interrupt (OCIE1A)
    *U_SREG |= (1 << 7);     // Enable global interrupts (I-bit)

    while (1); // wait for ISR
}