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

#define SCL_PIN PD3
#define SDA_PIN PD4

void setup() {
  // put your setup code here, to run once:
    DDRD |= (1 << SCL_PIN) | (1 << SDA_PIN);
}

void loop() {
  // put your main code here, to run repeatedly:
   PORTD |= (1 << SCL_PIN);
   _delay_ms(1000);
   PORTD &= ~(1 << SCL_PIN);
   _delay_ms(1000);
}