// file: blink.c
#define F_CPU 16000000UL
#include <avr/io.h>
#include <util/delay.h>

int main(void) {
    DDRB |= (1 << DDB5); // PB5 (Arduino D13) as output
    DDRB |= (1 << DDB4); // PB5 (Arduino D13) as output
    while (1) {
        PORTB |= (1 << PORTB5);   // ON
        _delay_ms(250);
        PORTB &= ~(1 << PORTB5);  // OFF
        _delay_ms(250);
    
        PORTB |= (1 << PORTB4);   // ON
        _delay_ms(250);
        PORTB &= ~(1 << PORTB4);  // OFF
        _delay_ms(250);
    }
}