blink-3.c

#include <avr/io.h>

#define F_CPU 16000000
#include <util/delay.h>

int main (void)
{
  DDRD = 0x01;
  PORTD = 0x01;
  while (1)
    {
      while ((PIND & 0x02) == 0)
        ; /* just wait */
      PORTD ^= 0x01;
    }
  return 0;
}