#include <avr/io.h>    

int main(void) 
{	
    unsigned int z;
	
	DDRB = 0xFF;    

    for(z=0;z<50000;z++)
    {
      PORTB = 0x55;
      PORTB = 0xAA;
    }

    while(1);
	return 0;
}