#include <avr/io.h>          
  
  int main(void) 
  {  
      unsigned long z;
    
      ddrb = 0xff;    
  
      for(z=0;z<100000;z++)
      {
        portb = 0x55;
        portb = 0xaa;
      }
  
      while(1);
    return 0;
  }