#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;
  }