#include <avr/io.h>    
  
  void delay100ms(void);      
  
  int main(void) 
  {
    
    ddrb = 0xff;      
    while (1) 
    {
      portb = 0xaa;
      delay100ms();
      portb = 0x55;
      delay100ms();  
    } 
    return 0;
  }
  
  
  void delay100ms(void){
    
    unsigned int i ;
    for(i=0; i<42150; i++);
  }