#include <avr/io.h>        
  
  int main(void) 
  {  
      unsigned char mydata[] = {0x25,0x62,0x3f,0x52};
      unsigned char sum = 0;
      unsigned char x;
      unsigned char chksumbyte;
  
      ddra = 0xff;  
      ddrb = 0xff;  
      ddrc = 0xff;  
  
      for(x=0;x<4;x++)
        {
          porta = mydata[x];     
          sum = sum + mydata[x];  
          portb = sum;     
        }
      chksumbyte = ~sum + 1;    
      portb = chksumbyte;   
  
    return 0;
  }