#define N asm volatile("RJMP .+0")
void setup(){pinMode(3,1);cli();}
byte j=0;
void loop(){
 digitalWrite(3,0);
 delayMicroseconds(50);
 byte k=0,l=j++>>5;
 for (int i=0;i<256*24;i++) {
  PORTD=~0;
  if(k&8){k+=l;N;N;N;N;PORTD=0;N;N;N;}else{N;PORTD=0;k++;N;N;N;N;}
 }
}