#define N asm volatile("RJMP .+0")
void setup(){pinMode(3,OUTPUT);}
void loop(){
 digitalWrite(3,LOW);
 delayMicroseconds(50);
 noInterrupts();
 for(int i=0;i<256*24;i++){PORTD=~0;N;N;N;N;N;PORTD=0;N;N;N;}
 interrupts();
}