void setup() {
  // put your setup code here, to run once:
  DDRB |= 0b00111111;
  DDRD |= 0b11110000;
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int8_t MSB = 0; MSB < 0b01000000; MSB++) {
    for (int16_t LSB = 0; LSB < 0x0100; LSB += 0b00010000) {
      PORTB = PORTB & 0b11000000 | MSB;
      PORTD = PORTD & 0b00001111 | LSB;
      delay(50);
    }
  }
}