void setup() {
  pinMode(3, OUTPUT); //DIR
  pinMode(4, OUTPUT); //STEP
}

void loop() {
  uint8_t binary = (micros() >> 12) & 3;
  uint8_t gray = binary ^ (binary >> 1);
  uint8_t A = gray & 1;
  uint8_t B = gray >> 1 & 1;
  digitalWrite(3, A);
  digitalWrite(4, B);
}