// Encoder pins
const byte pinA = 0;   // PB0
const byte pinB = 1;   // PB1
// Outputs
const byte pinCLK = 2; // PB2
const byte pinDATA = 3; // PB3

volatile byte lastState = 0;

void setup() {
  pinMode(pinA, INPUT_PULLUP);
  pinMode(pinB, INPUT_PULLUP);
  pinMode(pinCLK, OUTPUT);
  pinMode(pinDATA, OUTPUT);
  
  // Initialize last state
  lastState = (digitalRead(pinA) << 1) | digitalRead(pinB);

  // Enable pin change interrupts on A and B
  GIMSK |= (1 << PCIE); 
  PCMSK |= (1 << pinA) | (1 << pinB);
  sei();
}

ISR(PCINT0_vect) {
  byte a = digitalRead(pinA);
  byte b = digitalRead(pinB);
  byte curr = (a << 1) | b;
  byte trans = (lastState << 2) | curr;
  lastState = curr;

  switch (trans) {
    case 0b0001: case 0b0111: case 0b1110: case 0b1000:
      digitalWrite(pinDATA, HIGH); // CW
      pulseCLK();
      break;
    case 0b0010: case 0b0100: case 0b1101: case 0b1011:
      digitalWrite(pinDATA, LOW);  // CCW
      pulseCLK();
      break;
    default:
      break;
  }
}

void pulseCLK() {
  digitalWrite(pinCLK, HIGH);
  delayMicroseconds(200);   // adjust
  digitalWrite(pinCLK, LOW);
}

void loop() {
  // Nothing needed; ISR handles everything
}