#define F_CPU 20000000L

#include <Arduino.h>
#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>

#define outputA PB3
#define outputB PB4

uint8_t counter = 0, aState = 0, aLastState = 0;

void setup() {

  DDRB &= ~((1 << outputA) | (1 << outputB));
  PORTB |= ((1 << outputA) | (1 << outputB));

  Serial.begin (115200);

  aLastState = (PINB & (1 << outputA));
}

void loop() {

  aState = (PINB & (1 << outputA));

  if (aState != aLastState) {

    if ((PINB & (1 << outputB)) != (aState)) {

      counter ++;
    }

    if ((PINB & (1 << outputB)) != (aLastState)) {

      counter --;
    }

    Serial.print("Position: ");
    Serial.println(counter);
  }

  aLastState = aState;
}