Knob stepper on an Arduino Uno

// Knob stepper: a rotary encoder steers a stepper motor, click for click.
//
//   encoder   CLK -> pin 2, DT -> pin 3, SW -> pin 4, + -> 5V, GND -> GND
//   ULN2003   IN1 -> pin 8, IN2 -> pin 9, IN3 -> pin 10, IN4 -> pin 11
//             + -> 5V, - -> GND
//
// The encoder has 20 clicks a turn and the 28BYJ-48 takes 2048 steps a turn,
// so each click moves the shaft about 102 steps, 18 degrees: turn the knob
// once and the shaft turns once. Press the knob to send the shaft home.
//
// # Reading the encoder
//
// CLK and DT are two switches that open and close a quarter of a click apart.
// Every change is looked up in a table of all sixteen moves: a real quarter
// step counts +1 or -1, and a jump of two states, which is what a bouncing
// contact looks like, counts nothing. Four quarter steps make a click.
//
// Pins 2 and 3 are the Uno's two interrupt pins, so the reading happens in an
// interrupt on every change. That matters here: while the motor is stepping,
// loop() is busy for a fifth of a second a click, and a knob turned in that
// time would be missed if loop() had to notice it.

#include <Stepper.h>

const int CLK = 2;
const int DT = 3;
const int SW = 4;
const int STEPS_PER_TURN = 2048;
const int CLICKS_PER_TURN = 20;

Stepper motor(STEPS_PER_TURN, 8, 10, 9, 11);

const int8_t QUARTER[16] = {0, -1, 1, 0, 1, 0, 0, -1, -1, 0, 0, 1, 0, 1, -1, 0};

volatile uint8_t state = 3;
volatile int8_t quarters = 0;
volatile long clicks = 0;
long position = 0;  // where the shaft is, in steps from home
long shownClicks = 0;
bool lastSw = HIGH;

uint8_t pair() {
  return (uint8_t)((digitalRead(CLK) << 1) | digitalRead(DT));
}

void onTurn() {
  uint8_t now = pair();
  if (now == state) return;
  quarters += QUARTER[(state << 2) | now];
  state = now;
  if (now == 3) {
    if (quarters >= 4) clicks++;
    if (quarters <= -4) clicks--;
    quarters = 0;
  }
}

void moveTo(long target) {
  if (target != position) {
    motor.step((int)(target - position));
    position = target;
  }
  for (int pin = 8; pin <= 11; pin++) digitalWrite(pin, LOW);
}

void setup() {
  pinMode(CLK, INPUT_PULLUP);
  pinMode(DT, INPUT_PULLUP);
  pinMode(SW, INPUT_PULLUP);
  motor.setSpeed(15);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: knob stepper");
  state = pair();
  attachInterrupt(digitalPinToInterrupt(CLK), onTurn, CHANGE);
  attachInterrupt(digitalPinToInterrupt(DT), onTurn, CHANGE);
}

void loop() {
  noInterrupts();
  long now = clicks;
  interrupts();
  if (now != shownClicks) {
    shownClicks = now;
    Serial.print("click ");
    Serial.println(now);
    moveTo(now * STEPS_PER_TURN / CLICKS_PER_TURN);
  }
  bool sw = digitalRead(SW);
  if (sw == LOW && lastSw == HIGH) {
    noInterrupts();
    clicks = 0;
    interrupts();
    shownClicks = 0;
    Serial.println("home");
    moveTo(0);
  }
  lastSw = sw;
}