IBT-2 motor speed control with a knob on an Arduino Uno

// IBT-2 (BTS7960) motor speed control with a knob: the middle of the knob is
// stop, turn it one way for forward and the other for reverse, faster the
// further it goes.
//
//   A0      the potentiometer's wiper (its ends on 5 V and GND)
//   IBT-2   RPWM -> pin 5, LPWM -> pin 6, R_EN and L_EN -> pin 7
//           VCC -> 5V, GND -> GND
//           B+ / B- -> a 12 V battery, M+ / M- -> the motor
//
// The IBT-2 is two BTS7960 half bridges, good for a big 12 V motor that would
// cook an L298N. With both enables high, PWM on RPWM drives M+ (forward) and
// PWM on LPWM drives M- (reverse). Never PWM both at once: the sketch always
// sets one side to 0 before it drives the other.
//
// Two things a bare analogWrite(knob / 4) gets wrong on a big motor:
//
// A dead band. A knob never sits exactly in the middle, and a motor that
// creeps at "stop" is a nuisance, so the middle DEAD counts are stop.
//
// A ramp. Snapping a heavy motor from full forward to full reverse slams the
// gearbox and pulls a current spike from the battery. The output moves at
// most STEP every 10 ms, so 0 to full takes about half a second, and a
// reversal always passes through stop.
//
// Press Run, then turn the knob either side of the middle.

const int KNOB = A0;
const int RPWM = 5;
const int LPWM = 6;
const int EN = 7;

const int DEAD = 50;  // counts either side of 512
const int STEP = 5;   // the most the output changes in 10 ms

int output = 0;  // -255 full reverse .. 255 full forward
int printed = 999;

void drive(int s) {
  if (s >= 0) {
    analogWrite(LPWM, 0);
    analogWrite(RPWM, s);
  } else {
    analogWrite(RPWM, 0);
    analogWrite(LPWM, -s);
  }
}

// The knob as a target from -255 to 255, stop in the middle.
int target() {
  int offset = analogRead(KNOB) - 512;
  if (abs(offset) <= DEAD) return 0;
  int s = map(abs(offset), DEAD, 511, 0, 255);
  if (s > 255) s = 255;
  return offset > 0 ? s : -s;
}

void setup() {
  pinMode(EN, OUTPUT);
  digitalWrite(EN, HIGH);
  drive(0);
  Serial.begin(9600);
  Serial.println("IBT-2 speed control: middle is stop, turn either way");
}

void loop() {
  int want = target();
  if (want > output) output = min(output + STEP, want);
  if (want < output) output = max(output - STEP, want);
  drive(output);

  // Print once the ramp has caught up with the knob.
  if (output == want && output != printed) {
    printed = output;
    if (output > 0) Serial.print("forward ");
    else if (output < 0) Serial.print("reverse ");
    else Serial.print("stop ");
    Serial.print(abs(output) * 100L / 255);
    Serial.println(" %");
  }
  delay(10);
}