Robot car drive on an Arduino Uno

// Robot car drive: two motors on an L298N, the way every robot car does it.
//
//   pin 5  ENA, the left motor's speed (the ENA jumper is off)
//   pin 10 ENB, the right motor's speed (the ENB jumper is off)
//   pin 6  IN1   pin 7  IN2     the left motor's direction
//   pin 8  IN3   pin 9  IN4     the right motor's direction
//   L298N  12V from the battery (7.4 V here), GND to the Uno's GND,
//          5V left alone: the regulator jumper is on and it makes its own
//
// A car has no steering. It turns by running one side slower than the other,
// and it spins on the spot by running the two sides opposite ways. So the
// whole of driving is four numbers: a direction and a speed for each side.
// drive() takes them as two signed speeds, -255 to 255.
//
// # The volts the bridge keeps
//
// The L298 is a bipolar chip and keeps about 2.5 V for itself: 1.35 V in the
// high side and 1.2 V in the low side. A 7.4 V pack leaves the motors under
// 5 V, which is why the car is slower than the motors' label says. And the
// regulator on the module needs 2 V of headroom to make its 5 V, which is why
// a 6 V pack of four AA cells runs nothing at all with the jumper fitted.
//
// # Why the ENA and ENB jumpers are off
//
// Fitted, a jumper ties the enable to 5 V and the motor only has full speed.
// Pulled off, the enable is a pin, and analogWrite on it is the throttle.

const int ENA = 5;
const int IN1 = 6;
const int IN2 = 7;
const int ENB = 10;
const int IN3 = 8;
const int IN4 = 9;

void side(int en, int a, int b, int speed) {
  if (speed > 0) {
    digitalWrite(a, HIGH);
    digitalWrite(b, LOW);
  } else if (speed < 0) {
    digitalWrite(a, LOW);
    digitalWrite(b, HIGH);
    speed = -speed;
  } else {
    // Both inputs low is a brake: the motor's own EMF is shorted out.
    digitalWrite(a, LOW);
    digitalWrite(b, LOW);
    speed = 255;
  }
  analogWrite(en, speed > 255 ? 255 : speed);
}

void drive(const char *what, int left, int right, unsigned long ms) {
  Serial.print("car: ");
  Serial.println(what);
  side(ENA, IN1, IN2, left);
  side(ENB, IN3, IN4, right);
  delay(ms);
}

void setup() {
  const int pins[] = {ENA, IN1, IN2, ENB, IN3, IN4};
  for (int i = 0; i < 6; i++) {
    pinMode(pins[i], OUTPUT);
    digitalWrite(pins[i], LOW);
  }
  Serial.begin(115200);
  Serial.println("Mokxi Uno: a robot car on an L298N");
}

void loop() {
  drive("forward", 255, 255, 2000);
  drive("gentle left: the left side slower", 110, 255, 1500);
  drive("spin right on the spot", 200, -200, 1500);
  drive("stop", 0, 0, 700);
  drive("reverse", -255, -255, 2000);
  drive("stop", 0, 0, 1000);
}