MX1508 two-motor drive on an Arduino Uno

// Two gear motors on an MX1508 module, from an Uno: a robot car's drive.
//
//   MX1508 IN1 -> pin 5   IN2 -> pin 6   (motor A)
//   MX1508 IN3 -> pin 9   IN4 -> pin 10  (motor B)
//   MX1508 + -> four AA cells,  - -> the cells' - and the Uno's GND
//
// Each motor has a pair of inputs: PWM on one with the other low runs it,
// both low lets it coast, both high brakes. The module runs from 2 to 9.6 V,
// chip and motors on the same supply.

const int A1_PIN = 5, A2_PIN = 6, B1_PIN = 9, B2_PIN = 10;

void motor(int p1, int p2, int speed) {
  if (speed >= 0) {
    analogWrite(p2, 0);
    analogWrite(p1, speed);
  } else {
    analogWrite(p1, 0);
    analogWrite(p2, -speed);
  }
}

void go(const char *what, int left, int right, int ms) {
  Serial.println(what);
  motor(A1_PIN, A2_PIN, left);
  motor(B1_PIN, B2_PIN, right);
  delay(ms);
}

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: MX1508 drives two motors");
}

void loop() {
  go("forward", 200, 200, 1500);
  go("turn left", -150, 150, 800);
  go("forward", 200, 200, 1500);
  go("back", -200, -200, 1000);
  go("coast", 0, 0, 1000);
}