TB6612 robot drive on an Arduino Uno

// TB6612 robot drive: two motors on a TB6612FNG, the way the SparkFun
// Inventor's Kit wires it, off a pack of four AA cells.
//
//   pin 5  PWMA, the left motor's speed      pin 6  PWMB, the right motor's
//   pin 2  AIN1   pin 4  AIN2                pin 7  BIN1   pin 8  BIN2
//   pin 9  STBY, which must be high or the driver does nothing
//   VCC    the Uno's 5 V (the logic side)    VM  the battery pack's red lead
//   GND    the Uno's GND and the pack's black lead, on one ground
//
// SparkFun_TB6612.h does the pin work: drive(speed) takes -255 to 255, the
// sign is the direction, and brake() shorts the motor through the bridge.
//
// # Why the pack, and not the Uno's 5 V
//
// Motors are noisy and hungry. A separate motor rail is how every robot is
// built, and the TB6612's MOSFET bridge keeps only a fraction of a volt of
// it, where an L293D keeps more than two. Four fresh AA cells are 6.4 V with
// nothing on them and a little less once the motors start: the battery sags
// by its own internal resistance, and the readout on the pack shows it.
//
// # Brake or coast
//
// On this chip PWM low is a short brake, not a coast, so analogWrite at half
// speed stops the motor firmly between pulses. brake() is the same thing held
// on; standby() lets both motors coast to a stop.

#include <SparkFun_TB6612.h>

const int AIN1 = 2;
const int AIN2 = 4;
const int PWMA = 5;
const int BIN1 = 7;
const int BIN2 = 8;
const int PWMB = 6;
const int STBY = 9;

// 1 or -1: flip one if a motor was wired the other way round.
const int offsetA = 1;
const int offsetB = 1;

Motor leftMotor = Motor(AIN1, AIN2, PWMA, offsetA, STBY);
Motor rightMotor = Motor(BIN1, BIN2, PWMB, offsetB, STBY);

void say(const char *what) {
  Serial.print("drive: ");
  Serial.println(what);
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: two motors on a TB6612FNG");
}

void loop() {
  say("forward, full speed");
  forward(leftMotor, rightMotor, 255);
  delay(1500);

  say("forward, half speed (PWM low brakes between pulses)");
  forward(leftMotor, rightMotor, 128);
  delay(1500);

  say("spin left on the spot");
  left(leftMotor, rightMotor, 255);
  delay(1200);

  say("reverse");
  back(leftMotor, rightMotor, 200);
  delay(1500);

  say("brake");
  brake(leftMotor, rightMotor);
  delay(1000);

  say("standby: both motors coast");
  leftMotor.standby();
  delay(1500);
}