source: rtos_arduino/trunk/arduino_lib/libraries/Robot_Control/examples/explore/R04_Compass/R04_Compass.ino@ 136

/* Robot Compass

 The robot has an on-board compass module, with
 which it can tell the direction the robot is
 facing. This sketch will make sure the robot
 goes towards a certain direction.

 Beware, magnets will interfere with the compass
 readings.

 Circuit:
 * Arduino Robot

 created 1 May 2013
 by X. Yang
 modified 12 May 2013
 by D. Cuartielles

 This example is in the public domain
 */

// include the robot library
#include <ArduinoRobot.h>
#include <Wire.h>
#include <SPI.h>

int speedLeft;
int speedRight;
int compassValue;
int direc = 180;  //Direction the robot is heading

void setup() {
  // initialize the modules
  Robot.begin();
  Robot.beginTFT();
  Robot.beginSD();
  Robot.displayLogos();
}

void loop() {
  // read the compass orientation
  compassValue = Robot.compassRead();

  // how many degrees are we off
  int diff = compassValue - direc;

  // modify degress
  if (diff > 180)
    diff = -360 + diff;
  else if (diff < -180)
    diff = 360 + diff;

  // Make the robot turn to its proper orientation
  diff = map(diff, -180, 180, -255, 255);

  if (diff > 0) {
    // keep the right wheel spinning,
    // change the speed of the left wheel
    speedLeft = 255 - diff;
    speedRight = 255;
  } else {
    // keep the right left spinning,
    // change the speed of the left wheel
    speedLeft = 255;
    speedRight = 255 + diff;
  }
  // write out to the motors
  Robot.motorsWrite(speedLeft, speedRight);

  // draw the orientation on the screen
  Robot.drawCompass(compassValue);
}