File content as of revision 0:de78c543d4ed:

/* Program Example: 13.2 Open Loop Compass 
                                                               */
#include "mbed.h"
//***** mbed objects *****
I2C compass(p28, p27);         // sda, scl
PwmOut PWM(p25);
AnalogIn Ain(p20);
Serial pc(USBTX, USBRX);        // tx, rx
Ticker s100hz_tick;             // 100 Hz (10ms) ticker
Ticker s5hz_tick;               // 5 Hz (200ms) ticker
//***** variables *****
const int addr = 0x42;          // define the I2C write Address
char cmd[3];
float pos;                      // measured position
float ctrlval;                  // PWM control value
//***** function prototypes *****
void s100hz_task(void);         // 100 Hz task
void s5hz_task(void);           // 5 Hz task
//***** main code *****
int main() {
  // initialise and setup data
  PWM.period(0.02);
  cmd[0] = 0x47;                 // 'G' write to RAM address
  cmd[1] = 0x74;                 // Operation mode register address
  cmd[2] = 0x72;                 // Op mode = 20H, S/R, continuous 
  compass.write(addr,cmd, 3);    // Send operation
  s100hz_tick.attach(&s100hz_task,0.01);     // attach 100 Hz task              
  s5hz_tick.attach(&s5hz_task,0.2);          // attach 5 Hz task 
  while(1){
  // loop forever
  }
}

//***** function 100hz_task *****
void s100hz_task(void) {
  compass.read(addr, cmd, 2);     // read the two-byte compass data
  pos = 0.1 * ((cmd[0] << 8) + cmd[1]);        //convert to degrees
    if (pos>180)
      pos=pos-360;                            // convert to ±180deg
  ctrlval=Ain;          // set control value (also try ctrlval=Ain/4)
  PWM=ctrlval;          // output control value to PWM
}

//***** function 5hz_task *****
void s5hz_task(void) {
  pc.printf("deg = %.1f    PWM = %.4f\n", pos, ctrlval);
}