File content as of revision 2:d8db9db36fb7:

#include "mbed.h"
#include "echo_sensor.h" // echo sensor library

#define LM1_ D3
#define LM2_ D7
#define RM1_ D6
#define RM2_ D5
#define RX_  D0
#define TX_  D1

PwmOut LM1(LM1_); // motors configuration
PwmOut LM2(LM2_);
PwmOut RM1(RM1_);
PwmOut RM2(LM2_);

//Serial link(TX_, RX_);   // bluetooth serial port
Serial link(USBTX, USBRX); // usb serial port
 
void analogWrite(mbed::PwmOut *prt, char spd) // write to PWM
{
  float a = spd / 2.56; // convert 0..256 to '%'
  *prt = a;
}

void GoForward(char spd)
{
  analogWrite(&LM1,spd);
  analogWrite(&LM2,0);
  analogWrite(&RM1,spd);
  analogWrite(&RM2,0);
}

void GoBack(char spd)
{
  analogWrite(&LM1,0);
  analogWrite(&LM2,spd);
  analogWrite(&RM1,0);
  analogWrite(&RM2,spd);
}

void TurnRight(char spd)
{
  analogWrite(&LM1,spd);
  analogWrite(&LM2,0);
  analogWrite(&RM1,0);
  analogWrite(&RM2,spd);
}

void TurnLeft(char spd)
{

  analogWrite(&LM1,0);
  analogWrite(&LM2,spd);
  analogWrite(&RM1,spd);
  analogWrite(&RM2,0);
}

int charToInt(char a, char b, char c)
{
  int ans;
  if (a>'9' || b>'9' || c>'9') return 0;
  if (a<'0' || b<'0' || c<'0') return 0;
  ans = (a - 48)*100 + (b - 48)*10 + (c - 48);
  if (ans > 255) ans = 255;
  return ans;
}

char str[4];
void intToChar(int dat)
{
  int d = dat;
  
  if (dat<100) str[0] = '0';
  else
  {
    str[0] = d/100 + 48;
    d %= 100;
  }
  if (dat<10) str[1] = '0';
  else
  {
    str[1] = d/10 + 48;
    d %= 10;
  }
  str[2] = d + 48;
}

char toupper(char ch)
{
  if (ch >= 'a' && ch <= 'z') return ch;
  if (ch >= 'A' && ch <= 'Z') return ch - 'A' + 'a';
  return 0;
}

int main(void)
{
   char dat[5], b, i;
   link.baud(9600);
   initEcho();   // initialize echo sensor
   GoForward(0); // stop all motors

   while (!link.readable());    // trying to fount the starting byte
   while (link.getc() != 'Z');

   while (true)
   {
     if(link.readable()) // read data from blutooth serial port
     {
        for (i=0; i<5; i++)
        dat[i] = link.getc();
     }
     
     b = toupper(dat[0]);
   
     int spd = charToInt(dat[1],dat[2],dat[3]);
   
     if (b == 'F') GoForward(spd); // movements
     if (b == 'B') GoBack(spd);
     if (b == 'R') TurnRight(spd);
     if (b == 'L') TurnLeft(spd);
     if (b == 'N') GoForward(0);
   
     if (b == 'E') // echo sensor
     {
       intToChar(getEcho());
       link.printf(str);
     }
   }
}