File content as of revision 2:e0a553b64315:

#include "mbed.h"

DigitalOut led1(LED1);

Serial pc(SERIAL_TX, SERIAL_RX);

// used for motor
//AnalogOut M1_enable(D6);
//AnalogOut M2_enable(D5);
DigitalOut M1_enable(D6);
DigitalOut M2_enable(D5);
DigitalOut M1_in1(D12);
DigitalOut M1_in2(D11);
DigitalOut M2_in3(D10);
DigitalOut M2_in4(D9);
const int MOTOR_M1=0;
const int MOTOR_M2=1;
const int DIR_FORWARD= 0;
const int DIR_BACKWARD=1;
const int PWR_STOP = 0;

// used for IR sensors
AnalogIn leftIR(A1);
AnalogIn middleIR(A3);
AnalogIn rightIR(A5);

long map(long x, long in_min, long in_max, long out_min, long out_max)
{
    return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

void DriveSingleMotor(int m, int speed, int dir);

// main() runs in its own thread in the OS
int main()
{
    bool left = false;
    bool middle = false;
    bool right = false;
    
    // here I use 500 as threshold
    int threshold = 500;

    while (true) {    
        // not on track: > 500
        // on track (black): < 500
        left = leftIR.read_u16() < threshold ? true : false;
        middle = middleIR.read_u16() < threshold ? true : false;
        right = rightIR.read_u16() < threshold ? true : false;

        led1 = 1; // light on on-board LED

        // if middle detects black
        if(middle) {
            if(left && right) { // cliff
                // stop
                pc.printf("cliff\r\n");
                DriveSingleMotor(MOTOR_M1, PWR_STOP, DIR_FORWARD);
                DriveSingleMotor(MOTOR_M2, PWR_STOP, DIR_FORWARD);
                // pause for 0.5 sec
                wait_ms(100);

                // then go backward
                DriveSingleMotor(MOTOR_M1, 2, DIR_BACKWARD);
                DriveSingleMotor(MOTOR_M2, 2, DIR_BACKWARD);
            } else if(left) { // turn left
                pc.printf("turn left\r\n");
                DriveSingleMotor(MOTOR_M1, PWR_STOP, DIR_FORWARD);
                DriveSingleMotor(MOTOR_M2, 2, DIR_FORWARD);
            } else if(right) { // turn right
                pc.printf("turn right\r\n");
                DriveSingleMotor(MOTOR_M1, 2, DIR_FORWARD);
                DriveSingleMotor(MOTOR_M2, PWR_STOP, DIR_FORWARD);
            } else { // go straight
                pc.printf("go straight\r\n");
                DriveSingleMotor(MOTOR_M1, 2, DIR_FORWARD);
                DriveSingleMotor(MOTOR_M2, 2, DIR_FORWARD);
            }

            //wait_ms(1000);
        }

        // if all detects white, which means the car is out of track
        if(!middle && !left && !right) { // go backward
            pc.printf("out of track, go backward\r\n");
            DriveSingleMotor(MOTOR_M1, 2, DIR_BACKWARD);
            DriveSingleMotor(MOTOR_M2, 2, DIR_BACKWARD);
            //wait_ms(1000);
        }

        pc.printf("pause\r\n");
        DriveSingleMotor(MOTOR_M1, PWR_STOP, DIR_FORWARD);
        DriveSingleMotor(MOTOR_M2, PWR_STOP, DIR_FORWARD);
        led1 = 0; // turn of on-board LED
        wait_ms(100);
    }
}

// m: 0 => M1      1 => M2
// power 0~10     dir: 0=>正向　1=>反向
void DriveSingleMotor(int m, int speed, int dir)
{
    /*int _speed = 0;

    //　設定速度
    if (speed>10) speed=10;
    _speed = map(speed, 1, 10, 100, 255);

    if (speed<=0) {
        speed=0;
        _speed=0;
    } else
        _speed = map(speed, 1, 10, 100, 255);*/

    if (m == MOTOR_M1) {
        //　設定方向
        if (speed == PWR_STOP) {
            M1_in1 = 0;
            M1_in2 = 0;
        } else if (dir ==  DIR_FORWARD) {
            // right wheel go forward
            M1_in1 = 1;
            M1_in2 = 0;
        } else {
            // right wheel go backward
            M1_in1 = 0;
            M1_in2 = 1;
        }
        //M1_enable.write_u16(_speed); // 驅動馬達  右輪
        M1_enable.write(1);

    } else if (m == MOTOR_M2) {
        //　設定方向
        if (speed == PWR_STOP) {
            M2_in3 = 0;
            M2_in4 = 0;
        } else if (dir == DIR_FORWARD) {
            //左輪前進
            M2_in3 = 1;
            M2_in4 = 0;
        } else {
            //左輪倒轉
            M2_in3 = 0;
            M2_in4 = 1;
        }
        //M2_enable.write_u16(_speed); // 驅動馬達  左輪
        M2_enable.write(1);
    } else {
        //M1_enable.write_u16(PWR_STOP); // right wheel
        //M2_enable.write_u16(PWR_STOP); // left wheel
        M1_enable.write(0);
        M2_enable.write(0);
    }
}