File content as of revision 0:8123026ac7a4:

#include "mbed.h"

PwmOut motor1(p26);
PwmOut motor2(p25);
PwmOut motor3(p24);
PwmOut motor4(p23);
DigitalOut myled(LED1);

#define PERIOD 0.00005

int motor(int right, int left)
{
    if (right > 0) {
        motor1.pulsewidth(0.0);
        motor2.pulsewidth(PERIOD *    right  / 200);
    } else {
        motor1.pulsewidth(PERIOD * (- right) / 200);
        motor2.pulsewidth(0.0);
    }
    if (left > 0) {
        motor4.pulsewidth(0.0);
        motor3.pulsewidth(PERIOD *    left / 200);
    } else {
        motor4.pulsewidth(PERIOD * (- left) / 200);
        motor3.pulsewidth(0.0);
    }
    return 0;
}


int main() {
    motor1.period(PERIOD);
    motor1.pulsewidth(0.0);
    motor2.period(PERIOD);
    motor2.pulsewidth(0.0);
    motor3.period(PERIOD);
    motor3.pulsewidth(0.0);
    motor4.period(PERIOD);
    motor4.pulsewidth(0.0);
    int right = 0, left = 0, del = 10;
    while(1){
        myled = 1;
        wait(0.2);
        myled = 0;
        wait(0.2);
        right += del;
        left  += del;
        if ((right >= 100)||(right <= -100)) del *= -1;
        motor(right, left);
    }
}