11 years, 2 months ago.

Problem changing PWM on the fly

Hi all,

I am currently utilizing a PWM pin to be a "simulator" for some hardware that I am going to build. Basically I am going to use the PWM to drive a stepper driver (from Pololu) and I'm using an opto-interrultor connected to a pin on the mbed and that pin is configured as an interrupt. This way I can count the interrupt time and be certain that the motor is actually moving at the RPM expected.

In the meantime I have the PWM directly driving the interrupt pin. (I'm only counting a "pulse" every 200th interrupt since the stepper takes 200 steps per revolution.)

The problem I am having is that when I go to change the period of the pulses on the PWM, the interrupts stop happening.

Here are is a snippet of my code:

    motorPulse.rise(&revolution); // tet up ISR
    motorSim.period_us(1000); // just set an inital timing for the PWM
    motorSim.pulsewidth_us(2); // motor driver requires only a 2uS pulse to acheive a step
    wait(10); // Let me clear the serial terminal
    char buf[1024];
    int ptr = 0;
    while(1)
    {
        myled = 1;
        wait(0.2);
        myled = 0;
        wait(0.2);
        if(pc.readable())
        {
            buf[ptr] = pc.getc();
            if(buf[ptr] == '\r')
            {
                buf[ptr] = '\0';
                ptr = 0;
                double period = ((double)atof(buf) / 60.0) * 1000.0;
                int period_ms = (period / 200.0) * (double) 1000.0;
                pc.printf("\x1b[3;0fbuf = %s, period = %f, period_ms = %i\n", buf, period, period_ms);
                motorSim.period_ms(period_ms);
            }
            else
            {
                ptr++;
            }
        }
    }

So once I get input on the serial line and convert it into a number I calculate and change the period using period_ms(); but when that happens my interrupt code stops firing. So I am assuming that my change to the period is stopping the PWM output.

Any suggestions as to how I can change the period without killing the PWM?

Thanks in advance!

    motorSim = new PwmOut(p21);
    motorPulse.rise(&revolution);
    motorSim->period_us(1000);
    motorSim->pulsewidth_us(2);
    wait(10);
    char buf[1024];
    int ptr = 0;
    //pc.printf("\r\n\r\n"); //Position to where input will be
    displayUpdater.attach(&updateDisplay, 1.0);
    delete motorSim;
    motorSim = new PwmOut(p21);
                motorSim->period_ms(2000);
                motorSim->pulsewidth_us(2);