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Event-Driven Programming (GPIO)

  • Hello,

    I am trying to do a simple pushbutton(physical)-LED circuit. As always, when the pushbutton pushed, LED will on and when it released, LED will off.

    Here is my main loop looks like;

    mmap_gpio gpio;
    gpio.setMode(4, mmap_gpio::output);
    gpio.setMode(17, mmap_gpio::input);
    while (1) {
        if (gpio.read(17) == mmap_gpio::low) {
            if (gpio.read(17) == mmap_gpio::low) {
                gpio.write(4, mmap_gpio::high);
                while (gpio.read(17) == mmap_gpio::low)
        gpio.write(4, mmap_gpio::low);

    But when I loop like that, the program just consumes whole CPU cycles.. When I monitor with htop, one of the processor is always working %100. I would like to make it by native way, so I can understand how Qt works.

    I use Raspberry Pi 2 Model B.

    Thank you,

  • Lifetime Qt Champion


    If you want to understand how Qt works you should rather read its sources. The event dispatcher will be one of the part you're interested in.

  • @SGaist Thank you SGaist, I will definetly check this. I found a way but not sure if it is a native one.. So here it is how this way is;

    QCoreApplication-derived mmapApplication, on the constructor, QBasicTimer is allocated on heap and interval is setted to 50ms,

    mmapApplication::mmapApplication(int &argc, char && argc) : QCoreApplication(argc, argv)
         QBasicTimer pTimer = new QBasicTimer;
         pTimer->start(50, this);

    and on the mmapApplication::timerEvent(QTimerEvent *event) override, can call a GPIO function to check if any of the pins state has changed or not. If changed, it can post a custom event which has the information about the state of the pins.


  • Lifetime Qt Champion

    What is your definition of native ?

    A while loop is the same thing on Linux, OS X or Windows.

    From a pure design point of view, I'd rather create a dedicated QObject derived class rather than subclass QCoreApplication.

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