Event-Driven Programming (GPIO)

kafanti

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) {
        usleep(20000);
        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,
Sina

SGaist

Hi,

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.

kafanti

@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.

Sina

SGaist

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.