Qt precise timer timing accuracy

Question

I write a simple demo to check the single-shot timer accuracy. Here below are my codes:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    //two singleshot timers  
    QTimer::singleShot(1000,Qt::PreciseTimer,this,SLOT(myslot1()));
    QTimer::singleShot(10000,Qt::PreciseTimer,this,SLOT(myslot2()));

    //metrics of timing accuraccy
    t = 0;
    timer = new QTimer();
    timer->setInterval(100);
    timer->setTimerType(Qt::PreciseTimer);
    connect(timer,SIGNAL(timeout()),this,SLOT(timerhandler()),Qt::DirectConnection);
    timer->start();
}

void MainWindow::myslot1()
{
    qDebug()<<"myslot1 called: t="<<100*t;
    ui->lcdNumber1->display(1);
}

void MainWindow::myslot2()
{
    qDebug()<<"myslot2 called: t="<<100*t;
    ui->lcdNumber2->display(2);
}

void MainWindow::timerhandler()
{
    t++;
    ui->lcdNumber->display(t*100);
}

Run these codes gives:

myslot1 called: t= 900
myslot2 called: t= 9900

It seems to me there is a timing error of about 100 ms, for the precise timer. Are I right?? Is the apparent error due to the time interval I set for timer? According to some post (QTimer not accurate at all?), QTimer may not be so accurate at all.

Your times are measured using a `QTimer` (the variable `timer`) that is explicitly configured to have an interval of 100ms -- so I'm not sure what else you would expect. Apologies if I've missed something. — G.M., Mar 14 '19 at 17:41
Try adding debug print to timerhandler as well. Also, use QElapsedTimer (same for all, reset in the constructor) to print timestamp in the debug prints. I think that should clarify things up, especially regarding to when and at what points in time the slots get called. — hyde, Mar 14 '19 at 17:44
`QTimer` is not meant to implement clocks. If you need a clock, i.e. something that measures wall-time, or its passage (a stopwatch), then you forget about `QTimer`'s existence, and all is well again: in such circumstances, it's but a distraction. — Kuba hasn't forgotten Monica, Mar 14 '19 at 18:08

score 2 · Accepted Answer · answered Mar 14 '19 at 17:48

If you want the time between 2 events you must use QElapsedTimer:

Example:

#include <QtCore>

class Foo: public QObject
{
    Q_OBJECT
public:
    Foo(QObject *parent=nullptr):
        QObject(parent),
        timer(new QTimer())
    {
        QTimer::singleShot(1000,Qt::PreciseTimer,this, &Foo::myslot1);
        QTimer::singleShot(10000,Qt::PreciseTimer,this, &Foo::myslot2);

        timer->setInterval(100);
        timer->setTimerType(Qt::PreciseTimer);
        connect(timer,&QTimer::timeout,this,&Foo::timerhandler,Qt::DirectConnection);
        timer->start();

        timer_measure.start();
    }
private:
    Q_SLOT void myslot1(){
        qDebug()<< __PRETTY_FUNCTION__ << timer_measure.elapsed();
    }
    Q_SLOT void myslot2(){
        qDebug()<< __PRETTY_FUNCTION__ << timer_measure.elapsed();
    }
    void timerhandler()
    {
        qDebug()<< __PRETTY_FUNCTION__ << timer_measure.elapsed();
    }
    QElapsedTimer timer_measure;
    QTimer *timer;
};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    Foo foo;
    return a.exec();
}
#include "main.moc"

Output:

void Foo::timerhandler() 100
void Foo::timerhandler() 200
void Foo::timerhandler() 301
void Foo::timerhandler() 400
void Foo::timerhandler() 500
void Foo::timerhandler() 600
void Foo::timerhandler() 700
void Foo::timerhandler() 800
void Foo::timerhandler() 900
void Foo::myslot1() 1000
void Foo::timerhandler() 1001
void Foo::timerhandler() 1100
void Foo::timerhandler() 1200
void Foo::timerhandler() 1300
void Foo::timerhandler() 1401
void Foo::timerhandler() 1500
void Foo::timerhandler() 1600
void Foo::timerhandler() 1700
void Foo::timerhandler() 1801
void Foo::timerhandler() 1900
void Foo::timerhandler() 2000
void Foo::timerhandler() 2100
void Foo::timerhandler() 2201
void Foo::timerhandler() 2300
void Foo::timerhandler() 2400
void Foo::timerhandler() 2500
void Foo::timerhandler() 2600
void Foo::timerhandler() 2700
void Foo::timerhandler() 2800
void Foo::timerhandler() 2901
void Foo::timerhandler() 3000
void Foo::timerhandler() 3100
void Foo::timerhandler() 3200
void Foo::timerhandler() 3301
void Foo::timerhandler() 3400
void Foo::timerhandler() 3500
void Foo::timerhandler() 3600
void Foo::timerhandler() 3701
void Foo::timerhandler() 3800
void Foo::timerhandler() 3900
void Foo::timerhandler() 4000
void Foo::timerhandler() 4101
void Foo::timerhandler() 4200
void Foo::timerhandler() 4300
void Foo::timerhandler() 4400
void Foo::timerhandler() 4501
void Foo::timerhandler() 4600
void Foo::timerhandler() 4700
void Foo::timerhandler() 4800
void Foo::timerhandler() 4901
void Foo::timerhandler() 5000
void Foo::timerhandler() 5100
void Foo::timerhandler() 5200
void Foo::timerhandler() 5300
void Foo::timerhandler() 5400
void Foo::timerhandler() 5500
...

Qt precise timer timing accuracy

1 Answers1