What difference between while(true) and QTimer?

[isan]

I want to read something form ADC connected to raspberry pi

value = analogRead  (PCF + 0) ;

When I do this with while(true) in function ,I get more numbers than when do this with QTimer that connected to read function
I start a timer in mainWindow

connect(tm, SIGNAL(timeout()),this, SLOT(readFunction()));
     tm->start();

why?
I don't want to use while(true) in my code ,what should I do to ,read more numbers at a faster rate?

[SGaist]

Hi,

Your timer runs at a fixed interval.

An infinite loop runs as fast as it can (but will block the event loop).

Either use a smaller interval for your timer or implement a thread that will use your infinite loop.

[isan]

@SGaist When start timer without the interval.like:

tm->start();

Is the interval 0?if yes, how can set smaller interval?

[mrjj]

@isan
Hi
It cannot be smaller than 1 ms.
tm->start(1);

If you need it faster, a thread - as suggested is the way to go.

https://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

[JKSH]

@isan said in What difference between while(true) and QTimer?:

When start timer without the interval.like:

tm->start();

Is the interval 0?

The interval used here is the one reported by tm->interval(). http://doc.qt.io/qt-5/qtimer.html#start-1

The default interval value is 0. You can change it by calling setInterval().

@isan said in What difference between while(true) and QTimer?:

When I do this with while(true) in function ,I get more numbers than when do this with QTimer that connected to read function
I start a timer in mainWindow

connect(tm, SIGNAL(timeout()),this, SLOT(readFunction()));
     tm->start();

why?

Because a timer with a 0 interval yields the CPU when your slot finishes running. This allows the CPU to do other things between reading your ADC. (see https://en.wikipedia.org/wiki/Yield_(multithreading) ).

while(true) {...} does not yield unless you call sleep() inside the loop. There is a chance it will consume 100% of the CPU core and never let other threads use it.

Also, there is a bit of overhead when using signals and slots (but I don't know how much). You can try to reduce the overhead by using the new signal-slot syntax (see http://doc.qt.io/qt-5/signalsandslots-syntaxes.html ):

connect(tm, &QTimer::timeout, this, &MyClass::readFunction); // replace "MyClass" with your real class name

I don't want to use while(true) in my code ,what should I do to ,read more numbers at a faster rate?

What rate do you want?

[aha_1980]

What rate do you want?

and what do you want to do with the ADC samples?

[isan]

@JKSH when read number with while(true) I get 120 numbers per second
and with QTimer I get 40 numbers per second
so for now ok to get 120 numbers persecond

[isan]

@aha_1980 I want to draw a chart with numbers and when the numbers lost the chart is not valid enough
I know if change i2c baud rate the sample rate is change ,but also its important how to read

[mrjj]

@isan
Hi
I think you will be most happy if you spend the extra time and
make a tread to do the reading as it wont block main and will
be as fast as while(true).

[isan]

@mrjj said in What difference between while(true) and QTimer?:

I think you will be most happy if you spend the extra time and
make a tread to do the reading as it wont block main and will
be as fast as while(true).

I do this surely. tank you

[JKSH]

@isan said in What difference between while(true) and QTimer?:

@JKSH when read number with while(true) I get 120 numbers per second
and with QTimer I get 40 numbers per second
so for now ok to get 120 numbers persecond

120 samples per second with while(true) is quite slow. A QTimer should be able to reach that rate, so I don't think that CPU yielding is the reason for the slowness.

1. Can you show more code?
2. What does tm->interval() return?
3. What ADC is it?
4. Where is the analogRead() function from?

@isan said in What difference between while(true) and QTimer?:

@aha_1980 I want to draw a chart with numbers

Drawing a chart is very expensive. I agree with @mrjj -- you should see an improvement if you read the ADC in a dedicated thread and draw the chart in the GUI thread.

[isan]

@JKSH I dont use tm->interval()

ADC is PCF8591 with 100k scl

I use wiringpi library ,that has <pcf8591.h>

I try to use reading analog pin in thread tnx.

#include "chart.h"
#include <QtCharts/QAbstractAxis>
#include <QtCharts/QSplineSeries>
#include <QtCharts/QValueAxis>
#include <QtCore/QDebug>
#include <wiringPi.h>
#include <pcf8591.h>
#define PCF       120

Chart::Chart(QGraphicsItem *parent, Qt::WindowFlags wFlags):
    QChart(QChart::ChartTypeCartesian, parent, wFlags),
    m_series(0),
    m_axis(new QValueAxis),
    m_step(0),
    m_x(5),
    m_y(1)
{
    QObject::connect(&m_timer, &QTimer::timeout, this, &Chart::handleTimeout);
    m_timer.setInterval(1000);

    tm=new QTimer(this);
     connect(tm, SIGNAL(timeout()),this, SLOT(readFun()));
     tm->start();
    m_series = new QSplineSeries(this);
    QPen green(Qt::red);
    green.setWidth(3);
    m_series->setPen(green);
    m_series->append(m_x, m_y);

    addSeries(m_series);
    createDefaultAxes();
    setAxisX(m_axis, m_series);
    m_axis->setTickCount(5);
    axisX()->setRange(0, 10);
    axisY()->setRange(60,180);
    wiringPiSetup () ;
        // Setup pcf8591 on base pin 120, and address 0x48
     pcf8591Setup (PCF, 0x48) ;

    m_timer.start();
}