oscilloscope example,how to add a square wave on the canvas

[jetsu]

hi,everyone
oscilloscope program give me methods to sample the voltage from ADC of s3c2440 platform.Now I have got it.
Then I want to add a square wave on the canvas which could be a comparison with the sampling curve.
I don't know how to implement it.
I know how to add a curve in plot.cpp:

p_curve = new QwtPlotCurve();
    p_curve->setStyle(QwtPlotCurve::Steps);
    p_curve->setPen(QPen(Qt::green));
    p_curve->setRenderHint(QwtPlotItem::RenderAntialiased, true);
    p_curve->setPaintAttribute(QwtPlotCurve::ClipPolygons, false);
    p_curve->setData(SquareWaveData());
    p_curve->attach(this);

p_curve = new QwtPlotCurve();
    p_curve->setStyle(QwtPlotCurve::Steps);
    p_curve->setPen(QPen(Qt::green));
    p_curve->setRenderHint(QwtPlotItem::RenderAntialiased, true);
    p_curve->setPaintAttribute(QwtPlotCurve::ClipPolygons, false);
    p_curve->setData(SquareWaveData());
    p_curve->attach(this);

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The problem is how to produce the data for the square wave which is synchronous with the sampling curve?