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320*240 resistive touch screen calibration problem

Hi every one.

I have 320*240 LCD with resistive touch screen and touch screen controller of AD7843 ( All is together in one LCD from topway: KM2088S). I want to know what is the best way to calibrate the touch screen? when I push the touch screen a little bit it shows me a number, but when I push it harder it shows me a different number!! how can I solve it? Is it going to solve by calibration? I really appreciate if you guys can help me.

Regards

Arash

  • Hi every one

    I have found this application note : AN-1021 for calibration. But still i don`t know what to do for pressure on touch screen problem? Please help me.

    Regards

    Arash

  • Hi Arash,

    Nice to hear from you.

    AN-1021 describes the calibraion for the mechanical misalignments and scaling factors of resistive-type touch screens which will affect the X and Y coordinates produced by the touch screens. That is different issue from your problem.

    Per my personal understanding, AD7843 is an ADC to sample the voltage output of the resistor divider (resistive touch screen is basically a resistor divider). So your problem is that when you touch the small point on touch screen, touch presure will affect the X/Y coordinates. It is because that the high touch pressure will make a big contact of the two layer of touch screen and affect the resistor value. In other word, high touch pressure doesn't touch only one point, it touches a big area, and make an ambiguity output.

  • Hi and thanks for reply. Actually I found AD7873 which calculate the pressure also and we can use it to say if the pressure is below some value, just ignore it. But I want to know what can we do when we have ad7843. I mean let say I have AD7843 and i want to use it as a touch screen controller. But the pressure of the people is different for a amount of area ( not just one point). what can I do then? How can I solve this pressure issue by ad7843? how can we find out the number is correct and the number shows the right position?

    Regards

    Arash

  • Hi Arash-

    I checked with my contact here for resistive touch screen controllers and they provided the following information:

    I would suggest measuring pressure as well as the X and Y coordinates. This can only be done with the AD7873 but this part is pin-to-pin compatible with the AD7843. Would it be possible to replace the AD7843 with the AD7873?

    Also, to improve the accuracy of the AD7843, usually, some averaging of the X and Y position is a good idea. The touch screens, due to their mechanical construction, can bounce a bit, so most customers perform some sort of averaging on the result. For the AD7843, the averaging must be done in the host.

    Since there is not a specific community on EngineerZone to support this, if you have further questions on this topic, I suggest contacting your local Analog Devices support center listed at http://www.analog.com/support.

    Regards,

    AndyR

    EngineerZone Community Manager

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