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ADS7843 PENIRQ Pullup resistor

Jigar Patel1
Prodigy 130 points

I am using ADS 7843. Touch screen is working almost 100% time, with bench setup. But once we install setup in noisy environment we have random button press near x = 0 location. Software check PENIRQ pin 16 times before it decide that pin is really pulled low.

PENIRQ pin is tied to 5 V (Vdd) with 100 K Oh pull up resistor.

If nothing is pressed on touch panel, I am getting ~3.5 V on PENIRQ pin (with 100K Pull up). (Measured with scope). If I decrease pull up resistor (to 10 K)  I am getting ~4. 8 V on PENIRQ pin.

I am able to get it working in noisy environment with 10 K Pull up on PENIRQ pin.

My question is,

If I reduce pull up resistor, how it will affect Vol ? Do I have to add any additional precaution in software or hardware after this change?  (Currently I am doing average of three out of nine samples)

Regards,

Jigar

  • 0
    TI__Guru 52115 points

    Hi Jigar,

    Do you have any idea of where the noise is from?  Noise from LCD or power noise?  Based on my own experience of fighting against touch noise. It it better to nail down the source of noise first. If the noise you were suffering from is due to analog input noise, I would recommend that you add some noise decoupling capacitors from Y+, Y-, X+ and X- to ground. If the noise is from power,  I suggest you increase the capacitor near the VDD, or even add a small resistor (like 10 Ohms) to from a RC low filter.  

    I hope my comments may help you a little bit.

    Andy

  • 0 in reply to Andy Liu (BMS)
    Prodigy 130 points

    Thank You for reply..

    We have LCD is in production, so we can not add extra capacitors on X or Y pins. We have to go through lay out change. But I tried to add extra capacitor on  VDD pin, which did not help.

    Does lowing pullup resistor between 10 to 20 K acceptable ? Datasheet suggest pull up value from 10K to 100K which is wide range. What are drawback to reduce pullup resistor ? I do not care about power consumption.

    -Jigar

  • 0 in reply to Jigar Patel1
    TI__Guru 52115 points

    Hi Jigar,

    From my point of view, there is no obvious risk for you to use the 10k pullup resistor, given that power consumption is not your concern at all. I will double check with the IC designer to confirm whether there is any defect.

    Andy

  • 0
    TI__Prodigy 130 points

    Hi, Jigar,

    The Vol will depend on the the maximal touch panel resistance and the chosen pull-up resistance.

    Use the following equation to estimate the pull-up resistance you are going to use.

    Rpull : Pull-up resistance

    Rtp : Touch panel max resistance for each plate (X or Y plate)

    Vdd = 5V

    Vol = 0.4V as described in the datasheet as max value

    If you have Rtp = 10k ohms panel for each plate, then you need

    Rpull = 2*Rtp*(Vdd-Vol)/Vol = 230k ohms

    at least as a pull-up resistor to meet the Vol requirement in the datasheet.

    Hopefully, this will help you.

    Regards,

    James

     

  • 0 in reply to James-Tucson Wang
    TI__Prodigy 130 points

    If the touch panel resistances for X or Y plate are different, then the equation is slightly changed to

    Rpull = (Rtp_x+Rtp_y)*(Vdd-Vol)/Vol