Linux/AM4378: Erratic touchscreen pen-up events

Hey Nick,

Yes that is correct - I am using a resistive touch screen connected with ADC pins. Currently, my tsadc node in my device looks like this:

&tscadc {
    status = "okay";
    compatible = "ti,am3359-tscadc";
    tsc {
        ti,wires = <4>;
        ti,x-plate-resistance = <500>;
        ti,coordinate-readouts = <5>;
        ti,wire-config = <0x00 0x11 0x22 0x33>;
        ti,charge-delay = <0x4000>;
    };
};

Any ideas?

Tanner,

From the bindings document included in the SDK:

Optional properties:
- child "tsc"
        ti,charge-delay: Length of touch screen charge delay step in terms of
                         ADC clock cycles. Charge delay value should be large
                         in order to avoid false pen-up events. This value
                         effects the overall sampling speed, hence need to be
                         kept as low as possible, while avoiding false pen-up
                         event. Start from a lower value, say 0x400, and
                         increase value until false pen-up events are avoided.
                         The pen-up detection happens immediately after the
                         charge step, so this does in fact function as a
                         hardware knob for adjusting the amount of "settling
                         time".

These values usually need to be "tuned" to each touchscreen, and even each board.

I hope this is helpful to you.

Sorry, meant to mention that the document is in Documentation/devicetree/bindings/input/touchscreen/ti-tsc-adc.txt and may provide some other useful information for you as you tune your touchscreen.

Yes, I saw that documentation (it was linked in my original post). But how high can the charge delay go? As you can see, I already have it set at 0x4000. I actually tried going all the way up to 0x9000, but even then I still was not getting 100% accuracy.

Tanner,

I'm sorry I missed the link in your original post. 

I took a look at the driver, and it is using this value to program the TS_CHARGE_DELAY register at line 183 of drivers/input/touchscreen/ti_am335x_tsc.c. Here is an overview of the register:

So it seems you could theoretically go up to 0x40000.

I hope this helps.

Tanner,

Yes, we could definitely improve that doc to make it more clear. I'm glad you were able to find a value that works well.

Thanks again for the post.