linux-kernel - Re: [PATCH v1] dt-bindings: touchscreen: add touchscreen-read-duration-us and touchscreen-settling-time-us properties

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Date:   Sat, 21 Nov 2020 06:56:42 -0600
From:   Rob Herring <robh@...nel.org>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Mark Brown <broonie@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        linux-input@...r.kernel.org,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Benjamin Gaignard <benjamin.gaignard@...com>,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH v1] dt-bindings: touchscreen: add
 touchscreen-read-duration-us and touchscreen-settling-time-us properties

On Thu, 12 Nov 2020 12:20:48 +0100, Oleksij Rempel wrote:
> According to the TI application bulletin [1] we deal with two generic
> mechanisms which would affect the precision of provided input events:
> 
> |TOUCH SCREEN SETTLING TIME
> |
> |When the touch  panel is pressed or touched, there are
> |two mechanisms that will affect the voltage level at the contact point of
> |the touch panel. These two mechanisms will cause the voltage across the
> |touch panel to “ring” (oscillate), and then slowly settle (decay)
> |down to a stable DC value.
> |
> |The two mechanisms are:
> | 1) Mechanical bouncing caused by vibration of the top layer sheet  of
> |    the touch  panel  when  the  panel  is  pressed.
> |
> | 2) Electrical  ringing  due  to  parasitic  capacitance  between the top
> |    and bottom layer sheets of the touch panel and at the  input  of  ADS7843
> |    that  causes  the  voltage  to  “ring”(oscillate).
> 
> Since both of this mechanisms are board specific and reflect the
> mechanical, and electrical properties of end product, it is better to
> provide a generic properties to address them.
> 
> The touchscreen-read-duration-us property should address 1. mechanism.
> This effect can be triggered by device specific design. The duration ma be
> dependent on the use case of the end device. For example a touch where
> writing is required may have other timing requirements as the device
> where only "buttons" should be pressed.
> 
> The touchscreen-settling-time-us property should address 2. mechanism
> where the size and construction of touch screen plates affect the parasitic
> capacitance and time needed between enabling power supply for the
> plates, and actual usable voltage level to detect the position of touch event.
> 
> [1] https://www.ti.com/lit/an/sbaa036/sbaa036.pdf
> 
> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> ---
>  .../bindings/input/touchscreen/touchscreen.yaml          | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 

Reviewed-by: Rob Herring <robh@...nel.org>