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

From: Rob Herring <robh@kernel.org>
To: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: devicetree@vger.kernel.org, Rob Herring <robh+dt@kernel.org>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Mark Brown <broonie@kernel.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	linux-kernel@vger.kernel.org, kernel@pengutronix.de,
	linux-input@vger.kernel.org,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Benjamin Gaignard <benjamin.gaignard@st.com>,
	David Jander <david@protonic.nl>
Subject: Re: [PATCH v1] dt-bindings: touchscreen: add touchscreen-read-duration-us and touchscreen-settling-time-us properties
Date: Sat, 21 Nov 2020 06:56:42 -0600	[thread overview]
Message-ID: <20201121125642.GA2083872@robh.at.kernel.org> (raw)
In-Reply-To: <20201112112048.12134-1-o.rempel@pengutronix.de>

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@pengutronix.de>
> ---
>  .../bindings/input/touchscreen/touchscreen.yaml          | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 

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