| Message ID | 20201112112048.12134-1-o.rempel@pengutronix.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v1] dt-bindings: touchscreen: add touchscreen-read-duration-us and touchscreen-settling-time-us properties | expand |
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>
diff --git a/Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml b/Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml index a771a15f053f..8ba845f68d5c 100644 --- a/Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml +++ b/Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml @@ -55,6 +55,15 @@ properties: values dependent on the controller) $ref: /schemas/types.yaml#/definitions/uint32 + touchscreen-read-duration-us: + description: Averaged time to sample each read to detect waves within + specified duration (valid values dependent on the controller) + + touchscreen-settling-time-us: + description: Time it takes for the touchscreen ADC to produce valid samples + again after switching between axes (valid values dependent on the + controller) + touchscreen-inverted-x: description: X axis is inverted type: boolean
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(+)