4-wire Touch Screen interface? Read a touchscreen, and emulate one too.

helloseth

Has anyone used a prop to interface with a 4 wire touchscreen? (I don't know the specifics)

Ideally I would like to be able to read the touchscreen and also emulate the signal to forward on to the existing OEM hardware.

I would like to put a propeller in as a 'carputer' in my '06 Acura TSX. I found a link where a guy 'patched' in a PC with the Acura nav system, and hooked up a 4-pole relay to tap into the 4-wire touchscreen which is in the nav system. The 4-wire interface was then connected to the PC side via a USB touchscreen controller.

He has a toggle switch to switch between the OEM and PC touchscreen controller.

What I would like to do is have the prop be able to read the 4-wire from the screen, and then generate the equivalent signal for the OEM side.
I don't want pass thru, but want to have the prop control some functions on the OEM, so it needs to emulate a person touching the screen.

Thanks
Seth

El Paisa

I use a ezLCD-004 graphics LCD with buildin touch screen.
I am including a example to program the ezLCD-004 using the propeller a photo of the actual screen.

helloseth

I've considered the ezLCD products for other projects, but for this one I need to integrate to an existing LCD/Touchscreen that came with the car. The video is no problem, but I would like to be able to use the touchscreen and have the prop drive the existing navigation system by generating 'fake'/emulated touches.

Seth

Lawson

Is this a resistive or capacitive touch screen? (capacitive touch screens usually only respond to fingers, resistive screens work with anything that contacts them)

if it's a resistive screen the datasheet for this www.national.com/pf/LM/LM8300.html part should give some idea how they're normally read.

Bingo! google found this focus.ti.com/lit/an/slyt209a/slyt209a.pdf pdf file that gives an overview of resistive touch screens.
and another one www.hampshiretouch.com/doc/appnotes/AN103%20How%20an%20Analog%20Resistive%20Touch%20Screen%20Works.pdf

I bet a couple of digital potentiometers could emulate a resistive screen really well. connect the two wipers together and then the ends go to the car.

I bet the Prop's counters running as Sigma-Delta ADCs could be modified to read a resistive touch screen. (hm... the prop counter ADCs will load the circuit, this will make reading the position more complex.)

have funish
marty

Paul Baker

The issue with the Propeller and touchscreens is that touchscreens are analog and have a potential bias that needs to be accounted for. The Propeller's analog capabilities aren't quite up to snuff, however if you use a touch screen digitizer chip such as Burr-Brown's ADS7843 or Analog Device's AD7843 as your analog front end, you can easily have the Propeller act as whatever device you need it to mimic for your downstream electronics.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.