GLCD and TouchScreen trouble

[SteveB]

Here are some more details.

If I place a piece of plastic, or rest it on some rubber bands, or both, I get this:


Now, if I place the TS on a couple of ordinary pencils, it works fine:


Also, if the TS is only about 1/4 of the way on the GLCD, it seems to be OK as well:


Any Ideas whats causing this? I will be playing with it to see if I can get a minimum clearance, but I like to know whats going on.

SteveB

[skimask]

Here are some more details.

Any Ideas whats causing this? I will be playing with it to see if I can get a minimum clearance, but I like to know whats going on.

SteveB

Capacitive touchscreen type thing? Or resistive?
If it's capacitive, maybe it (or the LCD) isn't not grounded well enough and inducing a charge back into the LCD. Or maybe you have to TOTALLY isolate the two...who knows. Looks pretty goofy to me. Ya gotta wonder how all those PDA's handle it.

[SteveB]

Using some various piece of thin plasic, I can get no closer than 1/4". If I reduce the gap by 1/32", it blanks out.

Capacitive touchscreen type thing? Or resistive?

It's 4-wire resistive.

[SteveB]

I don't know if it will make a difference in any help, but here is the code relevent to scanning the TS:

XRaw VAR WORD
YRaw VAR WORD

Xp VAR PORTA.3
Xm VAR PORTA.5
Yp VAR PORTA.2
Ym VAR PORTA.4

ADCON1 = $0D 'Set only PORTA.0-1 Analog


'Scan X
    HIGH Xp
    LOW Xm
    INPUT Yp
    INPUT Ym
    PAUSEUS 5
    ADCIN 0, XRaw
    LOW Xp
'Scan Y
    HIGH Yp
    LOW Ym
    INPUT Xp
    INPUT Xm
    PAUSEUS 5
    ADCIN 1, YRaw
    LOW Yp

and a schematic of the hookup:

[skimask]

I keep going back to that grounding thing, static or EMI...
Neither of the datasheets has any helpful hints as to what might be going on.
Maybe try hooking a 'grounding wire' up to the touchscreen to the LCD frame, first with a 1K resistor (maybe less), then maybe a direct connection.
I'm thinking it's some sort of static buildup causing the controllers on the GLCD to go nuts.
Maybe try it sitting on a regular character LCD and see what happens? Not that a touch panel on a character LCD is any good, but it might lead to something...

[SteveB]

Maybe try it sitting on a regular character LCD and see what happens? Not that a touch panel on a character LCD is any good, but it might lead to something...

I just happened to have a 4X20 breadboarded up with this project, and the TS has no effect on it.

I'll look into the resistor idea tomorrow (or whenever I can get back to it this week). Not a lot of options to connect it to though, the lcd bezel is painted and there is nothing exposed on the TS. The "FrameGround" pin on the lcd is connected to GND. (I've also tried it not connected)

SteveB

[Ron Marcus]

I bet that the touch screen is creating an EM field due to the high frequency switching going on. Just for a test, take some of the static protective translucent poly bag material, and sandwich it between the two. If this doesn't relieve the problem, it should reduce the distance. You do have a wierd ground loop problem going on there. What is the hookup of your GLCD?
Ron