resistive touchscreen problem

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Hi all,

A friend bought one of those fake Apple iPad clones, that actually run android, in China and brought it with him. The thing even has the Apple logo on the back and all. Turns out that it's bricked because no touch input is accepted.

I've opened the thing up and found a 4-wire resistive touchscreen. It is read by some vague IC from a Chinese semiconductor company, that is controlled over SPI.
Chip is "SEMCO CS7146", i found a Chinese datasheet if anyone's interested.

Now I'm trying to pin point the actual problem, but i have no experience with resistive touchscreens. I do have a bit of experience with capacitive touchscreens though.

I'm measuring the resistance across the X and Y terminals, but it doesn't seems to change when I touch the display. Can i assume the foil is damaged ?

In idle condition (jitters a bit when touched) :
Resistance across X terminals : 731 Ohm
Resistance across Y terminals : 240 Ohm

I guess i could hook up a logic analyzer to the SPI bus and look at what it's reading, but i thought I'd check here first for some insights on resistive touchscreens.

So, shouldn't the resistance change when i apply pressure on the foil ? Or is the difference very small perhaps ?

I also found AVR341, which has example code to read a 4-wire resistive touchscreen, but since i have no experience with these foils, and i don't know if the foil is damaged or not, this route might be a waste of time.

It would be nice if I could get any use out of this device, except as a coaster, since it's actually a pretty neat thing.

Perhaps I can hook up an USB mouse to the thing, but i'm not sure if the android OS has a default driver for this.

Any insights or tips are appreciated. Thanks.

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it"

Last Edited: Sun. Jun 5, 2011 - 09:33 PM
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Ah, thanks. That article sums it up nicely. I see now that i cannot simply measure the resistance across the terminals.

Is there any simple or direct method i could employ to determine if the foil is damaged or not ?

Perhaps I'll try that code in AVR341 to test it.

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it"

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first if both x and y terminals isn't open they should be ok.
take it out and force 3 or 5 V over Y and put a meter on X. then flip to the other side.

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I'm trying that right now, but of course i don't have the correct connector handy..

But indeed as you said, since they are not open, it's likely the foil will be alright. Maybe I should look elsewhere for the problem.

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it"

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I have a board with a resistive touchscreen, and its chip has the same pinout as yours (comparing to your Chinese datasheet). It is part number XPT2046 but the board's schematic shows another number similar to yours. If you want to have a look at the datasheet and can't locate it, let me know.

Chuck Baird

"I wish I were dumber so I could be more certain about my opinions. It looks fun." -- Scott Adams

http://www.cbaird.org