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Touch screen problems

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2PAC Mafia

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Hi guys,

I´m trying to solve a problem with a marine display which uses touch screen. The unit was working switching on correctly but not reacting to touch screen so I replaced it and then it was working. After some days I received the phone call from my customer saying the touch screen was failing again.

Now I took some measurements on the touch screen chip which is UCB1300. I measured resistence changes when I press between pin 27, 29 and pin 28, 30. If I measure at 28 or 30 against ground I see voltage from H to L, but when I measure at pin 27 or 29 I always see 0V.

I don´t know if there is some way to measure output or if these values measured are OK, or the chip is failing. I read the datasheet but it´s a little bit confusing for me. Any help would be good. Thanks.
 

Ian Rogers

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Two things that happen to me...

First! When I mounted the touch screen it worked really good.. But!! in the morning when cold it looked like it was failing... By 12 (ish) it was fine again.... I found the bezel was pressing on the touch panel... I hat to allow a clearance..

Second! The port pin of my Pic32 kept on going low!! No idea to this day why.. I reprogram and its fine again... I have checked all errata's, but its still an ongoing issue... At first I thought that capacitance was locking up the gate latch... It'll come to light one day..
 

2PAC Mafia

Member
Thanks. In my case it was working too but now is not working again, even if I wait time. My question is, are these values I´m measuring correct?

I attach the datasheet, at page 14 starts touch screen section and at page 19 there is a table. I followed 4 of the 8 lines from touch screen directly to the chip, lines 27, 28, 29 and 30. Now I would like to know what should I expect on those pins when I touch the panel.

As I understand, I should get some voltage value on pin 27 or 29 when I touch (position measurement), page 14.
 

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Ian Rogers

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I write my own touch ( resistive ) driver... Two analogue pins and two digital pins are used..
upload_2017-6-28_12-12-17.png

If A = 3v3 and C = 0v then B will see 0~3v3v. ( Called Xval )
If B = 3v3 and D = 0v then C will see 0~3v3v. ( Called Yval )


BUT!! The read sequence is usually only initiated when the Zval is above a certain point.

So when reading the live points you will need a scope...

A will be 3.3v D will be 0 v and the two analogue pins C and B give the pressure ( Zval )
This all happens quickly as C and B are tri stated several times during the reading process..
 

2PAC Mafia

Member
Thanks, finally I discovered the problem comes when power supply is off. I was in WinCE system and made recalibration of touch screen, then everything worked again, I also replaced the UCB1300 chip. When I thought everything was solved I saw when I switched off and on again I had same problems, so same behavieur connecting a mouse on the system to recalibrate again the touch screen. It looses the calibration of position...
 
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