Resistive Touch Screen for xMega or ARM

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The boards I made in 2013 for resistive touch screens didn't work very well, barely got by, because the xMega couldn't measure higher than VCC/1.6, so "full scale" was just over 2/3 of the way.

 

Now, it's time to do it again, perhaps for SAM parts this time, haven't decided for sure. Since the Reference voltage can't be higher than VCC-0.6v, I thought I'd use a diode to generate a reference .7v below VCC. Of course, the reference input to the processor is pretty high impedance... Oh, use the touch panel itself as a pull down!

 

What do you guys think of this?

 

 

To measure "Y," you have to pull B and E low, set A high. Now the reference, as well as the top of the screen are 1 diode drop below AVCC, and you can measure C or F.

 

To measure "X," you pull A and F low, set B high. Now the reference, as well as the side of the screen are 1 diode drop below AVCC and you can measure D or E.

 

To turn the whole works off and not draw any current, set A and B both high.

 

Any better ideas?

 

Any idea where I can find 2.8 inch "Transflective" TFT displays in quantities of 1 to 10? 

 

Anybody know, if you want it cooler, do you turn the air conditioner "up" or "down"?

 

Oh. Yes. Probably don't need pins C and D.

 

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

Last Edited: Sat. Aug 1, 2015 - 09:32 PM
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If Reference voltage is too much important to you, then maybe a shunt regulator diode for example TL431 would be better.

Torby wrote:

The boards I made in 2013 for resistive touch screens didn't work very well, barely got by, because the xMega couldn't measure higher than VCC/1.6, so "full scale" was just over 2/3 of the way.

Why didn't you use a Resistive Touch Controller?

 

P.S. Yor script is pretty nice and beautiful!smiley

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!

Last Edited: Sun. Aug 2, 2015 - 11:56 AM
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Perhaps only because I haven't successfully soldered one of those leadless packs.

I work on my script.

 

 

 

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

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Ok,

I have a resistive touch screen + TFT LCD 2" and I want to use it.

What part number do you want to use for MOSFET?

I was rummaging whole of net I saw this:

 

 

source

IMO that's good circuit.

What's your opinion?

 

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!

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I'd have to see the text of the web page where you found the diagram.

 

For the MOSFETS in my drawing, I have a bunch of SOT-23 size PFETs on hand. Let's see....

 

DMP32D4S-13

 

Very convenient to have around.

 

Resistive panels are quite easy to use in a processor where you can measure voltages near the VCC and GND. To find the Y position, disable the two Y outputs, put the X top high, and the x bottom low and measure with your ADC either of the Y pins. To get the X position, do the opposite. However, any voltage above the ADC reference reads as "full scale." Since the biggest internal reference of the xMega is VCC/1.6, like a third of the screen each way read as full scale. If you supply your own reference, it can't be higher than VCC-0.6v, so I was thinking of using a diode drop to provide the voltage for the panel and the reference.

 

I like those touch screen controller chips you pointed out above. I might try the STMPE811. I asked 'cause I thought some freak might have a better idea. Looks like you did wink

 

For soldering those leadless things, I think I want to A. Extend the solder pads a little ways out from the chip so I can see them. B. Don't put any traces under the chip.

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

Last Edited: Wed. Aug 5, 2015 - 01:22 PM
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Tom,

 

Amin has included the link to the webpage... it is below the diagram's Figure 2 wording and worded itself as "source".

 

Cheers,

 

Ross

 

Ross McKenzie ValuSoft Melbourne Australia

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Oh. I see

 

They're just providing BJT transistors to drive the touch panel, in case your micro doesn't have enough drive for it, or perhaps you can't turn the output drivers off. They seem to be 660 ohms one way and 330 the other. Anyhow, it wouldn't solve the problem of having to apply something at or below the max reference voltage of your ADC. For the high side, I just used a pfet. For the low, I just used an output pin. If you're using AVR, I don't think I'd buy it.

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

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Good luck Tomwink

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!