Were you able to calibrate it to match the PIXEL resolution of the LCD
screen beneath?

Can I expect to get that much resolution? I guess it depends on how well the
ADC is connected in terms of voltage drop, input impedance, voltage range,
number of bits on the ADC.

I thought about lighting a cross hair on the LCD and asking the user to
touch the intersection, do this in top left and bottom right and then divide
the voltage differential by the resolution of the pixels between the cross
hairs. This would only need to be done once per unit at manufacture I think.

I intend to use the ADC of a 16F877 PIC, or maybe just have a separate LCD
and touch screen controller chip using 16F84.

I'm also going to use an inverted LED backlit transflective LCD display to
get better contrast of the graphics and text.

Pete



> -----Original Message-----
> From: EXT Andrew Errington [mailto:ame@SYNAPTICS.COM]
> Sent: Wednesday, July 05, 2000 9:39 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Touch Screens
>
>
> Hi,
>
> > Anybody used a touch screen before over an LCD display? Got
> any interface
> > PIC code?
>
> I have done this too.  If you are using a 4-wire touchscreen
> it does in fact
> operate as you inferred.
>
> I was using a custom micro, with a 10-bit ADC (sort of- it
> was a DAC and a
> comparator on-chip).  As you say, you have to set up a
> voltage gradient on
> one side of the resistive pad and read the voltage off one of the
> connections on the other side, then swap the voltage gradient
> and readout
> pins.  There are a couple of pages about this in the
> Dragonball application
> notes from Motorola.
>
> A word of warning: some resistive pads have a low surface
> resistance, of the
> order of a few hundred ohms.  This means that the I/O's on
> your micro may
> not be able to sink or source the current needed to set up the voltage
> gradient as it could be 10mA or more, so you might need
> driver transistors.
> Also the input impedance to the ADC must be very high (which
> they typically
> are) otherwise you will get a voltage drop on the pickup
> side, and if your
> ADC does not work rail to rail then you might need four extra
> resistors to
> narrow the voltage range across the pad.
>
> Finally, resistive touch screens don't work very well with
> fingers.  For
> best result you need a stylus, or use your fingernail.
>
> Andy
>
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