alan.b.pearce@stfc.ac.uk wrote:
> Has anyone used resistive touch screens with the ADC on a PIC, i.e. witho=
ut using external chips such as the AR1000 series devices?
>  =20
Yes, we had to drive a resistive touchscreen as part of a uni project.

The screen we were given had two layers at right angles, one layer had=20
contacts at left and right the other at top and bottom.

The pics can both drive digital levels and read analog voltages on the=20
same pins. I used this ability to bias the screen in different ways and=20
by using different combinations of biasing and readout locations I was=20
able to first detect a touch and then determine the position of the=20
touch. IIRC it went something like.

Touch detection:
Drive 5V on top and bottom contacts, 0V on left contact and float on=20
right contact. Read voltage from right contact
Drive 5V on top and bottom contacts, 0V on right contact and float on=20
left contact. Read voltage from left contact
If either ADC reading is substantially above 0V then a touch has been=20
detected, move on to position finding.

Touch position finding
Drive 0V on top contact, 5V on bottom contact and float on left and=20
right contacts. Read voltage from left contact
Drive 0V on left contact, 5V on right conect and float top and bottom=20
conects. Read voltage from top contact
ADC readings give coordinates, some calibration is likely to be needed=20
for a good mapping to the display underneath.

It worked though iirc the response may have been a little slow (though=20
that may have been I didn't have the interrupt routine happening=20
frequently enough)
--=20
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