You can't do multitouch very well. You can sorta detect that there are two fingers pressed, but thats about it.
Just as an overview of how 4 wire resistive touch screens work:
You essentially have two big resistors overlaid on one another. The X resistor has its endpoints vertically at each end, and vice versa for the Y. When you touch the screen, you bridge the two resistors at the point you touch, with another resistor, labelled Rt below.
The following is an example with 1 point of contact.
Code: Select all
X+ Y+
| |
\ \
/ Rxp Ryp /
\ \
/ /
| Rt |
+----/\/\/\----+
| |
\ \
/ /
\ Rxm Rym \
/ /
| |
X- Y-
From this you can determine the X/Y and pressure of contact; for example, drive X+ / X- with 5 + 0 respectively, and measure either Y+ or Y- to find the X point of contact, same deal for Y. You can then drive X+ and Y-. and measure Y+ and X- to get the Rt voltage drop, which is inversely proportional to pressure.
Effectively when you add a second touch point, you add a Rt2 in parallel, which decreases the Rt voltage drop below what you can normally get, even when pressing on the touchscreen extremely hard.
So in summary, you can detect multiple touches occuring, but not exactly where.
Cheers,
--David Carne
