Electronic – TFT Resistive screen made multitouch


Can a TFT Resistive screen be made/used as a multitouch screen?

I have a Android 2.2 SuperPad III tablet which comes with a resistive screen and I am developing an app that needs a few features of a multitouch screen. When testing the tablet for multitouch it fails: it doesn't register 2 different points simultaneously. I know as a fact that Android OS has support for multitouch since I have performed the same tests on a capacitive screen and all tests were successful.

Would it be possible that Android OS disables the multitouch functionality on an resistive screen tablet or is it a hardware limitation? If needed, I don't mind modifying the Android kernel to reflect my needs however I would like to know that the hardware can handle it.

Best Answer

It's a hardware limitation. Resistive screens measure the resistance from the top and the left, and bottom and right of the pressed point, and derive X and Y coordinates from these.

Image portraying physical touchscreen and how the resistances are created

Pressing two points simultaneously can't give you multiple coordinates. It can confuse the controller, however, resulting in a complete irrelevant set of coordinates.
No matter what technology, to get multiple sets of coordinates you have to scan the grid, looking at one point at a time, which resistive doesn't.

The application schematic for the MXB7843 touchscreen controller shows the electrical model of the touchscreen as the 4 resistors on the left.

example resistive screen reader application schematic

When you are looking at the image you can see that it only has physical inputs for one point so asking for two is just not there.