4-wire resistive Touch screen
A four-wire resistive touch screen panel consists of two flexible layers uniformly coated with a transparent resistive material and separated by an air gap. Electrodes placed along the edges of the layers provide a means for exciting and monitoring the touch screen.
Block Diagram of Touch Screen Interface
Touch screen Controller/Digitizer
When a position is measured on a 4-wire touch screen, voltage is applied across the screen in the Y direction; and a touch presses the layers together, where a voltage can be read from one of the X electrodes. The contact made as a result of the touch creates a voltage divider at that point, so the Y coordinate can be determined; the process then repeats with the X direction being driven, and a reading is taken from one of the Y electrodes. A touch-screen controller is simply an ADC that has built-in switches to control which electrodes are driven and which electrodes are used as the input to the ADC.
An Analog Devices AD7843 scans the X and Y axes and determines the unique voltage drop for each axis. The four electrodes for scanning are labeled X+, X-, Y+, and Y-. These electrodes are connected to the AD7843 touch screen controller and the touch sensor is scanned and the analog voltages read.
The four touch electrodes are connected to the inputs X+, X-, Y+, and Y- of the AD7843. A selected axis (X or Y) pair of electrodes is energized with a static voltage and the voltage of the positive electrode of the other pair in the 4 wire touch panel is measured. The sensed voltage is measured and converted to either an 8 bit or 12 bit resolution. A digital word representing the voltage at the contacting point on the touch panel is created and sent out via a high speed SPI serial interface.
Graphic LCD with Touch Screen
These GLCD have common display drivers like KS0108 and T6963C and 4 wire resistive touch screen .
There is no need for touch screen digitizer/controller for micro controllers having on chip ADC with four analog channels. Just connect the four wire of touch screen to analog inputs and read the respective digital data for X and Y direction of touched point.
