Before you read on further, read the working of 4 wire resistive touch screen. This post is for the newbies; technicalities will be addressed later.
It is always better to use a dedicated controller for touch screens, instead of designing your own. The best option according to me is to use one of the product of MAXIM, MXB7843.
MXB7843 is controller dedicated for 4 wire resistive touch screens. It has a configurable ADC of 8/12 bit resolution. Overall pictorial representation says that to interface a 4 wire resistive touch screen, what you have to have a microcontroller apart from touch screen controller and a touch screen itself. Touch Screen is directly connected to the microcontroller. The microcontroller which is controlling all the affairs gets the coordinates of the touch screen, whenever it is interrupted, via SPI communication. SPI communication is used for high speed synchronous communication. Once the data is received in 8 or 12 bit format, you can play around as you desired.
The schematic of MXB7843 is given below:
More to follow!