Touch Screen Touch Detection

- May 09, 2018 -

All touch screens can detect if a touch has occurred by pulling up one of the layers with a weak pull-up resistor and pulling down the other with a strong pull-down resistor. If the measured voltage of the pull-up layer is greater than a certain logic threshold, there is no touch, and if not, there is a touch. The problem with this approach is that the touch screen is a huge capacitor, and in addition it may be necessary to increase the capacitance of the touch screen leads in order to filter out the noise introduced by the LCD. A weak pull-up resistor connected to a large capacitor will increase the rise time and may result in the detection of a false touch.

Four-wire and eight-wire touch screens can measure the contact resistance, RTOUCH in Figure 5. RTOUCH is approximately proportional to touch pressure. To measure touch pressure, you need to know the resistance of one or two layers in the touch screen. The formula in Figure 6 gives the calculation method. It should be noted that if the measured value of Z1 is close to or equal to 0 (when the touch point is close to the grounded X bus in the measurement process), some problems will occur in the calculation, and this problem can be effectively improved by adopting a weak pull-up method.