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TOUCH SCREEN OPERATION

4-WIRE TOUCH SCREEN COORDINATE PAIR MEASUREMENT

ConductiveBar

InsulatingMaterial(Glass)

Silver

Ink

TransparentConductor(ITO)

BottomSide

Transparent

Conductor(ITO)

TopSide

X+

X-

Y+

Y-

ITO=IndiumTinOxide

R

TOUCH

X

Postition

4096

Z

2

Z

1

* 1

(1)

TSC2007

SBAS405 – MARCH 2007

OVERVIEW (continued)

A resistive touch screen operates by applying a voltage across a resistor network and measuring the change in

resistance at a given point on the matrix where the screen is touched by an input (stylus, pen, or finger). The

change in the resistance ratio marks the location on the touch screen.

The TSC2007 supports the resistive 4-wire configurations, as shown in Figure 20. The circuit determines

location in two coordinate pair dimensions, although a third dimension can be added for measuring pressure.

A 4-wire touch screen is typically constructed as shown in Figure 20. It consists of two transparent resistive

layers separated by insulating spacers.

Figure 20. 4-Wire Touch Screen Construction

The 4-wire touch screen panel works by applying a voltage across the vertical or horizontal resistive network.

The A/D converter converts the voltage measured at the point where the panel is touched. A measurement of

the Y position of the pointing device is made by connecting the X+ input to a data converter chip, turning on the

Y+ and Y– drivers, and digitizing the voltage seen at the X+ input. The voltage measured is determined by the

voltage divider developed at the point of touch. For this measurement, the horizontal panel resistance in the X+

lead does not affect the conversion because of the high input impedance of the A/D converter.

Voltage is then applied to the other axis, and the A/D converter converts the voltage representing the X position

on the screen. This process provides the X and Y coordinates to the associated processor.

Measuring touch pressure (Z) can also be done with the TSC2007. To determine pen or finger touch, the

pressure of the touch must be determined. Generally, it is not necessary to have very high performance for this

test; therefore, 8-bit resolution mode may be sufficient (however, data sheet calculations will be shown with the

12-bit resolution mode). There are several different ways of performing this measurement. The TSC2007

supports two methods. The first method requires knowing the X-plate resistance, the measurement of the

Equation 1 calculates the touch resistance:

13

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