TSC2117 datasheet(67/192 Pages) TI

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5.7.1

The Four-Wire Resistive Touch Screen

TransparentConductor(ITO)

TopSide

TransparentConductor(ITO)

BottomSide

ConductiveBar

SilverInk

InsulatingMaterial

(Glass)

Y+

X+

Y–

X–

ITO=Indium TinOxide

M0068-01

TSC2117

Low-Power Audio Codec With Embedded miniDSP, Stereo Class-D

Speaker Amplifier, and Smart Four-Wire Touch-Screen Controller

www.ti.com

SLAS550A – APRIL 2009 – REVISED JUNE 2009

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

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

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

The TSC2117 supports the resistive four-wire configurations (see Figure 5-37). The circuit determines

location in two coordinate-pair dimensions.

A four-wire touch screen is constructed as shown in Figure 5-37. It consists of two transparent resistive

layers separated by insulating spacers.

Figure 5-37. Four-Wire Touch-Screen Construction

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

network. The ADC converts the voltage measured at the point the screen is touched. A measurement of

the Y position of the pointing device is made by connecting the X+ input to an ADC, turning on the 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 due to the high input impedance of the ADC.

Voltage is then applied to the other axis, and the ADC converts the voltage representing the X position on

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

When the touch screen is pressed or touched and the drivers to the panel are turned on, the voltage

across the touch screen often overshoots and then slowly settles (decays) down to a stable dc value. This

is due to mechanical bouncing which is caused by vibration of the top layer sheet of the touch screen

when it is pressed. This settling time must be accounted for, or else the converted value will be in error.

Therefore, a delay must be introduced between the time the driver for a particular measurement is turned

on and the time measurement is made.

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