Datasheet

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TOUCH SCREEN OPERATION
4-WIRE TOUCH SCREEN COORDINATE PAIR MEASUREMENT
ConductiveBar
InsulatingMaterial(Glass)
Silver
Ink
TransparentConductor(ITO)
BottomSide
Transparent
Conductor(ITO)
TopSide
X+
X-
Y+
Y-
ITO=IndiumTinOxide
R
TOUCH
+ R
X−plate
@
X
Postition
4096
ǒ
Z
2
Z
1
* 1
Ǔ
(1)
R
TOUCH
+
R
X−plate
@ X
Postition
4096
ǒ
4096
Z
1
*1
Ǔ
*R
Y−plate
@
ǒ
1*
Y
Position
4096
Ǔ
(2)
TSC2005
SBAS379C – DECEMBER 2006 – REVISED MARCH 2008
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 TSC2005 supports the resistive 4-wire configurations, as shown in Figure 21 . 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 21 . It consists of two transparent resistive
layers separated by insulating spacers.
Figure 21. 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 TSC2005. 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, 10-bit resolution mode is recommended (however, data sheet calculations are shown using 12-bit
resolution mode). There are several different ways of performing this measurement. The TSC2005 supports two
methods. The first method requires knowing the X-plate resistance, the measurement of the X-Position, and two
additional cross panel measurements (Z
2
and Z
1
) of the touch screen (see Figure 22 ). Equation 1 calculates the
touch resistance:
The second method requires knowing both the X-plate and Y-plate resistance, measurement of the X-Position
and the Y-Position, and Z
1
. Equation 2 also calculates the touch resistance:
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