Datasheet

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The 4-Wire Touch Screen Coordinate Pair Measurement
Conductive Bar
Transparent Conductor (ITO)
Bottom Side
X+
X–
Y+
Y–
Transparent Conductor (ITO)
Top Side
Insulating Material (Glass)
ITO= Indium Tin Oxide
Silver Ink
R
TOUCH
 R
X–plate
X–position
4096

Z
2
Z
1
 1

(1)
TSC2301
SLAS371D – SEPTEMBER 2002 – REVISED AUGUST 2004
DETAILED DESCRIPTION (continued)
The TSC2301 supports the resistive 4-wire configuration (see Figure 44 ). 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 constructed as shown in Figure 45 . It consists of two transparent resistive layers
separated by insulating spacers.
Figure 45. 4-Wire Touch Screen Construction
The 4-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 where the panel is touched. A measurement of the Y
position of the pointing device is made by connecting the X+ input to the ADC input, driving Y+ to +VDD and Y-
to GND using switches internal to the TSC2301, 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.
Measuring touch pressure (Z) can also be done with the TSC2301. To determine pen or finger touch, the
pressure of the touch needs to be determined. Generally, it is not necessary to have very high performance for
this test, therefore, the 8-bit resolution mode is recommended (however, calculations are shown with the 12-bit
resolution mode). There are several different ways of performing this measurement. The TSC2301 supports two
methods. The first method requires knowing the X-plate resistance, measurement of the X-position, and two
additional cross panel measurements (Z2 and Z1) of the touch screen (see Figure 46 ). Using Equation 1
calculates the touch resistance:
The second method requires knowing both the X-plate and Y-plate resistance, measurement of X-position and
Y-position, and Z
1
. Using Equation 2 also calculates the touch resistance:
18