R

TOUCH

+

R

X−plate

4096

4096

Z

1

Y

Position

4096

(2)

X-Position

MeasureX-Position

MeasureZ -Position

1

Touch

X+

Y+

X

-

Y

-

Z -Position

1

Touch

X+

Y+

Y

-

X

-

MeasureZ -Position

2

Z -Position

2

Touch

X+

Y+

Y

-

X

-

TSC2008

The second method requires knowing both the X-plate and Y-plate resistance, measurement of X-Position and

Figure 22. Pressure Measurement

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

touch panel will often overshoot and then slowly settle down (decay) to a stable dc value. This effect is a result of

mechanical bouncing caused by vibration of the top layer sheet of the touch panel when the panel is pressed.

This settling time must be accounted for, or else the converted value is incorrect. Therefore, a delay must be

introduced between the time the driver for a particular measurement is turned on, and the time a measurement is

made.

In some applications, external capacitors may be required across the touch screen for filtering noise picked up by

the touch screen (for example, noise generated by the LCD panel or back-light circuitry). The value of these

capacitors provides a low-pass filter to reduce the noise, but creates an additional settling time requirement when

the panel is touched. The settling time typically shows up as gain error. The TSC2008 has a built-in noise filter

(see the Preprocessing section). These capacitors can be reduced to minimal value or not installed.

The TSC2008 touch screen interface can measure position (X,Y) and pressure (Z).

Copyright © 2008, Texas Instruments Incorporated

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Product Folder Link(s): TSC2008