R

TOUCH

+

R

X−plate

@ XPosition

4096

4096

Z

1

*1 *RY−plate @ 1*

Y

Position

4096

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

-

TSC2017

SBAS472 – DECEMBER 2009

www.ti.com

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

Y-position, and Z1. Equation 2 also calculates the touch resistance:

(2)

Figure 21. 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 often overshoots and then slowly settles down (decays) 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.

To solve this problem, the TSC2017 can be commanded to turn on the drivers only, without performing a

conversion. Time can then be allowed to perform a conversion before the command is issued.

The TSC2017 touch screen interface can measure position (X,Y) and pressure (Z).

14

Submit Documentation Feedback

Copyright © 2009, Texas Instruments Incorporated

Product Folder Link(s): TSC2017