TSC2102IDA datasheet(14/54 Pages) TI

14 / 54 page

TSC2102

SLAS379A− APRIL 2003 − REVISED JUNE 2004

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Measure X-Position

Y+

Y−

X-Position

X−

X+

Touch

Measure Z1-Position

Touch

Y+

Y−

X−

X+

Z1-Position

Y+

Y−

X−

X+

Touch

Z2-Position

Measure Z2-Position

Figure 11. 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 (decays) to a stable dc value. This is due to mechanical bouncing which is 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 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.

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

screen, i.e. 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 causes an additional settling time requirement when the panel is touched.

Several solutions to this problem are available in the TSC2102. A programmable delay time is available which sets the

delay between turning the drivers on and making a conversion. This is referred to as the panel voltage stabilization time,

and is used in some of the modes available in the TSC2102. In other modes, the TSC2102 can be commanded to turn on

the drivers only without performing a conversion. Time can then be allowed before the command is issued to perform a

conversion.

The TSC2102 touch screen interface can measure position (X, Y) and pressure (Z). Determination of these coordinates

is possible under three different modes of the A/D converter: (1) conversion controlled by the TSC2102, initiated by

detection of a touch; (2) conversion controlled by the TSC2102, initiated by the host responding to the PINTDAV signal;

or (3) conversion completely controlled by the host processor.

Touch-Screen A/D Converter

The analog inputs of the TSC2102 are shown in Figure 12. The analog inputs (X, Y, and Z touch panel coordinates, battery

voltage monitors, chip temperature and auxiliary input) are provided via a multiplexer to the successive approximation

register (SAR) analog-to-digital (A/D) converter. The A/D architecture is based on a capacitive redistribution architecture,

which inherently includes a sample/hold function.

A unique configuration of low on-resistance switches allows an unselected A/D input channel to provide power and an

accompanying pin to provide ground for driving the touch panel. By maintaining a differential input to the converter and a

differential reference input architecture, it is possible to negate errors caused by the driver switch on-resistances.

The A/D is controlled by an A/D converter control register. Several modes of operation are possible, depending upon the

bits set in the control register. Channel selection, scan operation, averaging, resolution, and conversion rate may all be

programmed through this register. These modes are outlined in the sections below for each type of analog input. The results

of conversions made are stored in the appropriate result register.

TSC2102IDA Datasheet(PDF) 14 Page - Texas Instruments

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