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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

-

TSC2005

SBAS379C – DECEMBER 2006 – REVISED MARCH 2008

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 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 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 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 will cause an additional settling time requirement

when the panel is touched.

The TSC2005 offers several solutions to this problem. A programmable delay time is available that sets the delay

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

stabilization time, and is used in some of the TSC2005 modes. In other modes, the TSC2005 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 TSC2005 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:

• TSMode1—conversion controlled by the TSC2005 initiated by TSC;

• TSMode2—conversion controlled by the TSC2005 initiated by the host responding to the PENIRQ signal; or

• TSMode3—conversion completely controlled by the host processor.

12

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