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Z
2
–Position
Touch
X+
X–
Y+
Y–
X-Position
Touch
Measure X-Position
Measure Z
1
-Position
X+
X–
Z-Position
Touch
Y+
Y–
X+
X–
Y+
Y–
Measure Z
2
-Position
TSC2302
SLAS394–JULY 2003
Figure 41. 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 (decay) down 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 is 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 TSC2302. 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 TSC2302. In other modes, the TSC2302 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 TSC2302 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: conversion controlled by the TSC2302,
initiated by detection of a touch; conversion controlled by the TSC2302, initiated by the host responding to the
PENIRQ signal; or conversion completely controlled by the host processor.
A/D CONVERTER
The analog inputs of the TSC2302 are shown in Figure 42. The analog inputs (X, Y, and Z touch panel
coordinates, battery voltage monitors, chip temperature, and auxiliary inputs) are provided via a multiplexer to the
successive approximation register (SAR) analog-to-digital converter (ADC). The A/D architecture is based on
capacitive redistribution architecture, which inherently includes a sample/hold function.
A unique configuration of low on-resistance switches allows an unselected ADC 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.
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