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TSC2101
SLAS392D JUNE 2003 REVISED MAY 2005
www.ti.com
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Measuring touch pressure (Z) can also be done with the TSC2101. To determine pen or finger touch, the
pressure of the touch needs to be determined. Generally, it is not necessary to have very high performance
for this test; therefore, the 8-bit resolution mode is recommended (however, calculations are shown with the
12-bit resolution mode). There are several different ways of performing this measurement. The TSC2101
supports two methods. The first method requires knowing the X-plate resistance, measurement of the
X-Position, and two additional cross panel measurements (Z 2 and Z1) of the touch screen (see Figure 26). Using Equation
(1) calculates the touch resistance:
RTOUCH + RX–plate
X–position
4096
Z2
Z1
–1
The second method requires knowing both the X-plate and Y-plate resistance, measurement of X-Position and
Y-Position, and Z1. Using Equation (2) also calculates the touch resistance: RTOUCH +
RXplate
Xposition
4096
Z1
* 1
* RYplate
1*
Yposition
4096
i
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 26. 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) 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 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 TSC2101. 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 TSC2101. In other modes, the TSC2101
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 TSC2101 touch screen interface can measure position (X, Y) and pressure (Z). Determination of these
coordinates is possible under three different modes of the ADC: (1) conversion controlled by the TSC2101,
initiated by detection of a touch; (2) conversion controlled by the TSC2101, initiated by the host responding to
the PINTDAV signal; or (3) conversion completely controlled by the host processor.
Touch Screen ADC Converter
The analog inputs of the TSC2101 are shown in
Figure 27. 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 ADC 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.
(1)
(2)