
TSC2111
SLAS495A JUNE 2006 REVISED OCTOBER 2007
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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 TSC2111. 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 TSC2111. In other modes, the TSC2111
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 TSC2111 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 TSC2111,
initiated by detection of a touch; (2) conversion controlled by the TSC2111, 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 TSC2111 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.
The ADC is controlled by an ADC 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.