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AN1368
TOUCH SCREEN
Some applications require the support of a touch
screen for the display. This is achieved by using a
separate touch screen on the display glass or by
selecting a display module with a touch screen. In both
cases, the touch signals must be handled by either the
microcontroller or a separate touch screen controller
(such as Microchip’s AR1000 series touch screen
controllers). These touch signals are analog and digital
signals which must be decoded to sense the touch
coordinates. Transparent touch screens are usually of
resistive type or capacitive type. Resistive touch
screens are the most commonly used and are
generally available in 4-wire or 5-wire configurations.
The touch point can be detected by measuring the
variation of the resistance of the touch screen. Only a
4-wire touch screen is explained here.
4-WIRE RESISTIVE TOUCH SCREEN
This touch screen has four signals, of which two are
purely digital signals. The other two signals are
alternately configured as both digital and analog signals.
The four signals can be directly connected to the
microcontroller I/O pins with two digital inputs and two
digital outputs, or analog pins. Figure 10 illustrates the
connections for this scheme.
When the user touches the screen, the resistance of
the screen changes. By measuring the resistance in
horizontal and vertical directions, and comparing them
with the calibrated values, the (x, y) coordinates of the
point of touch can be obtained.
When a point on the screen is touched, the x-coordinate
voltage is obtained by applying voltages across the y-
signal and measuring the analog voltage on the x-signal,
as shown in Figure 11 . The y-coordinate voltage is
obtained by applying voltage across x-signals and
FIGURE 10:
4-WIRE RESISTIVE TOUCH SCREEN
Y
PIC ? MCU
with ADC
Digital I/O
Digital I/O
Digital I/O
Digital I/O
DS01368A-page 10
X
Y
X
? 2011 Microchip Technology Inc.
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