AN1368 datasheet(10/34 Pages) MICROCHIP

10 / 34 page

AN1368

DS01368A-page 10

 2011 Microchip Technology Inc.

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

measuring the analog y-voltage, as shown in Figure 12.

FIGURE 10:

4-WIRE RESISTIVE TOUCH SCREEN

PIC® MCU

Digital I/O

with ADC

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