CP3UB26 datasheet(73/268 Pages) NSC

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15.2

TOUCHSCREEN INTERFACE

The ADC provides an interface for 4-wire resistive touch-

screens with the resolution necessary for applications such

as signature analysis. A typical touchscreen configuration is

shown in Figure 12.

Figure 12.

Touchscreen Interface

A touchscreen consists of two resistive plates normally sep-

arated from each other. The TSX+ and TSX- signals are

connected to opposite ends of the X plate, while the TSY+

and TSY- signals are connected to the Y plate. If the pen is

down, the plates will be shorted together at the point of pen

contact. The location of the pen is sensed by driving one

end of a plate to VCC, driving the opposite end to ground,

and sensing the voltage at the point of pen contact using the

other plate. This is done twice, once for each coordinate.

An external RC low-pass filter is used to remove noise cou-

pled to the touchscreen signals from the display drivers.

15.2.1

Touchscreen Driver Configuration

An equivalent circuit for the touchscreen interface is shown

in Figure 13.

Figure 13.

Touchscreen Driver Equivalent Circuit

Low-ohmic drivers are provided to pull the TSX+ and TSY+

signals to VCC and the TSX- and TSY- signals to GND. The

on-resistance of these drivers is specified to be 6 ohms.

Two measurements are used to produce one (x,y) position

coordinate pair. To measure the x-coordinate, the TSX+ sig-

nal is pulled to VCC, the TSX- signal is pulled to GND, and

the TSY+ and TSY- signals are undriven. A voltage divider

is formed across the X plate, with the center tap of the divid-

er being the point of pen contact, represented in Figure 13

by node A. With TSY+ and TSY- undriven, the voltage at

node A can be measured by sampling either of the TSY+ or

TSY- signals. This voltage will be proportional to the position

of the pen contact on the X plate.

The position of the pen contact on the Y plate is measured

similarly, by driving the TSY+ signal to VCC, the TSY- signal

to GND, and leaving the TSX+ and TSX- signals undriven.

The voltage at node B can be sampled from either the TSX+

or TSX- signals. The TOUCH_CFG field of the ADCGCR

used to sample node A and 001b used to sample node B.

Typically, two consecutive measurements are made of each

coordinate so that any interference coupled from the LCD

column drivers is averaged out.

The plate-to-plate resistance is shown in Figure 13 as RZ.

This measurement is used as an indication of the force of

pen contact. When 100b is loaded into the TOUCH_CFG

field, the TSY+ signal is pulled to VCC and the TSX- signal

is pulled to GND, to support measuring RZ.

DS186

TSX+/ADC0

TSY+/ADC1

TSX-/ADC2

TSY-/ADC3

MUXOUT0

ADCIN

DS187

Ω

RX1

X Plate

TSY

TSX

To ADC

TSY

RX2

VCC

RY1

Y Plate

RY2

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