National CP3BT26 manual Touchscreen Interface, Touchscreen Driver Configuration

16.2TOUCHSCREEN 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 25.

TSX+/ADC0

16.2.1Touchscreen Driver Configuration

An equivalent circuit for the touchscreen interface is shown in Figure 26.

VCC

CP3BT26

TSY+/ADC1

TSX-/ADC2

TSY-/ADC3

MUXOUT0

ADCIN

DS186

Figure 25. 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

6Ω 6Ω

TSY+

TSX+

To ADC

TSX-

TSY-

6Ω 6Ω

X Plate

RX1

ARZ

RX2

Y Plate

RY1

B

RY2

DS187

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.

Figure 26. 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 26 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 register specifies the configuration of the drivers, with 010b 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 26 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.

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