ACT5880 datasheet(73/87 Pages) ACTIVE-SEMI

73 / 87 page

ACT5880

Rev 2, 03-Sep-13

- 73 -

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TM and ActivePathTM are trademarks of Active-Semi.

I2CTM is a trademark of NXP.

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The ACT5880 has a shared SAR ADC with a state-

machine for deploying actions to do resistive touch

panel measurement as well as normal ADC

conversion. The features and performance of the

circuit are optimized for applications with hand-held

size touch panel and related housekeeping. The

shared ADC core has 12 or 8 bits resolutions,

assures that it fits both the position measurement

and the contact evaluation.

The touch screen measurement circuit employs an

analogue front end and a touch detection circuit

with acquisition management, makes the circuit a

full function TSC (i.e., the touch screen controller).

Refer to Figure 10 for the top level block diagram of

the TSC ADC circuit.

Figure 10:

The TSC ADC Block Diagram.

The analogue front end is designed to provide force

driving bias and signal routes for 4-wire type

resistive touch panel and the auxiliary inputs for

common purpose ADC. Both the touch panel

measurement and the common purpose ADC

conversion are organized to manipulate with

operation to same sets of registers, the state

machine controls scanning to get each signal

converted in a sequential frame or doing a single

channel conversion, as it is programmed. Refer to

Acquisition Configuration

for details. And refer to

Table 7 for the key specifications.

4-wire Touch Screen Interface

The TSC ADC has following internal connections

for touch screen force and sense, as showed in

Table 8.

Table 7:

The Key Feature of the TSC ADC.

Table 8:

The Touch Screen Interface Connections.

Notes:

PEN: The pen touch detection connects to this pin;

2.5V: The 2.5V reference buffer output;

ADC: One of the ADC channel input.

The respective force and sense to touch panel is

shown in Figure 11.

The resistors lay in vertical and horizontal represent

the equivalent resistances of the transparent films

split by the touch spot, the resistors lay oblique is

the equivalent of the films contacting in the touch

spot, the paths in bold are the force current goes

through. Considering the current into the ADC input

is neglecting comparing the force current and the

even gradient slope of potential in the area between

two bias bar, the channel 0 and channel 1 read

exact the electrical equivalent coordinates in the

axes (the electrical equivalent coordinates then are

scaled to graphic user interface coordinates by the

application software). Giving the 3 equivalent

resistors in the force paths for the channel 2 or

channel 3 connections make the Z axis, the channel

TSC ADC

PARAMETERS

Resolution (bits)

12 or 8

Input Impedance

11MΩ

Input Capacitance

32pF

Force Driving Voltage and ADC Reference

2.5V

Maximum Force Driving Current

18mA

AUX0, AUX1, AUX2

2.5V

AUX3(VD)

6.024V

Full Scale Input

Voltage

XPOS

CHANNEL DESCRIPTION

PEN

Standby

ADC

Y-coordinate

2.5V

X-coordinate

ADC

Z2-coordinate

Hi-Z

Z1-coordinate

XNEG

Hi-Z

GND

YPOS

Hi-Z

2.5V

ADC

2.5V

YNEG

GND

Hi-Z

ADC

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