ACT5880 datasheet(74/87 Pages) ACTIVE-SEMI

74 / 87 page

ACT5880

Rev 2, 03-Sep-13

- 74 -

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2 and channel 3 read the 2 coordinate equivalences

Z2 and Z1 on Z axis. These Z2 and Z1 are used for

contact evaluation; Refer to Contact Evaluation for

more details.

Figure 11:

The Simplified Circuits for Touch Screen

Measuring.

Touch Detection

When the function is enabled, the touch detection

circuit measures the contact between two film

layers of the touch screen panel. The detection

circuit only biases the XPOS in standby when no

any conversion in progress and no conversion

result waiting for system to read. When contact

between 2 film layers happens, a touch event is

detected and an interrupt asserts for request

system service. The layer to layer coupling may

cause false touch detection when applying the bias,

the first touch event right after applying should be

ignored.

A wake-up against pen touch detection function is

designed for further power saving in applications

like E-book, in which the main processor may

power off. To enable this function, the PENWKEN

bit needs to be set and the TSC ADC circuit needs

to be powered independently to the operation of the

ACT5880, such as being powered with battery

directly.

Contact Evaluation

The (Z1−Z2) read the resistive divide ratio of the

contact impedance over the whole force path

impedance. As the film resistors in the path are

coordinate

dependent,

the

(Z1−Z2)

are

consequently X coordinate and Y coordinate

dependent. The linear estimation on the film resistor

impedances are used to normalize the (Z1−Z2) to

proportional factor C to one of the two film

impedances. As a sample, giving the effective

dimensions of the touch films is 1:F for X-dimension

to Y-dimension ratio and the X film impedance is 1,

the Y film impedance is then F and the film

thickness is same; Also giving the full scale reading

is FS, the Y film resistor impedance in the path is

(FS−Y)·F, the C is derived as:

C=(Z1−Z2)·[X+(FS−Y)]⁄[FS−(Z1−Z2)].

The volume C represents the tightness of contact in

the touch spot, which is proportional to the touch

pressure and touch area size.

The high resolution conversion, the reference buffer

force driving and the flexible acquisition features

make the ACT5880 suitable for contact evaluation

applications like finger touch and fingers touch. The

finger touch has low contact impedance than the

round tip touch; the C volume can be used to detect

the finger touch.

With two additional I/O pins to pull the Hi-Z pins in

channel 2 and channel 3 connections to ground

separately, the equivalent span changes of the

touch spot on X axis and Y axis can be evaluated,

so forth the 2 fingers wiping shaped touch is

identified. See connections in Figure 12 for

reference.

Figure 12:

The 2 Fingers Touch Measuring.

Internal Reference

The TSC ADC circuit has an internal reference

buffer to generate a 2.5V reference voltage for the

ADC circuit and the force driving in the analogue

front end. This reference is user trim-able for getting

better accuracy is necessary. Refer to the

section for

more details.

YPOS:

2.5V

YNEG

GND

XNEG:

Hi-Z

XPOS:

ADC

Channel :0 Y-coordinate

YPOS:

ADC

YNEG

Hi-Z

XNEG:

GND

XPOS:

2.5V

Channel :1 X-coordinate

YNEG

Hi-Z

XPOS:

ADC

Channel :2 Z2-coordinate

YPOS :

2.5V

YNEG

ADC

XNEG:

GND

XPOS:

Hi-Z

Channel :3 Z1-coordinate

YPOS:

2.5V

XNEG:

GND

YNEG:

GND

XPOS :

ADC

Ytouch spot span evaluation ,

the YNEGgrounded during

Channel 2measurement .

YPOS:

2.5V

YNEG:

ADC

XNEG:

GND

XPOS:

GND

YPOS:

2.5V

XNEG:

GND

Xtouch spot span evaluation ,

theYNEGgrounded during

Channel 3 measurement.

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