ACT5880
Rev 2, 03-Sep-13
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.
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:
Figure 11:
The Simplified Circuits for Touch Screen
Measuring.
YPOS:
2.5V
YPOS:
ADC
XPOS:
ADC
XNEG: XPOS:
Hi-Z 2. 5V
XNEG:
GND
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.
YNEG
:
GND
YNEG
:
Hi-Z
Channel :0 Y-coordinate
Channel :1 X-coordinate
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.
YPOS:
2. 5V
YPOS :
2.5V
XPOS:
ADC
XPOS:
Hi-Z
XNEG:
GND
XNEG:
GND
YNEG
:
Hi-Z
YNEG
:
ADC
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.
Channel :2 Z2-coordinate
Channel :3 Z1-coordinate
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.
Figure 12:
The 2 Fingers Touch Measuring.
YPOS:
2.5V
YPOS:
2.5V
XPOS :
ADC
XPOS:
GND
XNEG:
GND
XNEG:
GND
YNEG:
GND
YNEG:
ADC
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.
Y touch spot span evaluation , X touch spot span evaluation ,
the YNEG grounded during
Channel 3 measurement.
the YNEG grounded during
Channel 2 measurement .
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
REGISTER AND BIT DESCRIPTIONS section for
more details.
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
www.active-semi.com
Copyright © 2013 Active-Semi, Inc.
Active-Semi Confidential―Do Not Copy or Distribute
ActivePMUTM and ActivePathTM are trademarks of Active-Semi.
I2CTM is a trademark of NXP.
- 74 -
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
