AK4186 Datasheet, PDF (10/33 Pages) Asahi Kasei Microsystems – Low Power Touch Screen Controller with I2C Interface

[AK4186]
■ Pen Pressure Measurement (Only 4-wire Touch Screen)
The touch screen pen pressure can be derived from the measurement of the contact resistor between two plates. The
contact resistance depends on the size of the depressed area and the pressure. The area of the spot is proportional to the
contact resistance.
This resistance (Rtouch) can be calculated using two different methods. The first method is that when the total
resistance of the X-plate sheet is already known. The resistance, Rtouch, is calculated from the results of three
conversions, X-position, Z1-position, and Z2-position, and then using following formula:
R _TOUCH
=
R _X-plate
⋅
X _Position
4096
⎜⎜⎝⎛
Z₂
Z₁
−1_{⎟⎟⎠⎞}
The second method is that when both the resistances of the X-plate and Y-plate are known. The resistance, Rtouch, is
calculated from the results of three conversions, X-position, Y-position, and Z1-position, and then using the following
formula:
R _TOUCH
=
R ⋅ X _{X-plate Position}
4096
⎜⎜⎝⎛
4096
Z₁
−1_{⎟⎟⎠⎞} −
R _Y-plate
⋅ ⎜⎛1 -
⎝
Y_Position
4096
⎟⎞
⎠
VDD
ON
YP
VDD
ON
YP
XP
VREF+
AIN+
ADC
VREF-
AIN-
XN
ON
YN
touch
XP
VREF+
AIN+
ADC
VREF-
AIN-
XN
ON
YN
touch
a) Z1-Position Measurement Differential Mode
b) Z2-Position Measurement Differential Mode
Figure 5. Pen Pressure Measurements
MS1068-E-04
- 10 -
2011/03