[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:
⎛
⎞
XPosition Z2
⎜
4096 Z1
⎝
⎜
⎟
⎟
RTOUCH = RX-plate
⋅
−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:
RX-plate ⋅XPosition
⎛
⎞
4096
Z1
YPosition
4096
⎛
⎝
⎞
⎟
⎜
⎜
⎟
−1 − RY-plate ⋅ 1-
⎜
RTOUCH
=
⎟
4096
⎠
⎝
⎠
VDD
ON
VDD
ON
YP
XP
YP
XP
touch
touch
VREF+
VREF-
AIN+
AIN-
VREF+
VREF-
AIN+
AIN-
ADC
ADC
XN
YN
XN
YN
ON
ON
b) Z2-Position Measurement Differential Mode
a) Z1-Position Measurement Differential Mode
Figure 5. Pen Pressure Measurements
MS1068-E-04
2011/03
- 10 -
AKM [ ASAHI KASEI MICROSYSTEMS ]
