[AK4184]
■ Pen Pressure Measurement
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 when the total resistance of the X-plate sheet is already known. The resistance, Rtouch, is
calculated from the results of three A/D conversions: X-position, Z1-Position, and Z2-Position, using the following
formula:
Rtouch = (Rxplate) * (Xposition/4096) * [(Z2/Z1) – 1]
The second method is when both the resistances of the X-plate and Y-plate are known. The resistance, Rtouch, is
calculated from the results of three A/D conversions: X-position, Y-Position, and Z1-Position, using the following
formula:
Rtouch = (Rxplate*Xposition/4096)*[(4096/Z1) – 1] – Ryplate*[1 – (Yposition/4096)]
AVDD
YP-Driver SW ON
AVDD
YP-Driver SW ON
YP
XP
YP
XP
Rtouch
Rtouch
VREF+
VREF-
AIN+
AIN-
VREF+
VREF-
AIN+
AIN-
ADC
ADC
XN
YN
XN
YN
XN-Driver SW ON
XN-Driver SW ON
b) Z2-Position Measurement
a) Z1-Position Measurement
Figure 4. Pen Pressure Measurements
MS0603-E-00
2007/04
11
AKM [ ASAHI KASEI MICROSYSTEMS ]
