AK4184 Datasheet, PDF (11/36 Pages) Asahi Kasei Microsystems – TSC with Keypad Scanner and GPIO Expander

[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
YP
AVDD
YP-Driver SW ON
YP
XP
VREF+
AIN+
ADC
VREF-
AIN-
XN
XN-Driver SW ON
YN
Rtouch
XP
VREF+
AIN+
ADC
VREF-
AIN-
XN
XN-Driver SW ON
YN
a) Z1-Position Measurement
b) Z2-Position Measurement
Figure 4. Pen Pressure Measurements
Rtouch
MS0603-E-00
11
2007/04