A20 User Manual (Revision 1.2) Copyright © 2013 Allwinner Technology Co., Ltd. All Rights Reserved. Page 203 / 812
coordinate measurements, with the X and Y plates interchanged. In Single Touch mode, only need to
test X+, Y+ signal. But In Dual Touch mode, it need to test X+, X-,Y+,Y- signal.
Single Touch X-Position Measurement
DUAL TOUCH DETECTION
The principle of operation is illustrated below, For an X coordinate measurement, the X+ pin is
internally switched to 3V and X- to GND. The X plate becomes a potential divider, and the voltage at
the point of contact is proportional to its X coordinate. This voltage is measured on the Y+ and Y-,
which carry no current (hence there is no voltage drop in R
Y
+ or R
Y
-). Due to the ratiometric
measurement method, the supply voltage does not affect measurement accuracy. The voltage
references VREF+ and VREF- are taken from after the matrix switches, so that any voltage drop in
these switches has no effect on the ADC measurement. the controller will need to test X+,X-,Y+,Y- ,
and record ΔX=|X+ - X-|, ΔY= | Y+ - Y-|. In practice, we can set a threshold. If ΔX or ΔY greater than
the threshold, we consider it as a dual touch, otherwise as a single touch.
Dual Touch X-Position Measurements
TOUCH-PRESSURE MEASUREMENT
The pressure applied to the touch screen by a pen or finger to filter unavailable can also be
measured by the controller using some simple calculations. The contact resistance between the X
and Y plates is measured, provide a good indication of the size of the depressed area and the applied
pressure. The area of the touch spot t is proportional to the size of the object touching it. And the
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