Allwinner A20 - Page 204

A20 User Manual (Revision 1.2) Copyright © 2013 Allwinner Technology Co., Ltd. All Rights Reserved. Page 204 / 812
value of this resistance (R
touch
) can be calculated using two different methods.
First Method
The first method requires the user to know the total resistance of the X plate tablet (R
XPLATE
). Three
touch screen conversions are required: measurement of the X position, X
POSITION
(Y+ input);
measurement of the X+ input with the excitation voltage applied to Y+ and X− (Z1 measurement);
and measurement of the Y− input with the excitation voltage applied to Y+ and X− (Z2 measurement).
These three measurements are illustrated in Figure 12. The controller have two special ADC channel
settings to configure the X and Y switches for the Z1 and Z2 measurements and store the results in
the Z1 and Z2 result registers. The touch resistance (R
TOUCH
) can then be calculated using the
following equation:
R
TOUCH
= (R
XPLATE
) × (X
POSITION
/4096) × [(Z2/Z1) − 1] (1)
X+
X-
Y-
Y+
TOUCH
TOUCH
X+
X-
Y+
Y-
X-POSITION
Z1-POSITION
Z2-POSITION
MEASURE
X-POSITION
MEASURE
Z1-POSITION
X+
X-
MEASURE
Z2-POSITION
Y+
Y-
Pressure Measurement Block Diagram
Second Method
The second method requires the user to know the resistance of the X-plate and Y-plate tablets. Three
touch screen conversions are required: a measurement of the X position (X
POSITION
), the Y position
(Y
POSITION
), and the Z1 position. The following equation also calculates the touch resistance (R
TOUCH
):
R
TOUCH
= R
XPLATE
× (X
POSITION
/4096) × [(4096/Z1) − 1] − R
YPLATE
× [1 − (Y
POSITION
/4096)] (2)
PEN DOWN DETECTION, WITH PROGRAMMABLE SENSITIVITY
Pen down detection is used as an interrupt to the host. R
IRQ
is an internal pull-up resistor with a
programmable value of 6~96 kΩ (default 48kΩ). The pen down IRQ output is pulled high by an
internal pull-up. In the pen down detection, the Y– driver is on and connected to GND, and the pen
down IRQ output is connected to the X+ input. When the panel is touched, the X+ input is pulled to
ground through the touch screen, and the pen down IRQ output goes low because of the current path
through the panel to GND, initiating an interrupt to the processor. During the measurement cycle for
X-, Y-, and Z-position, the X+ input is disconnected from the pen down IRQ pull-down transistor to
eliminate any pull-up resistor leakage current from flowing through the touch screen, thus causing no
errors.