Datasheet

719
32142D–06/2013
ATUC64/128/256L3/4U
29.7.1 Resistive Touch Screen Principles
A resistive touch screen is based on two resistive films, each one fitted with a pair of electrodes,
placed at the top and bottom on one film, and on the right and left on the other. Between the two,
there is a layer that acts as an insulator, but makes a connection when pressure is applied to the
screen. This is illustrated in Figure 29-2 on page 719.
Figure 29-2. Resistive Touch Screen Position Measurement
29.7.2 Position Measurement Method
As shown in Figure 29-2 on page 719, to detect the position of a contact, voltage is first applied
to X
P
(top) and X
m
(bottom) leaving Y
p
and Y
m
tristated. Due to the linear resistance of the film,
there is a voltage gradient from top to bottom on the first film. When a contact is performed on
the screen, the voltage at the contact point propagates to the second film. If the input impedance
on the Y
P
(right) and Y
m
(left) electrodes are high enough, no current will flow, allowing the volt-
age at the contact point to be measured at Y
p
. The value measured represents the vertical
position component of the contact point.
For the horizontal direction, the same method is used, but by applying voltage from Y
P
(right) to
Y
m
(left) and measuring at X
P
.
In an ideal world (linear, with no loss), the vertical position is equal to:
VY
P
/ VDD
To compensate for some of the real world imperfections, VX
P
and VX
m
can be measured and
used to improve accuracy at the cost of two more conversions per axes. The new expression for
the vertical position then becomes:
(VY
P
- VX
M
) / (VX
P
- VX
M
)
X
M
X
P
Y
M
Y
P
X
P
X
M
Y
P
VDD
GND
Volt
Horizontal Position Detection
Y
P
Y
M
X
P
VDD
GND
Volt
Vertical Position Detection
Pen
Contact