Application Note AC279
August 2015 1
© 2015 Microsemi Corporation
Interfacing an Microsemi Fusion Programmable
System Chip to a Four-Wire Resistive Touchscreen
Table of Contents
The Microsemi Fusion
®
field programmable gate array (FPGA) is well suited for controlling liquid crystal
displays and interfacing to resistive touchscreen panels. As the world's first mixed signal FPGA, Fusion
seamlessly integrates digital FPGA logic, Flash memory, a multi-channel 12-bit successive
approximation register (SAR) analog to digital converter (ADC), and on-chip oscillator and power
management circuits.
Touchscreen Basics
As illustrated in Figure 1, a four-wire resistive touchscreen panel consists of two flexible layers uniformly
coated with a transparent resistive material and separated by an air gap. Electrodes placed along the
edges of the layers provide a means for exciting and monitoring the touchscreen.
When the screen is touched, the layers make contact. If a reference voltage is applied across the
electrodes in the X layer, a resulting voltage proportional to the horizontal position is measured on either
Y electrode. Likewise, if the reference voltage is applied across the Y layer, then a voltage proportional to
the vertical position can be measured on the X+ electrode. If the voltage applied across the layer is equal
to the full scale reference voltage of an ADC, then position is directly indicated by the resulting ADC
value.
Touchscreen Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Fusion Based Touchscreen Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
List of Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 1 • Structure of a Simple Four-Wire Resistive Touchscreen
Y
Y
X
X
Interfacing an Microsemi Fusion Programmable System Chip to a Four-Wire Resistive Touchscreen
2
It is also possible to estimate the touch pressure (contact area) by measuring the contact resistance
between the X and Y electrodes. This is accomplished by first measuring the X and Y positions and then
applying the reference voltage across the X– and Y+ electrodes and measuring the resulting voltage at
the Y– electrode (Z). Knowing these three values, the touch pressure is estimated using EQ , where Rx
and Ry are the total plate resistance and N is the number of ADC bits used in the analog to digital
conversion.
EQ 1
Fusion Based Touchscreen Interface
Implementing the four-wire touchscreen interface using a Fusion device requires the use of one Analog
Quad (three voltage inputs), an external 3.3 V reference/excitation voltage, four general purpose output
pads from the FPGA logic, and four external MOSFET transistors. These are connected as shown in
Figure 2.
R
touch
R
x
X
pos
Z
------------------ -
R
x
X
pos
Z
N
------------------ -
– R
y
–
R
y
Y
pos
2
N
------------------ -
+=
Figure 2 • Fusion Connections to a Four-Wire Touchscreen
V
REF
= 3.3V
X+
Y+
Y-
X-
XP O
YP O
YN O
XN O
Fusion
AV
AC
AT
Touch