New Touch-Screen Controllers Offer Robust

New Touch-Screen Controllers
Offer Robust Sensing for
Portable Displays
a resistor divider along the X-plate. The voltage at the point of
contact, which represents the position on the X-plate, is sensed
through the Y+ electrode, as shown in Figure 2. The process is
then repeated by exciting the Y-plate and sensing the Y position
through the X+ electrode.
By Gareth Finn
Touch-screen displays that sense the occurrence and location
of a physical touch on the display area are increasingly being
used to replace mechanical buttons in a variety of devices,
including smartphones, MP3 players, GPS navigation systems,
digital cameras, laptop computers, video games, and laboratory
instruments. First generation devices were not very accurate,
suffered from false detection, and consumed too much power.
Newer touch-screen controllers,1 such as the AD7879, 2 offer
improved accuracy, lower power consumption, and result
filtering. They can also sense temperature, supply voltage,
and touch pressure, facilitating robust sensing for modern
touch-screen displays.
How Does a Touch Screen Work?
First, let’s look at how a resistive touch screen operates. Figure 1 shows
a basic diagram of the construction and operation of a touch screen.
X+
Y+
Y–
CONDUCTIVE ELECTRODE
ON BOTTOM SIDE
ADC
TOUCH
SCREEN
X–
GND
PLASTIC FILM WITH TRANSPARENT,
RESISTIVE COATING ON BOTTOM SIDE
Y+
DIGITIZER
VCC
Figure 2. X-position measurement.
X–
Y–
Next, the supply is placed across Y+ and X–, and two further screen
measurements are made: Z1 is measured as the voltage at X+, and
Z2 is measured as the voltage at Y–. These measurements can
be used to estimate the touch pressure in one of two ways. If the
resistance of the X-plate is known, the touch resistance is given by:
X+
CONDUCTIVE ELECTRODE
ON TOP SIDE
PLASTIC FILM WITH TRANSPARENT,
RESISTIVE COATING ON TOP SIDE
LCD SCREEN
If both X- and Y-plate resistances are known, the touch resistance
is given by:
PEN
Y+
RESISTIVE FILM
Y-PLATE
X+
RESISTIVE FILM
X-PLATE
+
DIGITIZER
Y–
Larger values of touch resistance indicate lighter touch pressure.
X–
AD7879 Touch-Screen Controller
–
SUPPLY
Figure 1. Construction of a resistive touch screen.
The screen is formed by two plastic films, each coated with a
conductive layer of metal—usually indium tin oxide (ITO)—
that are separated by an air gap. One plate, the X-plate in the
diagram above, is excited by the supply voltage. When the screen
is touched, the two conductive plates come together, creating
Analog Dialogue 44-02 Back Burner, February (2010)
The AD7879 touch-screen controller is designed to interface with
4-wire resistive touch screens. In addition to sensing touch, it also
measures temperature and the voltage on an auxiliary input. All
four touch measurements—along with temperature, battery, and
auxiliary voltage measurements—can be programmed into
its on-chip sequencer. Its wide supply voltage range (1.6 V to
3.6 V), small size (12-ball, 1.6 mm × 2 mm WLCSP; or 16-lead,
4 mm × 4 mm LFCSP), and low power dissipation (480 μA while
converting, 0.5 μA in shutdown mode) make it flexible for use in
a wide range of products.
www.analog.com/analogdialogue
1
Wake Up on Touch
The AD7879 can be configured to start up and convert when the
screen is touched and to power down after release. This can be
useful for battery-powered devices where power conservation is
important. After each conversion sequence, the AD7879 delivers
an interrupt to the host microcontroller, waking it from its lowpower mode to process the data. Thus, the microcontroller also
draws little power until the screen is touched. Figure 3 shows
the setup for the wake-up-on-touch function.
12-BIT SAR
ADC
MEDIAN
FILTER
AVERAGING
FILTER
CONVERTED
RESULTS
16 MEASUREMENTS
ARRANGED
AVERAGE OF
MIDDLE 8 VALUES
6
2
13
4
16
5
15
10
9
3
11
8
1
12
14
7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
VDD
ENABLE
Y+
50kΩ
X+
X–
DEGLITCH
ENABLE
Y–
ENABLE
AD7879
GND
Figure 3. Wake-up-on-touch setup.
When the screen is touched, the X- and Y-plates connect, pulling
the deglitch input low and waking the AD7879, which then
starts converting. An interrupt is sent to the host at the end of
the conversion.
Result Filtering
In a typical display, the resistive plates are placed on top of a
liquid-crystal display (LCD), which contributes a lot of noise
to the position measurement. This noise is a combination of
impulse noise and Gaussian noise. The AD7879 offers median
and averaging filters to reduce this noise. Instead of taking a
single sample for position measurement, the sequencer can
be programmed to take two, four, eight, or 16 samples. These
samples are sorted, median filtered, and averaged to give a lower
noise, more accurate result. The principle is illustrated more
clearly in Figure 4. Sixteen position measurements are taken
and are then ranked from lowest to highest. The four biggest
and four smallest measurements are discarded to eliminate
impulse noise; the remaining eight samples are averaged to
reduce Gaussian noise. This has the added benefit of reducing
the required amount of host processing and host-to-touch-screen
controller communication.
2
M = 16
A=8
Figure 4. Median and average filtering.
References
1
w ww.analog.com/en/analog-to-digital-converters/touchscreencontrollers/products/index.html.
2
w ww.analog.com/en/analog-to-digital-converters/touchscreencontrollers/ad7879/products/product.html.
Pratt, Susan. Ask The Applications Engineer—35, “Capacitance
Sensors for Human Interfaces to Electronic Equipment.” Analog
Dialogue. Volume 40, Number 4. www.analog.com/library/
analogdialogue/archives/40-10/cap_sensors.html.
Kearney, Paul. “The PDA Challenge—Met by the AD7873
Resistive-Touch-Screen Controller ADC.” Analog Dialogue.
Volume 35, Number 4. www.analog.com/library/analogdialogue/
archives/35-04/touchscreen.
Author
G a ret h Fi n n [ g a ret h.f i n n@ a n a log.com]
currently works as a staff analog design engineer
with the Integrated Portable Products Group
in Limerick, Ireland. After graduating from
University College Cork with a BE (Elec) in
1999, he spent five years as a designer with
the Consumer Products Group at S3 in Cork,
Ireland and two years as a designer in the Mixed-Signal
Automotive Group at TI in Munich, Germany. Gareth joined
the Transmit Signal Processing Group at ADI in October 2006.
Analog Dialogue 44-02 Back Burner, February (2010)
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