Volume 44 – February 2010
this article in PDF format. (292KB)
New Touch-Screen Controllers Offer Robust Sensing
for Portable 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, such as the AD7879,
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
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.
1. Construction of a resistive touch screen.
The screen is formed
by two plastic films, each coated with a conductive layer of metalusually
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
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.
2. X-position measurement.
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:
If both X- and Y-plate
resistances are known, the touch resistance is given by:
Larger values of
touch resistance indicate lighter touch pressure.
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 measurementsalong
with temperature, battery, and auxiliary voltage measurementscan
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.
Wake Up on Touch
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 low-power 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.
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.
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.
4. Median and average filtering.
Pratt, Susan. Ask
The Applications Engineer35, "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 ChallengeMet by the AD7873 Resistive-Touch-Screen Controller
ADC." Analog Dialogue. Volume 35, Number 4. www.analog.com/library/analogdialogue/archives/35-04/touchscreen.
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.|
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