MODULE -16
Touch screen
LCD panel interface design- details for designing touch
sense based embedded system:
The touch sense interface is becoming must in handheld
portables as well as mobile computers. Any smartphone or
a tablet PC is sure to have a touch interface. Apple's iPad
and also many other tablet computers are in demand due to
many of its advanced features, one such key feature is touch
interface.
South Korea based market research firm Displaybank has
forecasted in terms of units the touch screen panel market
is expected to show a compound annual growth rate of 30
% by 2013. Displaybank says demand of more than 800 million
touch screen panel units in 2013, up from 280 million units
in 2009.
Another researcher Display Search forecasts the total touch
screen module market will grow to $9 billion by 2015, from
$3.6 billion in 2008, with a CAGR of 14%
Touch screen is more human way of interface to the computer.
Proper touch panel design can make the keyboard and mouse
obsolete. Touch panel displays will have a transparent touch
panel placed on the LCD. Touch panel supports direct finger
touch, Stylus touch or touch by any such physical object.
The touch panel sends voltage variables to determine x and
y co-ordinates of touch location to the touch controller
chip/IC which sends the digitized code to the microcontroller
or any such digital processor chip.
There are several touch technologies available now in the
market. The below table compares some of the popular touch
sense technologies used in the market.
Table:
Touch sense type |
Cost |
Applications |
Advantages |
Drawbacks |
Resistive touch sense |
Low |
Mobile phone and any such
handheld portable computing devices |
High resolution (300/inch),
Low price, popular, Can be operated with any pointing
device |
Not-rugged and reliable
for harsh environments |
Capacitive touch sense |
Medium |
Kiosks, mobile hand helds |
Fast response time (3 ms),
more durable, lasts longer and suitable for outdoor
use compared to resistive type |
Must be operated with finger,
or special pointing device |
Surface acoustic wave touch
sense |
High |
Military/Aerospace/Medical |
Highly durable, and most
suitable for outdoor environments |
Costly, does’t operate
when liquid comes into contact and must be operated
with finger |
Infrared Touch Sense |
High |
Military/Aerospace/Medical |
No LCD light is blocked,
extremely durable (based on the life of LED) |
Low resolution and is complex |
Touch sense in gaining more importance due to the convenience
it provides to the user. Its so humane. Keyboard soon going
to be eliminated by providing touch interface based virtual
keyboard. Apple Ipad is the good example of touch interface
based handheld computer which has touch based virtual QWERT
keyboard.
Touch interface requires a touch screen and controller
IC to convert the touch into proper digital data.
The types of touch interface:
Resistive: Resistive based touch sensors are most
popular touch sensors in the market. They are used in Smart
Phones, POS (point-of-sale) Terminals, industrial, medical,
and office automation, and in consumer electronics application.
There are sub variation in the resistive based touch screens,
however the basic fundamental remains same across all types
of touch panels.
The resistive touch panel consist of rigid glass plate
coated with a transparent conductive material and a flexible
Polyester sheet coated with transparent conductive material
such as Indium Tin Oxide (ITO). The polyester sheet is placed
over glass plate in such a way that the conductive layers
of glass plate and polyester sheet face each other. They
are however separated by a tiny insulating dots, so that
when there is no touch there is no contact between the layers.
If the finger or any physical object is placed on the flexible
polyester sheet, the conducting layers of both polyester
sheet and glass plate make contact resulting in change in
electrical parameters.
The variation in resistive touch panel types is based in
number of wires. There are 4 wire, 5 wire, 6 wire, 7 wire
and 8 wire types. Higher the number of wires better is the
reliability, life and number of touches the panels can sense.
How to find the exact location of touch: In a 4 -wire touch
panel, the conductive layers of both glass plate and the
polyester sheet are connected at the ends to a pair of bus
bars. The bus bars connecting to the glass plate's conduction
layer is in left - right direction (x-axis) and the bus
bars connecting to the polyester sheet in top-bottom direction
(y-axis). To find the x -axis position of the touch a voltage
is applied to the bus bars of glass plate and the voltage
is measured at bus-bars of polyester sheet. The various
levels of voltage detected tells the x axis position. Similarly
the y-axis position can be determined by applying input
voltage to the bus-bars of polyester sheet and the voltage
is measured at the busbars of glass plate.
Applying and measuring of voltages to the touch panel is
done by touch controller IC which has analog and digital
circuitry/functions such as comparators, Switches and ADCs
and the controller to sense and convert the voltage levels
to digital signals.
Typical specifications of resistive touch panel include:
Voltage and current rating
Light transmission
Contact bounce
Resolution
Linearity
Operating Force
Surface Hardness
Life expectancy (number of touches)
Operating Temp Range
Thickness
Resistive touch panels are most popular, the 2nd most popular
touch technology is Capacitive touch panel. Here is the
brief explanation of other touch sense technologies.
Capacitive touch sense panels: Capacitive touch
sense panels are made by coating a conducting layer below
a solid glass plate. AC Voltage is fed to the four corners
of conducting layer. When ever a finger is placed on the
plate, a small current flows into the finger from all the
four corners. The amount of current from the four corners
determine the location of the touch.
SAW based Touch sensor panels: In this type of panel
ultrasonic waves are made to flow on the surface of panel.
SAW transducers are used to generate and capture the waves.
The touch by the finger absorbs some ultrasonic waves. Based
on the analysis of differences in received signal strength
the exact location can be found.
Infrared LED array: It is similar to SAW based touch
sensor type, instead of ultrasonic waves the infrared light
is fed through infrared LEDs and photo-transistors at the
other end receive the infrared light. Light gets disturbed
whenever the display is touched using finger resulting in
analysis of x, y co-ordinates.
Other less important touch sense technologies include optical
imaging, on-cell, in-cell and digitizer.
Here below are some of the reference designs and application
notes from the semiconductor and electronic companies:
National Semiconductor's Reference Design board based on
LM8300 (3v@3.27MHz) or LM8500 (5v@10MHz) touchscreen controller
chip works with Panasonic EMU601A2 Resistive Touchpanel.
The reference design kit includes circuit diagram schematics,
driver software, PCB, touch panel, cables, and RS232 pc
cable.
To know more visit,
http://www.national.com/appinfo/mcu/LM8000-REF.html
In this application note from Texas Instruments, author
explains the basic principle of resistor touch sensor interface
and details on what happens inside resistive touch screen
controller ICs which includes configuration, screen setting
time, Timing diagram, and touch screen acquisition flowchart.
To read this paper visit http://focus.ti.com/lit/an/slyt209a/slyt209a.pdf
Altera has presented a white paper to develop multipoint
touch screens using CPLDs. To know more click below,
http://www.altera.com/literature/wp/wp-01086-multipoint-touchscreens-panels-cplds.pdf
Hitachi Display's application note of touch panes explain
the basics of touch panels of all types, to read this paper
visit,
http://www.hitachi-displays-eu.com/doc/AN-004_Resistive_Touch_Panel_Technology.pdf