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Date: 30-12-16

Embedded Systems Course- module 18: Touch screen LCD panel interface design

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

 

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