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`Touchscreens 101: Understanding touch-
`screen technology and design
`
`JUNE 29, 2009
`av PLANET ANALOG
`
`'4 COMMENT 1
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`
`
`(Editor‘s note : there is a list of related articles with links at the end, below the
`“About the authors" section.)
`
`Silent Switcher“?
`family reduces radiated
`EMl on any PCB
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`_
`Touchscreens (sometimes spelled as touch screen) are everywhere: they are
`embedded in phones, office equipment, speakers, digital photo frames, TV
`control buttons, remote controls, GPS systems, automotive keyless entry, and
`medical monitoring equipment As a component, they have reached into every
`industry, every product type, every size, and every application at every price
`point. In fact, if a product has an LCD or buttons, a designer somewhere is
`probably evaluating how that product, too, can implement touchscreen
`technology. As with any technology, there are many different ways to
`implementation approaches, many promises of performance, and many
`different technical considerations when designing a touchscreen.
`
`5m- UBBS‘SES
`
`(
`
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`pieces of the puzzle, often times combining several to create a value chain for
`the end customer. Figure 1 shows a blowup of the touchscreen ecosystem. This
`ecosystem is the same whether it is in the latest Notebook PC or the latest
`touch—enabled mobile phone.
`
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`Archives
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1007, IPR2021-01160 Page 1 of 6
`
`

`

`There are six key elements:
`I. Front panel or bezel : The front panel or bezel is the outermost skin of the end
`product. In some products, this bezel will encompass a protective clear overlay
`to keep weather and moisture out of the system, and to resist scratching and
`vandalism to the underlying sensor technology (see item 3 below). Other times,
`
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`the outmost bezel simply covers the edges of the underlying touch sensor; in
`controller. This IC can either be located on a controller board inside the system
`
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`this case, it is purely decorative.
`
`2. Touch controller : The touch—controller is generally a small microcontroller—
`based [C that sits between the touch sensor and the embedded system
`
`or it can be located on a flexible printed circuit (FPC) affixed to the glass touch
`
`sensor. This touch controller takes information from the touch sensor and
`translates it into information that the PC or embedded systeln controller can
`understand.
`
`3. Touch sensor: A touchscreen “sensor” is a clear glass panel with a touch—
`responsive surface. This sensor is placed over an LCD so that the touch area of
`the panel covers the viewable area of the video screen. There are many different
`touch-sensor technologies on the market today, each using a different method
`to detect touch input. Fundamentally, these technologies all use an electrical
`current running through the panel that, when touched, causes a voltage or
`
`
`
`signal change. This voltage change is sensed by the touch controller to
`
`determine the location of the touch on the screen.
`
`‘
`
`’
`
`"
`
`’
`
`4. Liquid crystal display : Most touchscreen systems work over traditional LCDs.
`LCDs for a touch~enabled product should be chosen for the same reasons they
`would in a traditional system: resolution, clarity, refresh speed, and cost. One
`major consideration for a touchscreen, however, is the level of electrical
`emission. Because the technology in the touch sensor is based on small
`electrical changes when the panel is touched, an LCD that emits a lot of
`electrical noise can be difficult to design around. Touch sensor vendors should
`be consulted before choosing an LCD for a touchscreen system.
`
`5. System software : Touchscreen driver software can be either shipped from the
`factory (within the embedded OS of a cell phone) or offered as add—on software
`(like adding a touchscreen to a traditional PC). This software allows the
`touchscreen and system controller to work together and tells the product's
`operating system how to interpret the touch—event information that is sent
`from the controller. In a PC—style application, most touchscreen drivers work
`like a PC mouse. This makes touching the screen similar to clicking the mouse
`at the same location on the screen. In embedded systems, the embedded
`controller driver must compare the information presented on the screen to the
`location of the received touch.
`
`The “big three” of touchscreen technology
`
`Fully calibrated MEMS
`
`IMU offers significant
`
`height reduction for
`industrial applications
`
`ANALOG
`DEVICES
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`other debris on the screen. Resistive touchscreens are usually the
`
`— Resistive touchscreens are the most common touchscreen technology.
`
`They are used in high—traffic applications and are immune to water or
`
`(
`
`599 “31515sz
`
`lowest—cost touchscreen implementation. Because they react to
`pressure, they can be activated by a finger, gloved hand, stylus, or other
`object, such as a credit card.
`— Surface—capacitive touchscreens provide a much clearer display than the
`plastic cover typically used in a resistive touchscreen. In a surface—
`capacitive display, sensors in the four corners of the display detect
`capacitance changes due to touch. These touchscreens can only be
`activated by a finger 01' other conductive object.
`— Projected—capacitive touchscreens are the latest entry to the market. This
`technology also offers superior optical clarity, but it has significant
`advantages over surface—capacitive screens. Projected capacitive sensors
`
`In
`
`with.
`
`SEARCH
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1007, IPR2021-01160 Page 2 of 6
`
`

`

`How touchscreens work
`
`Let's look inside the two most common touchscreen technologies.
`
`The most widely used touchscreen technology is resistive. Most people have
`used one of these resistive touchscreens already, in the ATM at the bank, in the
`credit card checkout in most stores, or even for entering an order in a
`restaurant. Projective—capacitance touchscreens, on the other hand, are not as
`
`broadly available yet, but are gaining market momentum. Many cellphones and
`portable music players are beginning to come to market with projective—
`capacitance interfaces. Both resistive and capacitive technologies have a strong
`electrical component, both use ITO (Indium—Tin—Oxide, a clear conductor), and
`both will be around for a long time to come.
`
`A resistive touchscreen (Figure 2 , left side) consists of a flexible top layer, then a
`layer of ITO (Indium—Tin—Oxide), an air gap and then another layer of ITO. The
`panel has 4 wires attached to the ITO layers: one on the left and right sides of
`the 'X' layer, and one on the top and bottom sides of the 'Y layer.
`
`Figure 2. Slackup ltiycrsfai' “I'r‘sislivc” (left) and "t‘z'ipucilivo" (right) screens
`
`(Click an image to enlarge)
`
`A touch is detected when the flexible top layer is pressed down to contact the
`lower layer. The location of a touch is measured in two steps: First, the 'X right'
`is driven to a known voltage, and the 'X left' is driven to ground and the voltage
`is read from a Y sensor. This provides the X coordinate. This process is repeated
`for the other axis to determine the exact finger position.
`
`Resistive touchscreens also come in S—wire, and 8—wire versions. The 5—wire
`
`version replaces the top ITO layer with a low—resistance "conductive layer" that
`provides better durability. The 8—wire panel was developed to enable higher
`resolution by enabling better calibration of the panel's characteristics.
`
`There are several drawbacks to resistive technology. The flexible top layer has
`only 7S%—80% clarity and the resistive touchscreen measurement process has
`several error sources. If the ITO layers are not uniform, the resistance will not
`vary linearly across the sensor. Measuring voltage to 10— or12—bit precision is
`required, which is difficult in many environments. Many of the existing
`resistive touchscreens also require periodic calibration to realign the touch
`points with the underlying LCD image.
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1007, IPR2021-01160 Page 3 of 6
`
`

`

`Conversely, projected—capacitive touchscreens have no moving parts. The only
`thing between the LCD and the user is ITO and glass, which have near 100%
`optical clarity. The projected—capacitance sensing hardware consists of a glass
`top layer (Figure 2 , right side), followed by an array of X sensors. an insulating
`layer, then an array of Y sensors on a glass substrate. The panel will have a wire
`for each X and Y sensor, so a 5 x 6 panel will have 11 connections (Figure 3),
`while a 10 x14 panel will have 24 sensor connections.
`
`Figural Signal intensity lll rows (llItl r'olinnns (lunolt' location Ufluurll
`
`(Click on llIltIyl’ to enlarge)
`
`As a finger or other conductive object approaches the screen, it creates a
`capacitor between the sensors and the finger. This capacitor is small relative to
`the others in the system (about 0.5 pF out of 20 pF), but it is readily measured.
`One common measuring technique known as Capacitive Sensing using a Sigma—
`Delta Modulator (CSD) involves rapidly charging the capacitor and measuring
`the discharge time through a bleed resistor.
`
`A projected capacitive sensor array is designed so that a finger will interact with
`more than one X sensor and more than one Y sensor at a time (See Figure 3).
`This enables software to accurately determine finger position to a very fine
`degree through interpolation. For example, if sensors I, 2 and 3 see signals of 3,
`10, and 7, the center of the finger is at:
`
`[(M3) o(2xio)«(7x3)|/(3 +10o7):2.2
`
`Since projected—capacitive panels have multiple sensors, they can detect
`multiple fingers simultaneously, which is impossible with other technologies.
`In fact, projective capacitance has been shown to detect up to ten fingers at the
`same time. This enables exciting new applications based on multiple finger
`presses, including multiplayer gaming on handheld electronics or playing an
`touchscreen piano.
`
`Without question, touchscreens are great looking. They have begun to define a
`new user interface and industrial design standard that is being adopted the
`world over. In everything from heart—rate monitors to the latest all—in—one
`printers, touchscreens are quickly becoming the standard of technology design.
`
`Beyond just looks, however, touchscreens provide an unparalleled level of
`security from tampering, resistance from weather, durability from wear, and
`even enable entirely new markets with unique features such as multi—touch
`touchscreens. With touchscreens making their way into so many types of
`products, it's imperative that design engineers understand the technology
`ecosystem and technology availability.
`
`About the authors
`
`Steve Kolokowsky is a Member of the Technical Staff in Cypress Semiconductor's
`Consumer and Computation Division (CCD). Steve's focus is high—speed USB
`peripheral products. He has participated in the design of many of today's
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1007, IPR2021-01160 Page 4 of 6
`
`

`

`'1'1‘evor Dams is currently the Director 01 Marketing for Cypress's Consumer and
`
`Computation Division (CCD) focused on Universal Serial Bus (USB) in consumer
`products Trevor received his undergraduate degree from the United States Air
`Force Academy and also holds his Masters in Business Administration. Trevor
`served as an Air Force Officer for five years before joining Cypress. Trevor lives in
`San Diego, CA and can be reached at tmd@cypress.com
`
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`1 COMMENT ON "TOUCHSCREENS 101: UNDERSTANDING TOUCHSCREEN
`TECHNOLOGY AND DESIGN"
`
`uinaino
`«211111
`I7. 201-}
`
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`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1007, IPR2021-01160 Page 5 of 6
`
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`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1007, IPR2021-01160 Page 6 of 6
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