| Embedded in phones, office equipment, speakers, | | | | touchscreens are also very exciting because they can |
| digital photo frames, TV control buttons, remote | | | | detect multiple touches simultaneously. |
| controls, GPS systems, automotive keyless entry, and | | | | How Touchscreens Work |
| medical monitoring equipment, touchscreens are | | | | We’ll take a look inside the two most common |
| everywhere. As a component, they have reached into | | | | touchscreen technologies. The most widely used |
| every industry, every product type, every size, and | | | | touchscreen technology is resistive. Most people have |
| every application at every price point. Touchscreens | | | | used one of these resistive touchscreens before in the |
| are everywhere. In fact, if a product has an LCD or | | | | ATM at the bank, in the credit card checkout in most |
| buttons, a designer somewhere is probably evaluating | | | | stores, or even for entering an order in a restaurant. |
| how they too can implement touchscreen technology. | | | | Projective capacitance touchscreens, on the other |
| As with any technology, there are many different | | | | hand, are not as broadly available yet, but are gaining |
| ways to implementation approaches, many promises | | | | market momentum. Many cellphones and portable |
| of performance, and many different technical | | | | music players are beginning to come to market with |
| considerations when designing a touchscreen. | | | | projective capacitance interfaces. Both resistive and |
| Anatomy of a Touchscreen | | | | capacitive technologies have a strong electrical |
| Knowing what you need is an important first step in | | | | component, both use ITO (Indium-Tin-Oxide, a clear |
| designing a touchscreen product. Vendors in the | | | | conductor), and both will be around for a long time to |
| touchscreen supply chain frequently offer different | | | | come. |
| pieces of the puzzle, often times combining several to | | | | A resistive touchscreen consists of a flexible top layer, |
| create a value chain for the end customer. | | | | then a layer of ITO (Indium-Tin-Oxide), an air gap and |
| The front panel or bezel is the outermost skin of the | | | | then another layer of ITO. The panel has 4 wires |
| end product. In some products, this bezel will | | | | attached to the ITO layers: one on the left and right |
| encompass a protective clear overlay to keep | | | | sides of the ‘X’ layer, and one on the top and |
| weather and moisture out of the system and to resist | | | | bottom sides of the ‘Y’ layer. |
| scratching and vandalism to the underlying sensor | | | | Stackup Layers for “Resistive” (Left) and |
| technology. Other times, the outmost bezel simply | | | | “Capacitive” (Right) Screens |
| covers the edges of the underlying touch sensor; in this | | | | A touch is detected when the flexible top layer is |
| case, it is purely decorative. | | | | pressed down to contact the lower layer. The location |
| #2 Touch Controller | | | | of a touch is measured in two steps: First, the ‘X |
| The touch-controller is generally a small | | | | right’ is driven to a known voltage, and the ‘X |
| microcontroller-based chip that sits between the touch | | | | left’ is driven to ground and the voltage is read |
| sensor and the embedded system controller. This chip | | | | from a Y sensor. This provides the X coordinate. This |
| can either be located on a controller board inside the | | | | process is repeated for the other axis to determine |
| system or it can be located on a flexible printed circuit | | | | the exact finger position. |
| (FPC) affixed to the glass touch sensor. This | | | | Resistive touchscreens also come in 5-wire, and |
| touch-controller takes information from the touch | | | | 8-wire versions. The 5-wire version replaces the top |
| sensor and translates it into information that the PC or | | | | ITO layer with a lowresistance “conductive |
| embedded system controller can understand. | | | | layer” that provides better durability. The 8-wire |
| #3 Touch Sensor | | | | panel was developed to enable higher resolution by |
| A touchscreen “sensor” is a clear glass panel | | | | enabling better calibration of the panel’s |
| with a touch responsive surface. This sensor is placed | | | | characteristics. |
| over an LCD so that the touch area of the panel | | | | There are several drawbacks to resistive technology. |
| covers the viewable area of the video screen. There | | | | The flexible top layer has only 75%-80% clarity and |
| are many different touch sensor technologies on the | | | | the resistive touchscreen measurement process has |
| market today, each using a different method to detect | | | | several error sources. If the ITO layers are not uniform, |
| touch input. Fundamentally, these technologies all use | | | | the resistance will not vary linearly across the sensor. |
| an electrical current running through the panel that, | | | | Measuring voltage to 10 or 12-bit precision is required, |
| when touched, causes a voltage or signal change. This | | | | which is difficult in many environments. |
| voltage change is sensed by the touch controller to | | | | Many of the existing resistive touchscreens also |
| determine the location of the touch on the screen. | | | | require periodic calibration to realign the touch points |
| #4 Liquid Crystal Display | | | | with the underlying LCD image. |
| Most touchscreen systems work over traditional | | | | Conversely, projected capacitive touchscreens have |
| LCDs. LCDs for a touch-enabled product should be | | | | no moving parts. The only thing between the LCD and |
| chosen for the same reasons they would in a | | | | the user is ITO and glass, which have near 100% |
| traditional system: resolution, clarity, refresh speed, and | | | | optical clarity. The projected capacitance sensing |
| cost. One major consideration for a touchscreen, | | | | hardware consists of a glass top layer (see figure 2), |
| however, is the level of electrical emission. Because | | | | followed by an array of X sensors, an insulting layer, |
| the technology in the touch sensor is based on small | | | | then an array of Y sensors on a glass substrate. The |
| electrical changes when the panel is touched, an LCD | | | | panel will have a wire for each X and Y sensor, so a |
| that emits a lot of electrical noise can be difficult to | | | | 5 x 6 panel will have 11 connections (as shown in |
| design around. | | | | Figure 3 below), while a 10 x 14 panel will have 24 |
| Touch sensor vendors should be consulted before | | | | sensor connections. |
| choosing an LCD for a touchscreen system. | | | | Signal Intensity at Rows and Columns Denote Location |
| #5 System Software | | | | of Touch |
| Touchscreen driver software can be either shipped | | | | As a finger or other conductive object approaches the |
| from the factory (within the embedded OS of a cell | | | | screen, it creates a capacitor between the sensors |
| phone) or offered as add-on software (like adding a | | | | and the finger. This capacitor is small relative to the |
| touchscreen to a traditional PC). This software allows | | | | others in the system (about .5pF out of |
| the touchscreen and system controller to work | | | | 20pF), but it is readily measured. One common |
| together and tells the product’s operating system | | | | measuring technique known as Capacitive Sensing |
| how to interpret the touch event information that is | | | | using a Sigma-Delta Modulator (CSD) involves rapidly |
| sent from the controller. In a PC-style application, most | | | | charging the capacitor and measuring the discharge |
| touchscreen drivers work like a PC mouse. This | | | | time through a bleed resistor. |
| makes touching the screen similar to clicking the | | | | A projected capacitive sensor array is designed so |
| mouse at the same location on the screen. In | | | | that a finger will interact with more than one X sensor |
| embedded systems, the embedded controller driver | | | | and more than one Y sensor at a time. This enables |
| must compare the information presented on the | | | | software to accurately determine finger position to a |
| screen to the location of the received touch. | | | | very fine degree through interpolation. |
| The Big Three of Touchscreen Technology | | | | Since projected capacitive panels have multiple |
| Resistive Touchscreens are the most common | | | | sensors, they can detect multiple fingers |
| touchscreen technology. They are used in high-traffic | | | | simultaneously, which is impossible with other |
| applications and are immune to water or other debris | | | | technologies. In fact, projective capacitance has been |
| on the screen. Resistive touchscreens are usually the | | | | shown to detect up to ten fingers at the same time. |
| lowest cost touchscreen implementation. Because | | | | This enables exciting new applications based on |
| they react to pressure, they can be activated by a | | | | multiple finger presses, including multiplayer gaming on |
| finger, gloved hand, stylus or other object like a credit | | | | handheld electronics or playing an touchscreen piano. |
| card. | | | | Without question, touchscreens are great looking. They |
| Surface Capacitive Touchscreens provide a much | | | | have begun to define a new user interface and |
| clearer display than the plastic cover typically used in a | | | | industrial design standard that is being adopted the |
| resistive touchscreen. In a surface capacitive display, | | | | world over. In everything from heart rate monitors to |
| sensors in the four corners of the display detect | | | | the latest all-in-one printers, touchscreens are quickly |
| capacitance changes due to touch. These | | | | becoming the standard of technology design. Beyond |
| touchscreens can only be activated by a finger or | | | | just looks, however, touchscreens provide an |
| other conductive object. | | | | unparalleled level of security from tampering, |
| Projected Capacitive Touchscreens are the latest | | | | resistance from weather, durability from wear, and |
| entry to the market. This technology also offers | | | | even enable entirely new markets with unique features |
| superior optical clarity, but it has significant advantages | | | | such as multi-touch touchscreens. With touchscreens |
| over surface capacitive screens. Projected capacitive | | | | making their way into so many types of products, |
| sensors require no positional calibration and provide | | | | it’s imperative that design engineers understand the |
| much higher positional accuracy. Projected capacitive | | | | technology ecosystem and technology availability. |