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Pointing a Finger at Touch Screen Displays
Compact touch screen displays such as those in the EA eDIP series from Electronic Assembly serve as compact control interfaces and can also show different portions of data at the same time.

Touch panel displays are an important part of many of today's electronic products. They provide a control interface for a wide range of electronic devices, as well as operating information for users in such products as portable computers and cellular telephones. Of course, as touch panel displays add features, they also become more complex, and choosing a touch panel display for an application becomes more of a challenge. These guidelines may help make that choice a bit easier.

Touch Panel Technologies
Touch panels are available in three varieties, namely optical, capacitive, and resistive types. Optical scanning works in a manner similar to a light barrier, reacting when a user places a finger in a certain location. Optical touch panels tend to be complex and they are susceptible to the effects of stray light.

Capacitive touch panels are currently very popular and widely used on smartphones. They offer the advantage of two-finger operation can be designed with a rugged, vandalism-proof surface. However, the response characteristics must be individually tailored to a customer's needs. These screens work effectively with bare fingers but not with gloved fingers or a conventional stylus. Capacitive touch panels can be difficult to use for operators with dry hands or long fingernails. This type of touch panel display is best used with a customer-specific design, typically with a front surface made completely of glass.

From an operational standpoint, resistive touch panels are the most reliable option. Pressure on different locations on the touch screen results in different resistance values for use by other circuits within an electronic device. Resistive touch panels can be operated with a finger or any other object, which provides a clear advantage for this type of touch screen in industrial applications. Even sealing the front of the display with a transparent film does not impair the operation of a resistive touch panel.

Capacitive touch panels have two conductors or electrodes mounted on a glass substrate and insulated from each other. When a user's finger touches the screen near an electrode, it distorts the electromagnetic (EM) field around the electrode, causing a small change in capacitance (less than 1pF), and that change can be used as part of electronic circuits within an electronic product. To avoid annoying delays, a scanning operation (frame) should not last too long on multi-touch contact. Depending on the application, the time limit should be around 20ms.

A resistive touch panel consists of two layers (normally glass and plastic), each of which has a conductive coating, normally indium-tin oxide (ITO). The layers are insulated from each other and are separated by spacers. Pressure from touching the screen produces a specific resistance value that is proportional to the position where the screen was touched. Unwanted effects on the screen, such as debouncing, can be minimized by corrections from hardware and software, which can also aid in reliably detecting such motions as sliding and wiping on the touch screen. After intensive use, the metallization on a resistive touch panel may exhibit signs of wear and require readjustment.
The the EA eDIP series of touch screen displays simplify integration into other electron products with their own operating system.

Displays of many varieties find demand in many different electronic device applications. For example, industrial personal computers (PCs) are a good fit for displays larger than 7-in. across. Such PCs are fully programmable in terms of the display and are usually powered by a standard operating system (OS), such as Microsoft Windows or Linux. By contrast, embedded PCs might be a better fit for smaller touch panel displays. They are relatively compact and economical, and work with a standard OS. But they still suffer from high power consumption and cost, making them not well suited for handheld and/or large-volume applications. The displays for these products are normally sourced from a third-party supplier and integration requires adaptation of the electronics and drivers. To minimize the required number of separate components in these applications, compact touch screen displays for consideration should have built-in light-emitting-diode (LED) drivers and touch panel controllers. Still, in applications for touch screen displays such as embedded PCs, modifications to the main computer board or display board, as well as product retirement, represent risk factors for a touch panel display developer/supplier.

ARM Controllers
Applications with ARM controllers represent another area of opportunities for touch panel displays. Such controllers are relatively inexpensive and operate with with low power consumption, but development time and effort is needed with such applications, such as for the DRAM and FLASH memory and peripheral interfaces. Software development can be simplified by means of a functional OS, although sch operating systems can also bring with them long boot times, license issues, and high-level programming requirements. If the software programming is too remote (and high level) from the hardware, power control functions can be affected and the reaction times of individual components can be slowed. If many different components are needed in these applications, procurement and inventory management can become more complex.

As a display programming alternative, intelligent EA eDIP series displays from ELECTRONIC ASSEMBLY were originally developed for microcontroller applications and programming is performed at a lower level, typically using SPI and I2C interfaces. Because of its combination of display, contro, and communications functions, touch panel displays are not always desirable for applications with high-overhead OS setups. For example, the serial port is used exclusively for image display and other communications. Short command sequences help draw and define the screen content, with keystrokes from a touch screen display sent back to the host via the RxD (or MISO) pin. Modularity is a key to managing these multiple-function operations with touch screen displays. Applications and the display should be kept separate, with the interface is clearly defined (in contrast to embedded systems). By using an exisitng system approach, the development process can process quickly, including when troubleshooting is needed and high reliability is desired.

When integrating touch screen displays, mechanical design and electromagnetic-compatibility (EMC) considerations are often left to the final stages of the design phase. EMC testing, especially at GHz frequencies such as used in many wireless systems, requires experience and the proper measurement equipment and tools. Existing and available human-machine-interface (HMI) systems can provide a huge boost when evaluating electronic designs with touch screen displays for proper EMC performance.

The EA eDIP series displays are designed for ease of integration into new designs, normally mounted to an electronic device's front plate with screws. When considering different touch screen displays for an embedded system design, mounting a display must also enable other subassemblies to be mounted to the product as well. Many displays are supplied as complete units but still without provision for mounting. In the EA eDIP series displays, the pins on the rear of the displays are also used for mounting the displays, saving production time and eliminating the need for extra (mounting) design effort. The displays are supplied with mounting brackets that mate with the left and right or top and bottom of the display for ease of mounting.

Sizing Up Displays
The minimum width for a touch panel display is 2.8-in. or 71.12 mm. The overall dimensions of these displays in terms of width and height are roughly 75 x 55 mm or 2.95 x 2.16-in. They can be used to display a series of measurement values, touch fields, or small images. In practice, small displays are often used initially as a replacement or upgrade for conventional text displays. Displays between 4.3-in. and roughly 5.7-in. in width are suitable for handheld and benchtop devices and small cabinet-mounted equipment. They offer sufficient display area for multiple images and support a user-friendly screen layout. Larger displays are used for process visualization in production control systems.

The amount of programming needed to install a touch screen display is often underestimated by product developers. Thin-film-transistor (TFT) touch screen displays support the full spectrum of colors, so programming should be based on a strong initial design. For larger applications running Windows or other major operating systems, programming a touch screen display will take place with the aid of a third-party graphical-unser-interface (GUI) library.

Some smaller electronic systems with microprocessor controllers may not include adequate memory for a touch screen display, such as the 260 kB display memory required for use with a 4.3-in. display. Lack of memory will result in a sluggish display or images that are highly pixelated. Without necessary memory, because graphical character sets are not included, all characters must be generated and converted for use with the touch screen display. Typical graphic-file formats, such as .jpeg files, are not commonly used in the display work and must be converted and modified for use with a display.

To simplify integration of EA eDIP series displays, everything needed for the displays, such as character sets, graphic functions, and buttons, are included with the displays. There is no OS with these displays, to avoid system conflicts and crashes because of the display and to enhance operational reliability. Creating screen content for these displays is much easier than for OS-based displays. Integration of an OS can be challenging and require a great deal of experience, and these EA eDIP series displays have been designed to simplify the integration process. They are designed to be reliable and to be compact touch screen displays that other electronic designers can build their products around.

Contact: Electronic Assembly GmbH, Zeppelinstrasse 19, D-82205 Gilching, Germany; +49-8105-778090 fax: +49-8105-778099 Web:

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