Designing and implementing a liquid-crystal display (LCD) module for small, handheld applications requires particular attention to issues such as size, viewability, weight, cost, ruggedness, and tolerance of temperature extremes. While available technologies have greatly advanced the LCD's performance in each of these areas, the advances themselves create problems. To avoid these "gotchas" and ensure a smoother design flow, it is helpful to break down the module design stages and examine them in the context of these recently introduced advances. The stages can be loosely defined as the glass design and layout, the driver and interconnection technology, and the backlight as both an optical and mechanical component.
Glass Design
To create a cost-effective module design, the issue foremost in the designer's mind should be how to create a glass layout that will use the least amount of glass area to implement the desired display format (Fig. 1). In addition to the active viewing area of the glass, the other physical constraints of interconnection ledge width, seal width, and the inactive area between the active viewing area and the environmental seal need to be minimized. This must be done without compromising the mechanical/environmental integrity of the display.
Pixel Size: At the heart of the design are the size of each pixel and the number of pixels that are required to present a suitable display to the user. Today's glass manufacturer can fabricate displays with pixel sizes that are smaller than can be suitably viewed by a human. When pixel size is below a threshold of approximately 0.25 mm, the usefulness of the display in the handheld environment begins to degrade. At issue is character size, especially in a single-pixel format. Below this 0.25-mm threshold, multiple pixels must be used to create alphanumeric characters, leading to an increase in hardware and software complexity.
Generally, the larger the pixel, the more readable the information on the display. As a result, in the small, handheld environment, pixel sizes are usually greater than 0.30 mm. This pixel size allows enhanced character readability, and leads to quicker recognition of characters by the user. Thus, the user experiences less fatigue, especially under conditions of high usage.
To provide some aspect of differentiation, some manufacturers have adopted an asymmetric pixel shape for their characters' dot-matrix display formats. They have opted for a pixel that is taller in the vertical dimension than the square pixel. This pixel format is due to the aspect ratio required by the display. Again, a larger display makes it easier for the user. Therefore, if any dimension can be increased, without compromising the quality of the display, it will further aid the user in recognizing the presented information. In these instances, it makes sense to increase only the vertical size of the pixel. Note, however, that the format will make it unsuitable for use as a graphics display, due to the asymmetry of the pixel.
Active Viewing Area: The next step in the design process is to determine the number of pixels necessary for proper display of the desired information. In small, handheld environments, the trend is to put as much information on the display as possible, while keeping it from appearing crowded. In most cases, the display size is limited to no less than two lines of 10 characters, and no more than eight lines of 20 characters, within the graphics area. Icons can either be included in the display, or generated in the graphics area. Of course, in the graphics mode, the character formats are user programmable, thus individual character format is a function of the application.
For the application of eight lines of approximately 14 characters, the total number of pixels in the graphics display will be at least 100 pixels horizontally by 64 pixels vertically. Standard LCD drivers are available which will drive formats of 102 by 65 pixels.
A viewable graphics area of 47.9 by 30.5 mm results when using the square-pixel format and a pixel size, trace, and space of 0.47 mm. The active area of the display is shown in Figure 1.
Note that the active area is much smaller than the actual glass area. This is to accommodate the area needed on the ends of the glass to route the row traces. In small, handheld display applications, only one ledge can be used to interconnect the row and column lines. In our example, 32 row lines must be routed on each edge of the glass. As the number of rows increases, the number of crossover connections between the glass plates must increase, leading to lower reliability and higher cost.
Display Modeling
Once the pixel format is chosen, the entire display format should be modeled. When modeling a display format, it is imperative for it to be scaled to actual size, to assess the readability of the information presented. Mylar film, the same material used in the pc-board industry, should be used to evaluate the display formats. CAD packages, such as AutoCAD, are more than adequate for producing a mylar model (Fig. 2).
Display format modeling will save headaches in the future, especially from the marketing and sales, as well as the human aspect of the product. Many projects have failed after the prototypes have been delivered because the utility of the display was compromised due to inadequate pixel and/or display size, or unacceptable information content. It is obvious that, at this end point, time and money have been expended for no useful purpose.
Viewing Area-To-Seal Distance: The next area of consideration is the distance between the active pixel area and the inside edge of the LCD seal (Fig. 3). This distance is usually defined to be approximately 1 mm minimum. However, the actual distance is determined by the bezel opening in the user's housing.
The edge of the bezel opening should be within this area. To properly hide the seal area, the bezel should be placed as closely to the glass surface as possible, while retaining mechanical integrity.
The factor that defines the active area-to-seal distance is the thickness of the glass itself. The thicker the glass, the greater the parallax between the glass surfaces. This presents the end user with the opportunity to view the seal through the upper glass layer. Seeing the edge seal is highly objectionable in most display applications, therefore the LCD module designer must plan accordingly to properly obscure the seal material. As the thickness of the glass decreases, the minimum distance required to obscure the seal is reduced, allowing a smaller distance between the edge of the active area and the seal edge.
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