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.