Liquid Crystal Displays

Liquid Crystal Displays (LCDs) are a passive display technology. This means that LCDs do not emit light but instead manipulate ambient light. By manipulating this light, LCDs can display images using very little power. This characteristic has made LCDs the preferred technology whenever low power consumption is critical. An LCD is basically a reflective part. It needs ambient light to reflect back to a user's eyes. In applications where ambient light is low or nonexistent, a light source can be placed behind the LCD. This is known as backlighting.

Backlighting can be accomplished by either using electroluminescent (EL) or LED light sources. EL backlights are very thin and lightweight and produce a very even light source. EL backlights for LCDs are available in a variety of colors with white being the most popular. EL backlights consume very little power but require high voltages (80 to 100 Vac). EL backlights also have a limited life of about 2,000 to 3,000 hours. LEDs are used for backlighting and are primarily used for character modules. LEDs offer a much longer life (at least 50,000 hours) and are brighter than ELs. Unfortunately, LEDs consume more power than ELs. LEDs are typically mounted in an array directly behind the display. LEDs come in a variety of colors but yellow-green LEDs are the most common.

Controlling LCDs is a little bit trickier than controlling LEDs. LCDs are almost always controlled with dedicated hardware. Figure 5.1 shows the three types of LCDs currently available:

1. Custom displays with individual segment controls (similar to LED displays). LCDs lend themselves very well to custom displays, as shown in Figure 5.1. You can design a display with just about any type of annunciation. Where software is concerned, these types of displays are similar to LED displays because each segment is controlled individually.

2. Alphanumeric or character displays. These types of displays are currently available in modules. A module contains the LCD and the drive electronics. Character displays are composed of one to four lines of 16 to 40 character blocks. Each character block consists of a 5x8 dot matrix that is used to display any ASCII character and a limited number of symbols.

3. Full graphics displays. As with character displays, full graphics displays are available in modules. Graphic modules offer the greatest flexibility in formatting data on the display. They allow for text, graphics, pictures, or any combinations of these. Because character size is defined by software, graphic modules allow any language or character font. Limitations are driven by the resolution. Graphic modules are organized in rows (horizontal) and columns (vertical) of pixels. Each pixel is addressed individually, which allows any pixel to be ON or OFF. Graphics displays are available in a wide variety of configurations from 64x32 to 640x480 pixels (columns x rows). From a software point of view, interfacing with graphics displays is at least an order of magnitude more complex than interfacing with the other two types of displays. I will not be covering graphics displays in this book.

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