Hardware Reference
In-Depth Information
Segment
Frontplane
Glass
Backplane
Black cardboard backing
Liquid crystal
Figure 7.26
■
A liquid crystal display (LCD)
signal in the range of 30 to 1000 Hz is applied to it. The polarity of the voltage must alternate,
or the LCD will not be able to change very quickly.
When a voltage is applied across the segment, an electrostatic field is set up that aligns the
crystals in the liquid. This alignment allows light to pass through the segment. If no voltage is
applied across a segment, the crystals appear to be opaque because they are randomly aligned.
Random alignment is assured by the AC excitation voltage applied to each segment. In a digital
watch, the segments appear darker when they are activated because light passes through the
segment to a black cardboard backing that absorbs all light. The area surrounding the activated
segment appears to grow brighter in color because the randomly aligned crystals reflect much of
the light. In a backlit computer display, the segment appears to grow brighter because of a light
placed behind the display; the light is allowed to pass through the segment when it is activated.
In recent years, the price of LCD displays has dropped to such an acceptable level that
all PC vendors bundle LCD displays instead of CRT displays with their PC systems. Note-
book computers used LCD as displays right from the beginning. Because of the price reduc-
tion of LCDs, the prices of notebook computers also have dropped sharply, and more and more
computer users have switched from desktop to notebook computers.
Although LCDs can display graphics and characters, only character-based LCDs are dis-
cussed in this text. LCDs are often sold in a module that consists of the LCD and its controller.
The Hitachi HD44780 (with two slightly different versions: HD44780U and HD44780S) is one
of the most popular LCD display controllers in use today. The following section examines the
operation and programming of this controller.
The block diagram of an LCD kit that incorporates the HD44780U controller is shown in
Figure 7.27. The pin assignment shown in Table 7.3 is the industry standard for character-based
LCD modules with a maximum of 80 characters. The pin assignment shown in Table 7.4 is the
industry standard for character-based LCD modules with more than 80 characters.
The DB7,DB0 pins are used to exchange data with the microcontroller. The E pin is an
enable signal to the kit. The R/W signal determines the direction of data transfer. The RS sig-
nal selects the register to be accessed. When the RS signal is high, the
data register
is selected.
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