The Microcontroller Idea Book - Jan Axelson's Lakeview Research

In Figure 8-5’s circuit, an 82(C)55 controls the 7218C. For complete control, the circuit
requires 10 outputs. Bits 0-4 of Port A determine the data to be written, including a decimal
point controlled by ID7. If you don’t need the decimal point, tie pin 7 of the 7218C low. Bits
5-7 of Port A select the digit to write to. If you have four or fewer displays, you can tie one
or more of these lines low and free up another port bit.
Port C, bit 4 controls WRITE. The display-mode input is tied high to select hexadecimal
mode. If you instead tie pin 9 to a port bit, you can turn off the display by bringing the bit
low. To allow selecting different modes, connect an additional output bit to the 7218D’s
MODE input.
Listing 8-3 uses the 7218D to display data, using Figure 8-5’s circuit.
7-segment LCDs
Displays
An alternative to LEDs is liquid-crystal displays (LCDs). Unlike LEDs, which consume
several milliamperes per segment, LCDs are voltage-controlled and require very little
operating current.
Compared to LEDs, LCDs are easy to read in bright light. However, because LCDs don’t
emit light as LEDs do, but merely absorb or transmit it, you need additional lighting to see
them in the dark. LCDs also tend to have narrower viewing angles than LEDs. So, whether
to use LEDs or LCDs may depend on where and how you will use the display.
Most 7-segment LCD modules contain two or more digits. Like the LEDs, a 7-segment LCD
creates a numeral by turning on selected segments.
Each LCD segment contains a thin layer of liquid crystal between two layers of glass. Liquid
crystals are organic compounds that act as electrically controlled light polarizers. In a
positive-image display (the most common type), applying a voltage across a segment causes
the segment to appear dark, or opaque, while removing the voltage causes the segment to
appear light-colored, or transparent. Negative-image displays are opaque when not powered,
and transparent when powered. By applying and removing voltages across individual
segments, you can display numeric, alphabetic, and other characters.
Applying a constant voltage to an LCD segment will eventually destroy it. Instead, you must
drive the segment with an alternating voltage, typically a square wave that alternately applies
+5 and -5V across the segment.
Single-digit driver. Figure 8-6 shows an LCD module driven by a 4543B LCD latch/decoder/driver.
The 4543 is a lot like the 4511 LED driver, with the addition of a phase input
that accepts a square wave for driving the segments. A typical drive frequency is around 100
Hertz. A 555 timer provides the phase input, or you can use any oscillator output.
The Microcontroller Idea Book 135