Liquid crystal display (LCD)
...................................................................................................................................
A liquid crystal display (commonly abbreviated LCD) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector. It is prized by engineers because it uses very small amounts of electric power, and is therefore suitable for use in battery-powered electronic devices
How They Work
The operation of character-based LCDs is analogous to the PC's video card and monitor. In the PC, information is sent by the program to the video card via the PC bus. The video card is responsible for the actual task of creating, updating, and refreshing the display. Information in the display in maintained in the video card memory. Our LCDs behave in the same manner: they accept data and commands via a bus, maintain character information in memory, and manage the built-in display. The brains behind the LCD is the Hitachi HD44780. In addition to managing the display (i.e., writing a character, clearing the display, moving the cursor, etc.), it contains three areas of memory: CGROM, DDRAM, and CGRAM.
The CGROM (Character Generator ROM) contains the dot patterns for the characters that can be displayed. This ROM contains most of the US ASCII character set and several Japanese kana characters and symbols.
The DDRAM (Data Display RAM) is the memory where the display contents are stored. The actual content of this memory are the character codes to be displayed. The HD44780 takes care of retrieving the dot patterns from the CGROM and putting them into the display. The DDRAM and its manipulation can be a bit confusing at first, since it usually contains many more characters than can be physically displayed.
The CGRAM (Character Generator RAM) is memory that we can manipulate to create our own characters. In a pinch, it can also be used as external, general-purpose memory. Use the CGRAM as external memory requires another control line (R/W, LCD pin 5).
DISPLAY BASICS
Use of the LCD is pretty straightforward. After power-up, wait a half second or so to let the LCD run its own initialization. Since the default mode is eight bits, we'll have to reinitialize it to accept our data via the four-bit bus. When the four-bit initialization is complete, we can send our characters or commands. The RS line is set high for characters, low for LCD commands.
The initialization code is required to allow the LCD to operate in four-bit mode. After setting the four-bit interface, this section of code turns the display on, turns off the underline cursor, and causes the cursor to increment after each character is written. Just to ensure that there is no garbage left from any previous operations, the Display Clear command is sent to the LCD.
Writing a character or command is done in these steps:
1. Set the RS line (HIGH for character, LOW for command).
2. Place the high nibble of the character/command byte on the bus.
3. Strobe the Enable line (cause a HIGH-to-LOW transition).
4. Place the low nibble on the bus.
5. Strobe the Enable line one more time.
Character code of LCD
BACKLITE:
Some LCDs come with 2 extra Pins (pin nos 15 and 16) for backplane Illumination.
