LCD stands for Liquid Crystal Display. LCD is finding wide spread use replacing LEDs (seven segment LEDs or other multi segment LEDs) because of the following reasons:
1. The declining prices of LCDs.
2. The ability to display numbers, characters and graphics. This is in contrast to LEDs, which are limited to numbers and a few characters.
3. Incorporation of a refreshing controller into the LCD, thereby relieving the CPU of the task of refreshing the LCD. In contrast, the LED must be refreshed by the CPU to keep displaying the data.
4. Ease of programming for characters and graphics.
These components are “specialized” for being used with the microcontrollers, which means that they cannot be activated by standard IC circuits. They are used for writing different messages on a miniature LCD
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Fig LCD Display
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A model described here is for its low price and great possibilities most frequently used in practice. It is based on the HD44780 microcontroller (Hitachi) and can display messages in two lines with 16 characters each. It displays all the alphabets, Greek letters, punctuation marks, mathematical symbols etc. In addition, it is possible to display symbols that user makes up on its own. Automatic shifting message on display (shift left and right), appearance of the pointer, backlight etc. are considered as useful characteristics.
Pins Functions
There are pins along one side of the small printed board used for connection to the microcontroller. There are total of 14 pins marked with numbers (16 in case the background light is built in). Their function is described in the table below:
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| Table Pin Functions |
LCD screen
LCD screen consists of two lines with 16 characters each. Each character consists of 5x7 dot matrix. Contrast on display depends on the power supply voltage and whether messages are displayed in one or two lines. For that reason, variable voltage 0-Vdd is applied on pin marked as Vee. Trimmer potentiometer is usually used for that purpose. Some versions of displays have built in backlight (blue or green diodes). When used during operating, a resistor for current limitation should be used (like with any LE diode).
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Fig LCD screen
LCD
Basic Commands
All data transferred to LCD
through outputs D0-D7 will be interpreted as commands or as data, which depends
on logic state on pin RS:
RS = 1 - Bits D0 - D7 are
addresses of characters that should be displayed. Built in processor addresses
built in “map of characters” and displays corresponding symbols. Displaying
position is determined by DDRAM address. This address is either previously
defined or the address of previously transferred character is automatically
incremented.
RS = 0 - Bits D0 - D7 are
commands which determine display mode. List of commands which LCD recognizes
are given in the table below:
I/D 1 =
Increment (by 1) R/L
1 = Shift right
0 = Decrement (by 1) 0
= Shift left
S 1 =
Display shift on DL 1 = 8-bit interface
0 = Display shift off 0 = 4-bit interface
D
1 = Display on N
1 = Display in two lines
0 = Display off 0 = Display in one line
U
1 = Cursor on F
1 = Character format 5x10 dots
0 = Cursor off 0 = Character format 5x7 dots
B 1 =
Cursor blink on D/C
1 = Display shift
0 = Cursor blink off 0 = Cursor shift
Table LCD Basic Commands
LCD Initialization
Once the power supply is
turned on, LCD is automatically cleared. This process lasts for approximately
15mS. After that, display is ready to operate. The mode of operating is set by
default. This means that:
1. Display is cleared
2. Mode
DL = 1 Communication through 8-bit interface
N = 0 Messages are displayed in one line
F = 0 Character font 5 x 8 dots
3. Display/Cursor on/off
D = 0 Display off
U = 0 Cursor off
B = 0 Cursor blink off
4. Character entry
ID = 1 Addresses on display are automatically
incremented by 1
S = 0 Display shift off
Automatic reset is mainly
performed without any problems. Mainly but not always! If for any reason power
supply voltage does not reach full value in the course of 10mS, display will start perform completely
unpredictably. If voltage supply unit can not meet this condition or if it is
needed to provide completely safe operating, the process of initialization by
which a new reset enabling display to operate normally must be applied.
Algorithm according to the initialization is being performed depends on whether
connection to the microcontroller is through 4- or 8-bit interface. All left
over to be done after that is to give basic commands and of course- to display
messages.