07-06-2013, 12:52 PM
INTRODUCTION TO LIQUID CRYSTAL DISPLAY
[attachment=54933]
INTRODUCTION
Liquid crystal display a type of display used in digital watches and many portable computers.
LCD displays utilize two sheets of polarizing material with a liquid crystal solution between them. An electric
current passed through the liquid causes the crystals to align so that light cannot pass through them. Each
crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light.
LCDs have become very popular over recent years for information display in many smart
’appliances. They are usually controlled by microcontrollers. They make complicated equipment easier to
operate. LCDs come in many shapes and sizes but the most common is the 20 character x 4 line display
with no backlight. It requires only 11 connections – eight bits for data (which can be reduced to four if
necessary) and three control lines (we have only used two here). It runs off a 5V DC supply and only needs
about 1mA of current. The display contrast can be varied by changing the voltage into pin 3 of the display,
usually with a trim pot.
Description for interfacing of LCD:
To get the display working requires eight bits of data, a register select line (RS) and a strobe line (E). A third
input, R/W, is used to read or write data to/from the LCD. The eight bits of data are supplied from the
controller port data lines and two controller port control lines are used for RS (‘auto’) and E (‘strobe’).
Basically the LCD has two registers, a data register and a command register. Data is written into the
command register when RS is low and into the data register when RS is high. Data is latched into the LCD
register on the falling edge of Enable’.
The sequence for writing to the LCD is:
1. To begin, E is low.
2. Select the register to write to by setting RS high (Data) or low (Command).
3. To write RW is low.
4. Write the eight bits of data to the LCD.
5. Set the Enable signal high then low again.
There are certain minimum timing requirements that must be followed when writing to the LCD,
such as data setup times and Enable signal pulse width. These are in the order of tens and hundreds
ofnanoseconds. LCD displays have a ‘busy’ flag that is set while it is executing a control command. This
flag is accessible only when the R/W line is tied high (read). This imply wait for time to pass before
accessing the LCD again, the controller need to check the flag and thereby saving on an I/O line! Eg: The
‘Clear Display’ command has an execution time of approx. 1.6mS. After sending this command to the LCD
we simply wait until the busy flag is cleared before continuing on. This ensures that the command has
finished. Characters to be displayed are written into the LCD’s ‘data’ RAM memory.
The amount of RAM available depends on the type of LCD. The LCD used in this application has80
bytes of RAM. An internal address counter holds the address of the next byte to write to. These 80 bytes
are broken up into two blocks of 40 bytes. The address range of the first block is from 80h to 8Fh and the
second block from C0h to CFh. After power up and initialization address 80h is the first character of the top
line and address C0h is the first character of the bottom line The address counter is set to address 80h and
is automatically incremented after each byte is written. Only the first 16 bytes of each line are visible. If we
now start writing data to the LCD it will be stored from address 80h on but only the first 16 characters will
be visible. To display the rest of the characters we need to ‘scroll’ the display. Scrolling simply means
changing the start address of each line. If we scroll left one position address 01h becomes the first
character of the top line and address C1h the first character of the bottom line. Scrolling right does the
opposite – addresses 8Fh and CFh become the first characters of the top and bottom lines.
[attachment=54933]
INTRODUCTION
Liquid crystal display a type of display used in digital watches and many portable computers.
LCD displays utilize two sheets of polarizing material with a liquid crystal solution between them. An electric
current passed through the liquid causes the crystals to align so that light cannot pass through them. Each
crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light.
LCDs have become very popular over recent years for information display in many smart
’appliances. They are usually controlled by microcontrollers. They make complicated equipment easier to
operate. LCDs come in many shapes and sizes but the most common is the 20 character x 4 line display
with no backlight. It requires only 11 connections – eight bits for data (which can be reduced to four if
necessary) and three control lines (we have only used two here). It runs off a 5V DC supply and only needs
about 1mA of current. The display contrast can be varied by changing the voltage into pin 3 of the display,
usually with a trim pot.
Description for interfacing of LCD:
To get the display working requires eight bits of data, a register select line (RS) and a strobe line (E). A third
input, R/W, is used to read or write data to/from the LCD. The eight bits of data are supplied from the
controller port data lines and two controller port control lines are used for RS (‘auto’) and E (‘strobe’).
Basically the LCD has two registers, a data register and a command register. Data is written into the
command register when RS is low and into the data register when RS is high. Data is latched into the LCD
register on the falling edge of Enable’.
The sequence for writing to the LCD is:
1. To begin, E is low.
2. Select the register to write to by setting RS high (Data) or low (Command).
3. To write RW is low.
4. Write the eight bits of data to the LCD.
5. Set the Enable signal high then low again.
There are certain minimum timing requirements that must be followed when writing to the LCD,
such as data setup times and Enable signal pulse width. These are in the order of tens and hundreds
ofnanoseconds. LCD displays have a ‘busy’ flag that is set while it is executing a control command. This
flag is accessible only when the R/W line is tied high (read). This imply wait for time to pass before
accessing the LCD again, the controller need to check the flag and thereby saving on an I/O line! Eg: The
‘Clear Display’ command has an execution time of approx. 1.6mS. After sending this command to the LCD
we simply wait until the busy flag is cleared before continuing on. This ensures that the command has
finished. Characters to be displayed are written into the LCD’s ‘data’ RAM memory.
The amount of RAM available depends on the type of LCD. The LCD used in this application has80
bytes of RAM. An internal address counter holds the address of the next byte to write to. These 80 bytes
are broken up into two blocks of 40 bytes. The address range of the first block is from 80h to 8Fh and the
second block from C0h to CFh. After power up and initialization address 80h is the first character of the top
line and address C0h is the first character of the bottom line The address counter is set to address 80h and
is automatically incremented after each byte is written. Only the first 16 bytes of each line are visible. If we
now start writing data to the LCD it will be stored from address 80h on but only the first 16 characters will
be visible. To display the rest of the characters we need to ‘scroll’ the display. Scrolling simply means
changing the start address of each line. If we scroll left one position address 01h becomes the first
character of the top line and address C1h the first character of the bottom line. Scrolling right does the
opposite – addresses 8Fh and CFh become the first characters of the top and bottom lines.