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7.9 LCD Display
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More microcontroller devices are using 'smart LCD' displays to
output visual information. The following discussion covers the
connection of a Hitachi LCD display to a PIC microcontroller.
LCD displays designed around Hitachi's LCD HD44780 module, are
inexpensive, easy to use, and it is even possible to produce a
readout using the 8 x 80 pixels of the display. Hitachi LCD displays
have a standard ASCII set of characters plus Japanese, Greek and
mathematical symbols. |
 |
| A
16x2 line Hitachi HD44780 display |
For a 8-bit data bus, the display requires a +5V supply plus 11 I/O lines.
For a 4-bit data bus it only requires the supply lines plus seven extra
lines. When the LCD display is not enabled, data lines are tri-state which
means they are in a state of high impendance (as though they are
disconnected) and this means they do not interfere with the operation of
the microcontroller when the display is not being addressed.
The LCD also requires 3 "control" lines from the microcontroller.
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Enable (E) |
This line allows access to the display through R/W and RS
lines. When this line is low, the LCD is disabled and ignores signals from
R/W and RS. When (E) line is high, the LCD checks the state of the two
control lines and responds accordingly. |
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Read/Write (R/W) |
This
line determines the direction of data between the LCD and
microcontroller. When it is low, data is written to the LCD. When it
is high, data is read from the LCD. |
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Register select (RS) |
With the
help of this line, the LCD interprets the type of data on data
lines. When it is low, an instruction is being written to the LCD.
When it is high, a character is being written to the LCD. |
Logic status on control lines:
E 0 Access to LCD disabled
1 Access to LCD enabled
R/W 0 Writing data to LCD
1 Reading data from LCD
RS 0 Instruction
1 Character
Writing data to the LCD is done in several steps:
Set R/W bit to low
Set RS bit to logic 0 or 1 (instruction or character)
Set data to data lines (if it is writing)
Set E line to high
Set E line to low
Read data from data lines (if it is reading)
Reading data from the LCD is done in the same way, but control line R/W
has to be high. When we send a high to the LCD, it will reset and wait for
instructions. Typical instructions sent to LCD display after a reset are:
turning on a display, turning on a cursor and writing characters from left
to right. When the LCD is initialized, it is ready to continue receiving data or
instructions. If it receives a character, it will write it on the display
and move the cursor one space to the right. The Cursor marks the next
location where a character will be written. When we want to write a string
of characters, first we need to set up the starting address, and then send
one character at a time. Characters that can be shown on the display are
stored in data display (DD) RAM. The size of DDRAM is 80 bytes.
| The LCD display also
possesses 64 bytes of Character-Generator (CG) RAM. This memory is
used for characters defined by the user. Data in CG RAM is
represented as an 8-bit character bit-map. Each character takes up 8 bytes of CG RAM, so the total number of
characters, which the user can define is eight. In order to read in
the character bit-map to the LCD display, we must first set the CG
RAM address to starting point (usually 0), and then write data to
the display. The definition of a 'special' character is given in the
picture. |
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Before we access DD RAM after defining a
special character, the program must set the DD RAM address. Writing and
reading data from any LCD memory is done from the last address which was
set up using set-address instruction. Once the address of DD RAM is set, a
new written character will be displayed at the appropriate place on the
screen.
Until now we discussed the operation of writing and reading to an LCD as
if it were an ordinary memory. But this is not so. The LCD controller
needs 40 to 120 microseconds (uS) for writing and reading. Other
operations can take up to 5 mS. During that time, the microcontroller can
not access the LCD, so a program needs to know when the LCD is busy. We
can solve this in two ways.
One way is to check the BUSY bit found on
data line D7. This is not the best method because LCD's can get stuck, and
program will then stay forever in a loop checking the BUSY bit. The other
way is to introduce a delay in the program. The delay has to be long
enough for the LCD to finish the operation in process. Instructions for
writing to and reading from an LCD memory are shown in the previous table.
At the beginning we mentioned that we needed 11 I/O lines to communicate
with an LCD. However, we can communicate with an LCD through a 4-bit data
bus. Thus we can reduce the total number of communication lines to seven.
The wiring for connection via a 4-bit data bus is shown in the diagram
below. In this example we use an LCD display with 2x16 characters,
labeled LM16X212 by Japanese maker SHARP. The message 'character' is
written in the first row: and two special characters '~' and '}' are
displayed. In the second row we have produced the word 'mikroElektronika'.
Connecting an LCD
display to a microcontroller
File lcd.inc contains a group
of macros for use when working with LCD displays.
Using the macro for LCD support
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lcdinit |
Macro used to initialize
port connected to LCD. LCD
is configured to work in 4-bit mode. |
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Example: |
lcdinit |
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lcdtext |
lcdtext prints the text of up to 16
characters, which is specified as a macro parameter. First parameter
selects the line in which to start printing. If select is zero, text
is printed from the current cursor position. |
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Example: |
lcdtext 1, "mikroelektronika" |
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lcdtext 1, "Temperature1" ;Print
the text starting from line 1, character 1 |
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lcdtext 2, "temp="
;Print the text starting from line 2, character 1 |
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lcdtext 0, " C"
;Print C in the rest of the line 2 |
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lcdcmd |
Sends command instructions |
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LCDCLR |
= b'00000001' |
;Clear display, cursor home |
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LCDCH |
= b'00000010' |
;Cursor home |
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LCDCL |
= b'00000100' |
;Move the cursor to the left |
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LCDCR |
= b'00000110' |
;Move the cursor to the right |
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LCDSL |
= b'00011000' |
;Move the content of display to the
left |
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LCDSR |
= b'00011100' |
;Move the content of display to the
right |
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LCDL1 |
= b'10000000' |
;Select line 1 |
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LCDL2 |
= b'11000000' |
;Select line 2 |
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Example: |
lcdcmd LCDCH |
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lcdbyte |
Prints one byte variable and omits
leading zeros |
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Example: |
lcdbyte Temperature |
When working with a microcontroller the
numbers are presented in a binary form. As such, they cannot be displayed
on a display. That's why it is necessary to change the numbers from a
binary system into a decimal system so they can be easily understood. For
printing the variables lcdbyte and lcdword we have used the macros
digbyte and digword which convert the numbers from binary
system into a decimal system and print the result on LCD. Main program has
the purpose of demonstrating use of LCD display. At the start it's
necessary to declare variables LCDbuf, LCDtemp, Digtemp, Dig1, Dig2, and
Dig3 used by the macros for LCD support. It is also necessary to state the
port of microcontroller that LCD is connected to. Program initializes the
LCD and demonstrates printing text and 8-bit variable temp.
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