LCD Display
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.
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A 16x2 line
Hitachi HD44780 display |
Each of the 640 pixels of the display
must be accessed individually and this is done with a number of
surface-mount driver/controller chips mounted on the back of the display.
This saves an enormous amount of wiring and controlling so that only a few
lines are required to access the display to the outside world. We can
communicate to the display via an 8-bit data bus or 4-bit data bus.
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 impedance (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.
The Enable (E) 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.
The Read.Write (R/W) 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.
With the help of the Register select (RS) 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,
labelled 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.
Macro for LCD support
LCDinit macro used to initialize port connected to LCD. LCD
is configured to work in four-bit mode.
Example: LCDinit
LCDchar LCDarg Write ASCII character. Argument is ASCII
caracter.
Example: LCDChar 'd'
LCDw Write character found in W register.
Example: movlw 'p'
LCDw
LCDcmd LCDcommand Sending command instructions
Example: LCDcmd LCDCH
LCD_DDAdr DDRamAddress Set DD RAM address.
Example: LCD_DDAdr .3
LCDline line_num Set cursor to the beginning of 1st or 2nd row
Example: LCDline 2
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. Listings of two macros LCDval_08 and LCDval_16
are given below.
Macro LCDval_08 converts an eight-bit binary number into a decimal
number from 0 to 255 and displays it on the LCD display. It is necessary
to declare the following variables in the main program: TEMP1, TEMP2, LO,
LO_TEMP, Bcheck. An eight-bit binary number is found in variable LO. When
a macro was executed, the decimal equivalent of its number would be
displayed on the LCD display. The leading zeros before the number will not
be displayed.
Macro LCDval_16 converts 16-bit
binary number into decimal number from 0 to 65535 and displays it on LCD
display. The following variables need to be declared in the main program:
TEMP1, TEMP2, TEMP3, LO, HI, LO_TEMP, HI_TEMP, Bcheck. A 16-bit binary
number is found in variables LO and HI. When a macro was executed, a decimal
equivalent of this number would be displayed on LCD display. The leading
zeros before the number would not be displayed.
The main program is a demonstration of
using the LCD display and generate new characters. At the beginning of a
program, we need to declare variables LCDbuf and LCDtemp used
by subprograms for the LCD as well as the microcontroller port connected to
the LCD.
The program writes the message 'characters:' on the first row and shows two
special characters '~' and '}'. In the second row, 'mikroElektronika' is
displayed.
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