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Liquid Crystal Display also called as LCD is very helpful in providing user interface as well as for debugging purpose. The most common type of LCD controller is HITACHI 44780 which provides a simple interface between the controller & an LCD. These LCD's are very simple to interface with the controller as well as are cost effective.
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The LCD requires 3 control lines (RS, R/W & EN) & 8 (or 4) data lines. The number on data lines depends on the mode of operation. If operated in 8-bit mode then 8 data lines plus 3 control lines i.e. total 11 lines are required.
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In case of 4-bit mode only 4 data lines are required & the 8-bit data is sent by first sending the higher nibble followed by the lower nibble. Since we are operating the LCD in 4 bit mode only 7 port pins are required. Here is a program for interfacing a LCD through 8051 in 4 bit mode. The Higher 4 bits of P0 are connected to the higher 4 bits of the LCD i.e. P0.4 – P0.7 are connected to D4 – D7.
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The three control lines are RS, RW & Enable.
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RS: Register Select. When RS = 0 the command register is selected. When RS=1 the data register is selected.
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RW: Read or Write. When RW=0 write to LCD. When RW=1 read from LCD.
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Enable: Used to Latch data into the LCD. A HIGH TO LOW edge latches the data into the LCD.
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- RS: Register Select. When RS = 0 the command register is selected. When RS=1 the data register is selected.
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| LCD_RS |
BIT |
P2.5 |
| LCD_RW |
BIT |
P2.6 |
| LCD_E |
BIT |
P2.7 |
| LCD_PORT |
EQU |
P0 //the higher nibble i.e. P0.4 - P0.7 is used |
| ORG 0000h |
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| mov a,#20h
call lcd_command
mov a,#28h
call lcd_command
mov a,#06h
call lcd_command
mov a,#01h
call lcd_command
mov a,#0ch
call lcd_command
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mov a,#86h
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call lcd_command
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//1st line 6th character |
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mov a,#'D'
call lcd_datadisplay
mov a,#'N'
call lcd_datadisplay
mov a,#'A'
call lcd_datadisplay
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mov a,#0c3h
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| call lcd_command |
//2nd Line 3rd character |
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mov a,#'T'
call lcd_datadisplay
mov a,#'E'
call lcd_datadisplay
mov a,#'C'
call lcd_datadisplay
mov a,#'H'
call lcd_datadisplay
mov a,#'N'
call lcd_datadisplay
mov a,#'O'
call lcd_datadisplay
mov a,#'L'
call lcd_datadisplay
mov a,#'O'
call lcd_datadisplay
mov a,#'G'
call lcd_datadisplay
mov a,#'Y'
call lcd_datadisplay
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loop:
ajmp loop
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lcd_command:
mov b,a
call lcd_ready
clr lcd_rw
clr lcd_rs
orl lcd_port,#0f0h
mov a,b
orl a,#0fh
anl lcd_port,a
setb lcd_e
clr lcd_e
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orl lcd_port,#0f0h
mov a,b
swap a
orl a,#0fh
anl lcd_port,a
setb lcd_e
clr lcd_e
ret
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lcd_datadisplay:
mov b,a
call lcd_ready
clr lcd_rw
setb lcd_rs
orl lcd_port,#0f0h
mov a,b
orl a,#0fh
anl lcd_port,a
setb lcd_e
clr lcd_e
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orl lcd_port,#0f0h
mov a,b
swap a
orl a,#0fh
anl lcd_port,a
setb lcd_e
clr lcd_e
ret
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lcd_ready:
clr lcd_e
orl lcd_port,#0f0h
clr lcd_rs
setb lcd_rw
setb lcd_e
mov a,lcd_port
clr lcd_e
anl a,#0f0h
mov temp,a
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orl lcd_port,#0f0h
setb lcd_e
mov a,lcd_port
clr lcd_e
anl a,#0f0h
swap a
orl a,temp
jb acc.7,lcd_ready
ret
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