2x16 LCD And 4x4 Keypad Interfacing With 8051 in Assembly Language

Shashwat Srivastava

May 24, 2009 — 5 min read

Microcontrollers are just silicon wafers until we tell them what to do, program them according to our requirement. Similarly any user interface is incomplete without an Input. One, two pushbuttons can be easily interfaced however if more user inputs are required it can take up a lot of I/O lines. So here is a small tutorial to interface a 4x4 Matrix Keypad and displaying the key pressed on a LCD. The microcontroller used is AT89C51 and the coding has been done in assembly language.

The 4x4 Keypad has 16 keys and requires a single PORT or 8 I/O lines. Port 3 has been designed to handle keypad, LCD Data Bus D7-D0 is connected to PORT 1, while

(Enable) EN is connected to P2.0

(Register Select – Command or Data Register) RS is connected to P2.1

(Read/Write) RW is connected to P2.2

The LCD is based on Hitachi HD44780 Controller and datasheet is present online.

Working:

To check for the keystroke, a polling method has been used.

PORT 3.0	Key 1	Key 2	Key 3	Key 4
PORT3.1	Key 5	Key 6	Key 7	Key 8
PORT3.2	Key 9	Key 10	Key 11	Key 12
PORT3.3	Key 13	Key 14	Key 15	Key 16
	PORT3.4	PORT3.5	PORT3.6	PORT3.7

The connections are similar as shown over here. Now consider this, if I select the first column only, it has 4 keys, 1, 5,9,13. If a change of value (i.e. Binary 1 or 0) is made any one of these keys, it can be decoded and suitable message is displayed on the LCD. This is exactly what happens. Initially all the I/O lines are pulled high, then during Key Scan, every column linked is held low for a little time. If during that time a Key is pressed in that column a row I/O lines is also held low, thus the keystroke can be captured.

The obvious question would be what if we press the key on a particular column and at that particular moment that column has not been pulled low, thus making no signal changes?

The answer is simple, the microcontroller runs quite fast, even a convention 89c51 in which the internal frequency= external frequency clock/12 can achieve 2 MIPS at 24MHz. That is 2 Million instructions Per Second. This method is not foolproof, it has a drawback, while the Key Scan, It cannot perform other cumbersome operations which may take time and a Key Stroke could be missed. The program will work very well for small operations like activating a small relay or LED when a Key is pressed, but for people who want their systems to be near to perfect they may utilize other method.

Circuit Diagram

Each I/O line on PORT 1 should be pulled high with 4.7K Resistors

Program

EN equ P2.0

RS equ P2.1

RW equ P2.2

mov A,#38H            ; Setting Up the LCD

lcall command

mov A,#0EH            ; Display On

lcall command

mov A,#06H            ; Entry Mode  

lcall command

mov a,#82H

lcall command

lcall disp            ;      Function Disp Called

mov a,#02H            ; Setting DDRAM Address to Home position

lcall command

lcall delay1

 

; Displays BOTSKOOL SHOBHIT ON FIRST LINE OF LCD

mov a,#'B'

lcall datw

NOP

mov a,#'0'

lcall datw

NOP

mov a,#'T'

lcall datw

mov a,#'S'

lcall datw

NOP

mov a,#'K'

lcall datw

NOP

mov a,#'0'

lcall datw

NOP

mov a,#'0'

lcall datw

NOP

mov a,#'L'

lcall datw

NOP

mov a,#' '

lcall datw

NOP

mov a,#'S'

lcall datw

NOP

mov a,#'H'

lcall datw

NOP

mov a,#'O'

lcall datw

NOP

mov a,#'B'

lcall datw

NOP

mov a,#'H'

lcall datw

NOP

mov a,#'I'

lcall datw

NOP

mov a,#'T'

lcall datw

MOV A,#255   ; Moving Value 255 to PORT 3

MOV P3,A

; Keypad Scan Begins

sd:

lcall delay1

lcall key1

lcall delay

lcall key2

lcall delay

lcall key3

lcall delay

lcall key4

lcall delay

lcall sd

 

 

 

 

; Function to Send Commands to LCD

command:

clr RW

clr RS

setB EN

MOV P1,A

lcall delay

clr EN

RET

 

; Function to Clear the DDRAM Content

clear:

mov A,#01H

lcall command

lcall delay

mov A,#02H ; Set The DDRAM Address to Home Position

lcall command

lcall delay

RET

; Function to Display Data on LCD Screen

datw:

SETB RS

clr RW

SETB EN

MOV P1,A

lcall delay

clr EN

RET

 

;Function to Display The Key Pressed

datw1:

lcall delay1

lcall disp

lcall delay1

MOV A,R7

lcall datw

RET

 

; Generating Small Delay

delay:

mov r0,#255

loop: DJNZ r0,loop;

RET

 

; Generating a Bigger Delay

delay1:

mov r1,#255

loop1: mov r3,#120

loop2: djnz r3,loop2 

       djnz r1,loop1

RET

 

; Checking for Key Press on The First Column of 4x4 Matrix

 

KEY1:

MOV A,r5

MOV r6,A

clr p3.4

 

MOV A,p3

ANL A,#0FH

MOV r2,A

cjne r2,#14,n1

MOV r7,#'1'

lcall datw1

lcall delay1

 

n1: cjne r2,#13,n2

mov r7,#'5'

lcall datw1

lcall delay1

n2: cjne r2,#11,n3

mov r7,#'9'

lcall datw1

lcall delay1

n3: cjne r2,#7,n4

mov r7,#'D'

lcall datw1

lcall delay1

n4: lcall delay1

SETB P3.4

RET

 

; Checking for Key Press on the Second Column of 4x4 Matrix

KEY2:

clr p3.5

MOV A,p3

ANL A,#0FH

MOV r2,A

 

cjne r2,#14,q1

mov r7,#'2'

lcall datw1

lcall delay1

q1: cjne r2,#13,q2

mov r7,#'6'

lcall datw1

lcall delay1

q2: cjne r2,#11,q3

mov r7,#65; A=65

lcall datw1

lcall delay1

q3: cjne r2,#7,q4

mov r7,#'E'

lcall datw1

lcall delay1

q4: lcall delay

SETB p3.5

RET

 

; Checking for Key Press On The Third Column of 4x4 Matrix

 

KEY3:

clr p3.6

MOV A,p3

ANL A,#0FH

MOV r2,A

 

cjne r2,#14,w1

mov r7,#'3'

lcall datw1

lcall delay1

w1: cjne r2,#13,w2

mov r7,#'7'

lcall datw1

lcall delay1

w2: cjne r2,#11,w3

mov r7,#'B'

lcall datw1

lcall delay1

w3: cjne r2,#7,w4

mov r7,#'F'

lcall datw1

lcall delay1

w4: lcall delay1

SETB p3.6

RET

 

; Checking for Key Press on the Fourth Column of 4x4 Matrix

KEY4:

clr p3.7

MOV A,p3

ANL A,#0FH

MOV r2,A

cjne r2,#14,e1

mov r7,#'4'

lcall datw1

lcall delay1

e1: cjne r2,#13,e2

mov r7,#'8'

lcall datw1

lcall delay1

e2: cjne r2,#11,e3

mov r7,#'C'

lcall datw1

lcall delay1

e3: cjne r2,#7,e4

mov r7,#'G'

lcall datw1

lcall delay1

e4: lcall delay1

SETB p3.7

RET

 

disp:

mov a,#0c0H       ; Setting DDRAM Address on Second Line

lcall command

 

; Clearing the Previous Key Pressed Information from Screen

mov a,#' '

lcall datw

mov a,#' '

lcall datw

mov a,#' '

lcall datw

mov a,#' '

lcall datw

mov a,#' '

lcall datw

mov a,#' '

lcall datw

 

mov a,#0c0H    ; Setting DDRAM Address on Second Line To Display “Key Pressed”

lcall command

 

; Display "KEY" and Pressed Information

mov a,#' '

lcall datw

mov a,#'K'

lcall datw

mov a,#'E'

lcall datw

mov a,#'Y'

lcall datw

mov a,#' '

lcall datw

RET

END

The code was written when I was learning assembly language myself and therefore the code is not optimized, but it is easy to understand if someone is willing to check the instruction set.

People who want to optimize the code may wish to look into the DPTR Register and Addressing Modes Theory.