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Keyboard is a basic and essential element of an embedded or microcontroller system. For small and hobby projects a 4x4 (hex) matrix keyboard is sufficient. The keys in the matrix keyboard are arranged in a matrix arrangement in order to utilize the port pins of the microcontrollers efficiently. Here we can interface 16 keys by using just 8 pins of microcontroller. This is shown in the circuit diagram.

As shown in the circuit diagram, the rows are connected to 4 pins of a port, the columns are connected to other four pins, we configure rows as input and columns as output. By grounding one column and setting high all other columns, now by observing the status of the rows, we can come to know which button is pressed. For example, if we ground the first column and if the first row is low, then the first button is pressed. We know that microcontrollers are really fast, therefore they can detect the key press in microseconds, if we hold the switch for long time (for a microcontroller long time is in milliseconds), the microcontroller is triggered more than once, also there is a problem of switch debounce because of the spring action of the switch. To eliminate these problems we should introduce some amount of delay after every key press. The assembly source code for the circuit is given in the table below.

Circuit diagram for interfacing 4x4 keyboard to 8051(89s52):

4x4 matrix keyboard interface circuit


My prototype for connecting to the main development board:

4x4 matrix keyboard addon

The assembly program for interfacing 4x4 keyboard:

;Program: Blinking LED.
;Author: Srikanth
;Description: Displays the button pressed on the
;keyboard on a 7 segment display.
;keyboard format: 0 1 2 3
;                 4 5 6 7
;                 8 9 A B
;                 C D E F

rw0 equ P2.0;
rw1 equ P2.1;
rw2 equ P2.2;
rw3 equ P2.3;
cl0 equ P2.4;
cl1 equ P2.5;
cl2 equ P2.6;
cl3 equ P2.7;
;Main program

org 00h
ljmp main

org 30h
main: mov P2,#0ffh       ;Configure input
acall scan_key     ;Scan for keypress
acall display      ;display the key pressed
sjmp main          ;Loop

;Subroutine to scan keys
scan_key:mov P2,#0ffh
clr cl0
mov a, P2
anl a,#00001111b
cjne a,#00001111b,Row0
setb cl0
clr cl1
mov a, P2
anl a,#00001111b
cjne a,#00001111b,Row1
setb cl1
clr cl2
mov a, P2
anl a,#00001111b
cjne a,#00001111b,Row2
setb cl2
clr cl3
mov a, P2
anl a,#00001111b
cjne a,#00001111b,Row3
setb cl3

row0: mov dptr,#led_data
mov r6,#04h
clr c
rww0: rrc a
jc next0
sjmp over
next0:inc dptr
djnz r6,rww0
sjmp scan_key

row1: mov dptr,#led_data+4h
mov r6,#04h
clr c
rww1: rrc a
jc next1
sjmp over
next1:inc dptr
djnz r6,rww1
sjmp scan_key

row2: mov dptr,#led_data+8h
mov r6,#04h
clr c
rww2: rrc a
jc next2
sjmp over
next2:inc dptr
djnz r6,rww2
sjmp scan_key

row3: mov dptr,#led_data+0ch
mov r6,#04h
clr c
rww3: rrc a
jc next3
sjmp over
next3:inc dptr
djnz r6,rww3
sjmp scan_key

over: ret
;Display subroutine
display:clr a
movc a,@a+dptr
mov P0,a

;lookup table
led_data:db 0fch,066h,0feh,09dh; For row1:0 1 2 3
db 060h,0b6h,0f6h,0fdh; For row2:4 5 6 7
db 0dah,0beh,0efh,09fh; For row3:8 9 A B
db 0f2h,0e0h,0ffh,08fh; For row4:C D E F