Interfacing Keybord With Microcontroller

Keyboard is a basic and essential element of an embedded or microcontroller system. For small and hobby projects a 4×4 (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 4×4 keyboard to 8051(89s52):

My prototype for connecting to the main development board:

The assembly program for interfacing 4×4 keyboard:

;************************************************* ;Program: Blinking LED. ;Author: Srikanth ;Website: http://shree-electronics.com/ ;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 ;************************************************* ;Declarations 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 ret 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 ret ;************************************************* ;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 ;************************************************* end;

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