By Abdul Rehman
Today we are talking about interfacing a 4×3 keypad with 8051 microcontrollers in assembly language programming for the 8051 microcontrollers. The 8051 microcontroller is one of the simplest to use the 4×3 keypad or any other matrix keypad like 4×3 4×4 or any other matrix keypad. By calling the matrix keypad what I actually mean is the keypad which is made in terms of matrix-based switches. So every key on that keypad is basically the combination of rows and collumns. So to use that keypad with a 4×3 matrix, all we need to do is to use the 7 IO pins of the microcontroller and we are done.
If you like to do the traditional way of interfacing the 4×3 = 12 keys with any of the microcontrollers you need one IO Pin per switch so 12 IO pins will be used to read 12 keys. By doing the previous work and the learning of the microcontrollers you may already be aware that in Embedded systems the iO pins are very costly due to the low pin counts on most of the microcontrollers or the microprocessors. And sometimes it also becomes very costly and sometimes the requirement of the design doesn’t even allow to add external IO expanders ICs with the microcontroller or anything external shift registers as we did in one of our previous Arduino based 7 segment interfacing with the shift registers.
Here is the full assembly language code of the matrix 4×3 keypad interfacing with the 8051 microcontrollers.
Code language: PHP (php)
;================================================ ; ; THIS PROGRAM WILL READ 4X3 KEYPAD AND ; DISPLAY OUTPUT ON P0 AS BINARY OUTPUT ; ;================================================= KEYPAD_PORT EQU P1 LEDS_PORT EQU P0 ;================================================ ORG 00H ;=============================================== KEYSCAN: MOV KEYPAD_PORT,#00001111B MOV A,KEYPAD_PORT CJNE A,#00001111B,KEYSCAN K2: ACALL DELAY1 ;CHECK IF A KEY IS PRESSED MOV A,KEYPAD_PORT CJNE A,#00001111B,OVER SJMP K2 OVER: ACALL DELAY1 MOV A,KEYPAD_PORT CJNE A,#00001111B,OVER1 SJMP K2 OVER1: MOV KEYPAD_PORT,#11101111B ;TO CHECK IF PRESSED KEY IS FROM ROW0 MOV A,KEYPAD_PORT CJNE A,#11101111B,ROW_0 MOV KEYPAD_PORT,#11011111B MOV A,KEYPAD_PORT CJNE A,#11011111B,ROW_1 MOV KEYPAD_PORT,#10111111B MOV A,KEYPAD_PORT CJNE A,#10111111B,ROW_2 MOV KEYPAD_PORT,#01111111B MOV A,KEYPAD_PORT CJNE A,#01111111B,ROW_3 SJMP K2 ROW_0: MOV DPTR,#KCODE0 SJMP FIND ROW_1: MOV DPTR,#KCODE1 SJMP FIND ROW_2: MOV DPTR,#KCODE2 SJMP FIND ROW_3: MOV DPTR,#KCODE3 FIND: RRC A JNC MATCH INC DPTR SJMP FIND MATCH: CLR A MOVC A,@A+DPTR CPL A MOV LEDS_PORT,A SJMP KEYSCAN ;======== DELAYS ================================ DELAY1: MOV R7,#255 DJNZ R7,$ RET ;======== CODES ================================= KCODE0: DB 0FFH, 01H, 02H, 03H KCODE1: DB 0FFH, 04H, 05H, 06H KCODE2: DB 0FFH, 07H, 08H, 09H KCODE3: DB 0FFH, 0AH, 00H, 0BH END
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