Interfacing hex keypad using 8051

x keypad.

Hex key pad is essentially a collection of 16 keys arranged in the form of a 4×4 matrix. Hex key pad usually have keys representing numerics 0 to 9 and characters A to F. The simplified diagram of a typical hex key pad is shown in the figure below.

Hex keypad

The hex keypad has 8 communication lines namely R1, R2, R3, R4, C1, C2, C3 and C4.  R1 to R4 represents the four rows and C1 to C4 represents the four columns. When a particular key is pressed the corresponding row and column to which the terminals of the key are connected gets shorted. For example if key 1 is pressed row R1 and column C1 gets shorted and so on. The program identifies which key is pressed by a method known as column scanning. In this method a particular row is kept low (other rows are kept high) and the columns are checked for low. If a particular column is found low then that means that the key connected between that column and the corresponding row (the row that is kept low) is been pressed. For example if  row R1 is initially kept low and column C1 is found low during scanning, that means key 1 is pressed.

Interfacing hex keypad to 8051.

The circuit diagram for demonstrating interfacing hex keypad to 8051 is shown below.Like previous 8051 projects, AT89S51 is the microcontroller used here. The circuit  will display the character/numeric pressed on a seven segment LED display. The circuit is very simple and it uses only two ports of the microcontroller, one for the hex keypad and the other for the seven segment LED display.

Interfacing hex keypad to 8051

 The hex keypad is interfaced to port 1 and seven segment LED display is interfaced to port 0 of the microcontroller. Resistors R1 to R8 limits the current through the corresponding segments of the LED display. Capacitors C1, C2 and crystal X1 completes the clock circuitry for the microcontroller. Capacitor C3, resistor R9 and push button switch S1 forms a debouncing reset mechanism.

Program.

ORG 00H

MOV DPTR,#LUT // moves starting address of LUT to DPTR

MOV A,#11111111B // loads A with all 1's

MOV P0,#00000000B // initializes P0 as output port

BACK:MOV P1,#11111111B // loads P1 with all 1's

     CLR P1.0  // makes row 1 low

     JB P1.4,NEXT1  // checks whether column 1 is low and jumps to NEXT1 if not low

     MOV A,#0D   // loads a with 0D if column is low (that means key 1 is pressed)

     ACALL DISPLAY  // calls DISPLAY subroutine

NEXT1:JB P1.5,NEXT2 // checks whether column 2 is low and so on...

      MOV A,#1D

      ACALL DISPLAY

NEXT2:JB P1.6,NEXT3

      MOV A,#2D

      ACALL DISPLAY

NEXT3:JB P1.7,NEXT4

      MOV A,#3D

      ACALL DISPLAY

NEXT4:SETB P1.0

      CLR P1.1

      JB P1.4,NEXT5

      MOV A,#4D

      ACALL DISPLAY

NEXT5:JB P1.5,NEXT6

      MOV A,#5D

      ACALL DISPLAY

NEXT6:JB P1.6,NEXT7

      MOV A,#6D

      ACALL DISPLAY

NEXT7:JB P1.7,NEXT8

      MOV A,#7D

      ACALL DISPLAY

NEXT8:SETB P1.1

      CLR P1.2

      JB P1.4,NEXT9

      MOV A,#8D

      ACALL DISPLAY

NEXT9:JB P1.5,NEXT10

      MOV A,#9D

      ACALL DISPLAY

NEXT10:JB P1.6,NEXT11

       MOV A,#10D

       ACALL DISPLAY

NEXT11:JB P1.7,NEXT12

       MOV A,#11D

       ACALL DISPLAY

NEXT12:SETB P1.2

       CLR P1.3

       JB P1.4,NEXT13

       MOV A,#12D

       ACALL DISPLAY

NEXT13:JB P1.5,NEXT14

       MOV A,#13D

       ACALL DISPLAY

NEXT14:JB P1.6,NEXT15

       MOV A,#14D

       ACALL DISPLAY

NEXT15:JB P1.7,BACK

       MOV A,#15D

       ACALL DISPLAY

       LJMP BACK

DISPLAY:MOVC A,@A+DPTR // gets digit drive pattern for the current key from LUT

        MOV P0,A       // puts corresponding digit drive pattern into P0

        RET

LUT: DB 01100000B // Look up table starts here

     DB 11011010B

     DB 11110010B

     DB 11101110B

     DB 01100110B

     DB 10110110B

     DB 10111110B

     DB 00111110B

     DB 11100000B

     DB 11111110B

     DB 11110110B

     DB 10011100B

     DB 10011110B

     DB 11111100B

     DB 10001110B

     DB 01111010B

     END

About the program.

Firstly the program initializes port 0 as an output port by writing all 0′s to it and port 1 as an input port by writing all 1′s to it. Then the program makes row 1 low by clearing P1.0 and scans the columns one by one for low using JB instruction.If column C1 is found low, that means 1 is pressed and accumulator is loaded by zero and DISPLAY subroutine is called. The display subroutine adds the content in A with the starting address of LUT stored in DPTR and loads A with the data to which the resultant address points (using instruction MOVC A,@A+DPTR). The present data in A will be the digit drive pattern for the current key press and this pattern is put to Port 0 for display. This way the program scans for each key one by one and puts it on the display if it is found to be pressed.

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