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Interfacing the Keyboard to 8051 microcontroller

 By Nagabhooshan S.Shet     

           The key board here we are interfacing is a matrix keyboard. This key board is designed with a particular rows and columns. These rows and columns are connected to the microcontroller through its ports of the micro controller 8051. We normally use 8*8 matrix key board. So only two ports of 8051 can be easily connected to the rows and columns of the key board.

          When ever a key is pressed, a row and a column gets shorted through that pressed key and all the other keys are left open.  When a key is pressed only a bit in the port goes high.  Which indicates microcontroller  that the key is pressed. By this high on the bit key in the corresponding column is identified.

          Once we are sure that one of key in the key board is pressed next our aim is to identify that key.  To do this we firstly check for particular row and then we check the corresponding column the key board.

          To check the row of the pressed key in the keyboard, one of the row is made high by making one of bit in the output port of 8051 high .  This is done until the row is found out.  Once we get the row next out job is to find out the column of the pressed key. The column  is detected by contents in the input ports with the help of a counter. The content of the input port is rotated with carry until the carry bit is set.

          The contents of the counter is then compared and displayed in the display. This display is designed using a seven segment display and a BCD to seven segment decoder IC 7447.

          The BCD equivalent number of counter is sent through output part of 8051 displays the number of pressed key.

          Circuit diagram of INTERFACING  KEY BOARD TO 8051.

The programming algorithm, program and the circuit diagram is as follows. Here program is explained with comments.

Circuit diagram of INTERFACING  KEY BOARD TO 8051.

Keyboard is organized in a matrix of rows and columns as shown in the figure. The microcontroller accesses both  rows and columns through the port. 

  1. The 8051 has 4 I/O ports P0 to P3 each with 8 I/O pins, P0.0 to P0.7,P1.0 to P1.7, P2.0 to P2.7, P3.0 to P3.7. The one of the port P1 (it understood that P1 means P1.0 to P1.7) as an I/P port for microcontroller 8051, port P0 as an O/P port of microcontroller 8051 and port P2 is used for displaying the number of pressed key.

  2.  Make all rows of port P0 high so that it gives high signal when key is pressed.

  3.  See if any key is pressed by scanning the port P1 by checking all columns for non zero condition.

  4.  If any key is pressed, to identify which key is pressed make one row high at a time.

  5.  Initiate a counter to hold the count so that each key is counted.

  6.  Check port P1 for nonzero condition. If any nonzero number is there in [accumulator], start column scanning by following step 9.

  7.  Otherwise make next row high in port P1.

  8.  Add a count of 08h to the counter to move to the next row by repeating steps from step 6.

  9.  If any key pressed is found, the [accumulator] content is rotated right through the carry until carry bit sets, while doing this increment the count in the counter till carry is found.

  10.  Move the content in the counter to display in data field or to   memory location

  11.  To repeat the procedures go to step 2. 

Program to interface matrix keyboard to microcontroller 8051

Start of main program:   

to check that whether any key is pressed

                        start:   mov a,#00h

                                    mov p1,a             ;making all rows of port p1 zero

                                    mov a,#0fh    

                                    mov p1,a             ;making all rows of port p1 high

                        press:   mov a,p2

                                    jz press                ;check until any key is pressed


after making sure that any key is pressed


                                    mov a,#01h         ;make one row high at a time 

                                    mov r4,a

                                    mov r3,#00h        ;initiating counter

                        next:    mov a,r4

                                    mov p1,a             ;making one row high at a time

                                    mov a,p2             ;taking input from port A

                                    jnz colscan          ;after getting the row jump to check 


                                    mov a,r4

                                    rl a                       ;rotate left to check next row

                                    mov r4,a

                                    mov a,r3

                                    add a,#08h         ;increment counter by 08 count

                                    mov r3,a

                                    sjmp next           ;jump to check next row


after identifying the row to check the colomn following steps are followed


                  colscan:    mov r5,#00h

                           in:    rrc a               ;rotate right with carry until get the carry

                                    jc out             ;jump on getting carry

                                    inc r3             ;increment one count

                                    jmp in          

                         out:    mov a,r3

                                    da a                ;decimal adjust the contents of counter    

                                                                before display

                                    mov p2,a

                                    jmp start       ;repeat for check next key.


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