Working Principle and Programming of 4×4 Matrix Keyboard

1. A brief introduction to the working principle of the hardware

This experiment uses the 8-bit digital tube display circuit and 4×4 matrix keyboard circuit on ME300B. The working principles of these two parts are briefly introduced below:

1. Working principle of 4×4 matrix keyboard

The matrix keyboard is also called a determinant keyboard. It is a keyboard composed of 4 I/O lines as row lines and 4 I/O lines as column lines. A key is set at each intersection of the row and column lines. In this way, the number of keys in the keyboard is 4×4. This determinant keyboard structure can effectively improve the utilization rate of the I/O port in the single-chip microcomputer system.

Figure 1 is the circuit diagram of the ME300B matrix keyboard, with row lines connected to P1.4-P1.7 and column lines connected to P1.0-P1.3.

Figure 1   Matrix keyboard circuit

Figure 2   Button arrangement

2. Digital tube dynamic scanning display circuit

　　In the ME300B development system, an 8-bit digital tube dynamic scanning display is used. The 8 segment lines of all digital tubes are connected together accordingly and connected to the P0 port of AT89S51, and the field output is controlled by the P0 port. The common anode of each digital tube is controlled by the P2 port of AT89S51 to control Q20-Q27 to realize the bit output control of the 8-bit digital tube.

In this way, the dynamic scanning display of a group of digital tubes needs to be controlled by two groups of signals: one group is the glyph code output by the field output port, which is used to control the displayed glyph, called the segment code; the other group is the control signal output by the bit output port, which is used to select which number of digital tubes to work, called the bit code.

Since the segment lines of each digital tube are connected in parallel, the output of the segment code is the same for each digital tube. Therefore, if the bit selection lines of each digital tube are in the strobe state at the same time, the 8-bit digital tube will display the same character. If each digital tube is to display the character corresponding to the current position, a scanning display method must be used. That is, at a certain moment, only the bit selection line of a certain position is in the on state, while the bit selection lines of other positions are in the off state. At the same time, the font code of the character to be displayed in the corresponding position is output on the segment line. In this way, at the same time, only the selected position displays the character, while the other positions are off. In this way, each digital tube can display the character to be displayed.

Although these characters appear at different times, and at the same time, only one digit is displayed while the others are off, due to the afterglow characteristics of the digital tube and the persistence of vision of the human eye, as long as the display interval of each digital tube is short enough, the visual impression given to the human eye will be a continuous and stable display.

Figure 3   Digital tube circuit 

The time interval of different digits of the digital tube can be achieved by adjusting the delay length of the delay program. The time interval of the digital tube display can also determine the brightness of the digital tube display. If the display time interval is long, the brightness of the digital tube will be brighter when displayed. If the display time interval is short, the brightness of the digital tube will be darker when displayed. If the display time interval is too long, the digital tube will flicker when displayed. Therefore, when adjusting the display time interval, it is necessary to consider the brightness of the digital tube during display and to prevent the digital tube from flickering when displayed.

When using digital tube to display information in ME300B MCU development system, short-circuit the 2nd and 3rd terminals of JP2. See Figure 3

2. Programming method of demonstration program 

1. Programming method of 4×4 matrix keyboard:

1.1. First read the status of the keyboard and get the key feature code.

First, output low level from the upper four bits of P1 port, and output high level from the lower four bits, and read the keyboard status from the lower four bits of P1 port. Then output low level from the lower four bits of P1 port, and output high level from the upper four bits, and read the keyboard status from the upper four bits of P1 port. Combining the results of the two reads can get the characteristic code of the current key. Using the above method, we get the characteristic codes of 16 keys.

Here is an example of how to get the key feature code:

Assume that the "1" key is pressed, find the key's characteristic code.

The high four bits of P1 output low level, that is, P1.4-P1.7 are output ports. The low four bits output high level, that is, P1.0-P1.3 are input ports. The state of the low four bits of P1 is "1101", and its value is "0DH".

Then the high four bits of P1 port output high level, that is, P1.4-P1.7 are input ports. The low four bits output low level, that is, P10-P13 are output ports. The state of the high four bits of P1 port is "1110", and its value is "E0H".

The characteristic code of the key is obtained by performing a logical OR operation on the P0 port status values ​​read twice, which is "EDH".

The same method can be used to obtain the characteristic codes of the other 15 keys.

1.2. According to the characteristic code of the key, look up the table to get the sequence code of the key.

The feature codes of the 16 keys obtained by the above method are arranged in the order of the keys in Figure 2 to form a correspondence table between feature codes and sequence codes, and then the currently read feature code is used to look up the table. When the feature code appears in the table, its position is the corresponding sequence code.

1.3. Specific programming of matrix keyboard key value search program

     The main functions of this demo program are:

1. Identify whether any key is pressed on the keyboard. If no key is pressed, return.

2. If a key is pressed, find out the specific key value (sequence code).

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Matrix keyboard key value lookup program

The key value is stored in unit 30H

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KEY_SCAN:                          ; Identify whether a key is pressed on the keyboard subroutine

    MOV   P1,# 0F0H            ; Set the column line to 0 and the row line to 1

           MOV   A,P1                ; read P1 port

           ANL   A,#0F0H             ; take out the upper four bits

           MOV   B,A                 ; Temporarily save to B

           MOV   P1,#0FH             ; Set the column line to 1 and the row line to 0

           MOV   A,P1                ; read P1 port

           ANL   A,#0FH              ; take out the lower four bits

           ORL   A,B                 ; logical OR operation of the upper four bits and the lower four bits to recombine

           CJNE   A,#0FFH,KEY_IN1    ; 0FFH means no key is pressed

           RIGHT

KEY_IN1:                           ; Identify the specific key value subroutine

 MOV   B,A                 ; Temporarily store the key feature code in B

           MOV  DPTR,#KEYTABLE

           MOV  R3,#0FFH          

KEY_IN2: 

 INC   R3                   ; sequence code plus 1

           MOV   A,R3

           MOVC   A,@A+DPTR          ; Look up the table

           CJNE   A,B,KEY_IN3        ; Compare, if they are the same, find the key's feature code.

           MOV   A, R3                ; After finding the feature code, take the sequence code

           MOV   30H,A               ; store in unit 30H

           RIGHT

KEY_IN3:

  CJNE   A,#00H,KEY_IN2     ; Not finished yet, continue to check

           RET                       ; 00H is the end code

;Correspondence table between feature coding and sequence coding

KEY_TABLE:         

    DB   0EEH,0EDH,0EBH,0E7H,0DEH          ; 0,1,2,3,4,       sequence code

    DB   0DDH,0DBH,0D7H,0BEH,0BDH          ; 5,6,7,8,9,       sequence code

    DB   0BBH,0B7H,07EH,07DH,07BH, 077H    ; A,B,C,D,E,F      sequence code

    DB   00H                               ;                 End code

2. Programming method of 8-bit digital tube display program

Determine the scanning initial value and scanning direction according to the specific position of the digital tube to be used.

The number of scanned digits is determined by the number of digital tubes used.

Prepare the data to be displayed and put it into the corresponding display unit.

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 8-bit digital tube display subroutine

; Light up 8 digital tubes from right to left

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DISPLAY:

            MOV R1,#07FH           ; Scan the initial value and send it to R1

            MOV R2,#08H            ;Send the number of scan bits to R2

            MOV R0,#30H            ; Start filling the display unit

DISP1:       MOV A,@R0              ; Display content is sent to A

            MOV DPTR,#TABLE        ;Get the table header

            MOVC A,@A+DPTR         ; Look up the table to get the display data

            MOV P0,A               ; Display unit data

            MOV P2, R1              ; Start to display the current position

            MOV A,R1               ; prepare to display the next digit

            RR A                  

            MOV R1,A             

            INC R0                 ; get the next unit address

            LCALL DELAY2MS         ; Delay 2 MS

            DJNZ R2,DISP1          ; Repeat to display the next

            RET                    ; Display completed, return

Since only one digital tube is needed for key value display, the above display program can be optimized to obtain the following display program.

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Key value display subroutine

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KEY_PLAER：

           MOV   A,30H            ; key value data is sent to A

           MOV   DPTR,#TABLE      ;Get the segment code table address

           MOVC   A,@A+DPTR       ; Check the corresponding segment code of the displayed data

           MOV   P0,A             ; send segment code to port P0

           CLR   P2.7             ; the first digital tube display

           CALL  DELAY2MS

           SETB   P2.7

           RIGHT

TABLE：

 DB  0C0H,0F9H,0A4H,0B0H,99H,92H,82H,0F8H 

 DB  80H,90H,88H,83H,0C6H,0A1H,86H,8EH          ;0－F

3. Demonstration Program Functions

When you press any key on the matrix keyboard, the buzzer sounds and the corresponding key value is displayed on the digital tube. Figure 4 is a practical demonstration picture of the ME300B development system.

Figure 4   ME300B development system displays key value "E"