8×8 dot matrix LED driven by single chip microcomputer

　　LED Principle and Control Technology

　　1. What is LED?

　　LED is the abbreviation of light-emitting diode. In the PN junction of certain semiconductor materials, when the injected minority carriers recombine with the majority carriers, the excess energy will be released in the form of light, thereby directly converting electrical energy into light energy. When a reverse voltage is applied to the PN junction, it is difficult for minority carriers to be injected, so no light is emitted. This type of diode made using the principle of injection electroluminescence is called a light-emitting diode, commonly known as LED.

　　2. Characteristics and advantages of LED electronic display screen

　　As a modern electronic media, LED electronic display screen is irreplaceable by any other media due to its flexible display area (dividable and arbitrary assembly), high brightness, long life, large capacity, digitalization and real-time performance. LED electronic display screen makes 　　full use of modern information technology to integrate and perfectly combine sound, light, electricity, machinery and other disciplines, and is a high-tech information publishing terminal product that integrates video, animation, subtitles and pictures.
LED

Figure ( 1 ) shows the appearance and pin diagram of the 8 × 8 dot matrix LED . Its equivalent circuit is shown in Figure ( 2 ). As long as the corresponding X and Y axes are forward biased, the LED can be lit. For example, if you want to light up the upper left corner LED , then Y0=1 , X0=0 . When applied, the current limiting resistor can be placed on the X axis or Y axis.

Figure ( 1 ) 8 × 8 dot matrix LED appearance and pin diagram [page]

Figure ( 2 ) 8 × 8 dot matrix LED equivalent circuit

Figure ( 3 ) 8 × 8 dot matrix LED circuit principle

Introduction to dot matrix LED scanning method

Dot matrix LED generally adopts scanning display, and its actual application can be divided into three ways:

(1)        Point scanning;

(2)        Line scanning;

(3)        Column scanning.

If the first method is used, the scanning frequency must be greater than 16 × 64 = 1024Hz and the period must be less than 1ms.

Yes. If the second and third methods are used, the frequency must be greater than 16 × 8 = 128Hz , and the period must be less than 7.8ms to meet the visual retention requirements. In addition, when driving one column or one row ( 8 LEDs ) at a time , an external drive circuit is required to increase the current, otherwise the LED brightness will be insufficient. As shown in Figure 3 .

  The following is a routine that uses the P1 port to control scanning and the 74HC164 to control the display output, so that the 8 × 8 dot matrix LED displays an “×”. As shown in Figure ( 3 ). [page]

CLK EQU       P3.2

DINA      EQU       P3.3

DINB      EQU       P3.4

CLEAR   EQU       P3.5

ORG       0000H

AJMP    MAIN

ORG       0100H

MAIN:

       MOV      SP,#70H           ; Set stack pointer

       MOV       50H,#07EH      ; Set the column data to be sent (Y0~Y7)

       MOV       51H,#0BDH

       MOV      52H,#0DBH

       MOV       53H,#0E7H

       MOV      54H,#0E7H

       MOV       55H,#0DBH

       MOV       56H,#0BDH

       MOV       57H,#07EH

       CLR CLEAR ;                    Initialize I/O port

       SETB      CLK

       SETB      DINA

       SETB     DINB

       SETB      CLEAR

main loop:

       mov          r6,#8h ;               Set the number of scans

       mov         dptr,#09000h     ; read scan port data

       MOV       R1,#50H           ; Specify column data pointer  

flashcy:         

       MOV       A,@R1             ; read column data

       MOV      R0,A

       INC R1 ;                            column data pointer plus 1

       CLR       A

       MOVC     A,@A+DPTR

       MOV      P1,#0FFH

       LCALL    SENDTO

       MOV      P1,A

       INC        DPTR

       LCALL   DELAY

       DJNZ       R6,flashcy

       SJMP      MAINLOOP

sendto: ;                                      Send data subroutine

       PUSH      ACC

       CLR      CLK

BROADCAST TIME:

       MOV      R7,#08H

       MOV      A,R0

       CLR       C

SENDCY:             

       RRC       A

       MOV      DINA,C

       SETB      CLK

       CLRCLK       

       DJNZ       R7,SENDCY

       POP       ACC

       RIGHT

DELAY:   MOV      R7,#01         ; Delay subroutine

DELAY0: MOV     R4,#17

DELAY1: MOV     R3,#28

        DJNZ     R3,$

        DJNZ    R4,DELAY1

        DJNZ     R7,DELAY0

        RIGHT

ORG       09000H

DB    0FEH, 0FDH, 0FBH, 0F7H, 0EFH, 0DFH, 0BFH, 07FH       ; X0~X7 ( column scan )

end