Detailed explanation of the classic experimental examples of single-chip microcomputers (with source code) (Part 3)

　　Now let's use the two digital tubes on the experiment board to do an experiment to display the numbers 00 to 99 in a loop. First, complete the necessary hardware part.

　　Digital tubes are divided into common cathode and common anode types. Both can be driven by microcontrollers, but the driving methods are different, and the corresponding display codes of 0 to 9 are exactly the opposite.

　　First, let's introduce the single-chip microcomputer driving method of two common anode digital tubes. The circuit is as follows:

　　Netizens can see that: P2.6 and P2.7 ports control the power supply of tens and ones of the digital tube respectively. When the corresponding port becomes a low level, the corresponding transistor will be turned on, and +5V will supply power to the corresponding bit of the digital tube through the IN4148 diode and the driving transistor. At this time, as long as the P0 port sends out the digital display code, the digital tube can display the number normally.

　　Because two different digits need to be displayed, a dynamic scanning method must be used. That is, the ones digit is displayed for 1 millisecond first, and then the tens digit is displayed for 1 millisecond, and this cycle continues. In this way, as long as the scanning time is less than 1/50 second, the two different digits will be stably displayed due to the visual residual effect of the human eye.

　　Next, we will introduce a single-chip microcomputer driving method for common cathode digital tubes. The circuit is as follows:

　　Netizens can see that +5V directly supplies power to the 8 segments of the digital tube through a 1K resistor, and the P2.6 and P2.7 ports control the power supply of the tens and ones of the digital tube respectively. When the corresponding port becomes low level, the corresponding bit can absorb current. The data output by the P0 port of the microcontroller is equivalent to short-circuiting the digital segments that the digital tube does not want to display to the ground, so that the digital tube will display the required numbers.

　　Netizens can see that the hardware of common cathode digital tubes is simpler, so in mass production, the hardware overhead is small, PCB area is saved, welding workload is reduced, and overall cost is reduced. Therefore, the use of common cathode digital tubes is more conducive to mass production. The test boards sold now all use common cathode digital tubes.

　　The following is an assembly language program that uses the two-digit digital tube of the AT89C51 experimental board to display 00~99 in a loop.

　　a_bit equ 20h ; memory location for storing the single digit of the digital tube

　　b_bit equ 21h ; memory location for storing the ten digit of the digital tube

　　temp equ 22h ;Counter value is stored in memory location

　　star: mov temp, #0; initialize the counter, starting from 0

　　stlop: acall display; call display subroutine

　　inc temp; add 1 to the counter

　　mov a，temp

　　cjne a，#100，next ; Determine whether the counter is full of 100?

　　mov temp, #0; reset to zero when it reaches 100 and start over

　　next: ljmp stlop; loop execution if not satisfied

　　; Display subroutine

　　display: mov a, temp; convert the hexadecimal number in temp to decimal

　　mov b, #10; decimal/10=decimal

　　div ab

　　mov b_bit, a; the tens bit is in a

　　mov a_bit, b ; the ones bit is in b

　　mov dptr, #numtab; specify the start address of the table lookup

　　mov r0, #4

　　dpl1: mov r1, #250;

　　dplop: mov a, a_bit; get the unit digit

　　MOVC A, @A+DPTR; check the 7-segment code of the single digit

　　mov p0, a ; send out the 7-segment code of the unit digit

　　clr p2.7 ; open the single digit display

　　acall d1ms ;display 162 microseconds

　　setb p2.7; turn off the units display to prevent ghosting

　　mov a, b_bit ; get the tens digit

　　MOVC A, @A+DPTR; check the 7-segment code of the ten-digit number

　　mov p0, a ; send out the ten-digit 7-segment code

　　clr p2.6 ; open ten-digit display

　　acall d1ms ;display 162 microseconds

　　setb p2.6; turn off the ten-digit display to prevent ghosting

　　djnz r1, dplop; loop executed 250 times

　　djnz r0, dpl1; loop execution 250X4=1000 times

　　ret

　　;2+2X80=162 microseconds, delay is calculated based on 12MHZ

　　D1MS: MOV R7, #80

　　DJNZ R7, $

　　RET

　　;The 7-segment digital tube 0 to 9 digits on the experimental board display the common cathode code

　　numtab: DB 0CFH, 03H, 5DH, 5BH, 93H, 0DAH, 0DEH, 43H, 0DFH, 0DBH

　　end

　　;If it is a common anode digital tube display code

　　numtab: DB 30H, 0FCH, 0A2H, 0A4H, 06CH, 25H, 21H, 0BCH, 20H, 24H