51 single chip microcomputer system application

Application 1:  Suppose there is a 51 application system connected as shown in the attached figure. Please explain how the LED connected to the P1.0 port changes when the system executes the following program segments A and B respectively? What is the difference between the display changes of the corresponding program segments A and B?

Program segment A:

LP: CPL P1.0

    MOV R0,#0

     DJNZ R0,$

    JMP LP

Program segment B:

LP: SETB P1.0

    DJNZ R0,$

    CLR P1.0

    DJNZ R0,$

    DJNZ R0,$

    JMP LP

Answer: For both program segments A and B, the LED will periodically switch between on and off states. In each on-off switching cycle, for program segment A, the LED stays on for the same length of time as it stays off; for program segment B, the LED stays on for twice as long as it stays off. The LED on-off cycle for program segment B is approximately 1.5 times that for program segment A.

 

Application 2: Assume that the 51 application system is connected as shown in the figure of application 1, and the crystal frequency used in the system is 12MHz. Try to write a program segment to use the timer/counter 1 of the 51 microcontroller to realize that the display status of the LED connected to P1.0 is flipped every 10 seconds.

 

answer:

     MOV TMOD,#20H       ; T1 timing mode 2: 8-bit automatic reload

     MOV TH1,#(256-250); Timing period 250us (corresponding to 12MHz crystal oscillator, each machine cycle = 1us) 

     MOV TL1,TH1         ;

     CLR TF1             ; Clear T1 overflow flag

     SETB TR1            ; Start T1 timing

     MOV R0,#0           ; R0 is used to accumulate the number of times T1 overflows

     MOV R1,#0           ; R1 is used to accumulate the number of times R0 reaches 200

LP:   JBC TF1, L1          ; if TF1 is 1, clear TF1 and proceed to accumulate the number of overflows

     JMP LP              ; loop and wait for T1 to overflow

L1:   INC R0              ; cumulative T1 overflow times

     CJNE R0,#200,LP     ；If it does not reach 200 times, it will wait for the next overflow

     MOV R0,#0           ; Ensure that the value of R0 is between 0 and 199

     INC R1              ; the number of times the cumulative time reaches 50ms (250us*200=50ms)

     CJNE R1,#200,LP     ; If the accumulated time does not reach 10s, it will wait for the next overflow

     MOV R1,#0           ; Ensure that the value of R1 is between 0 and 199

     CPL P1.0            ; if the accumulated time reaches 10s, flip the P1.0 state once (50ms*200=10s)

     JMP LP              ; Continue looping to accumulate time