51 MCU Tutorial from Scratch—— 23 MCU Serial Port Communication Programming

1. Serial port mode 0 application programming The serial port mode 0 of the 8051 microcontroller is a shift register mode. By connecting an external serial-input and parallel-output shift register, a parallel port can be expanded.

Example: Use the 8051 microcontroller serial port to connect CD4094 to expand the 8-bit parallel output port. As shown in the figure, each bit of the 8-bit parallel port is connected to a light-emitting diode, and the light-emitting diode is required to be in the state of a running light. The data transmission of serial port mode 0 can be interrupted or inquired. No matter which method is used, it must rely on the TI or RI flag. When sending serially, the interrupt request can be caused by setting TI (after sending a frame of data), and the next frame of data can be sent in the interrupt service program, or by querying the status of TI. As long as TI is 0, the query will continue, and TI will end the query when it is 1, and the next frame of data will be sent. When receiving serially, RI will cause an interrupt or query RI to determine when to receive the next frame of data. No matter which method is used, the control register SCON must be initialized before starting communication. In mode 0, 00H can be sent to SCON.

-----------------MCU serial port communication program design example-------------------------

ORG 2000H

START: MOV SCON,#00H; Set the serial port working mode to 0

MOV A,#80H; The highest position light will light up first

CLR P1.0 ; turn off parallel output (to avoid the "dark red" phenomenon of each LED during transmission)

OUT0: MOV SBUF,A; Start serial output

OUT1: JNB TI, OUT1; Output completed?

CLR TI; Finished, clear the TI flag to prepare for the next send

SETB P1.0 ; Open parallel port output

ACALL DELAY; delay for a period of time

RR A ; Circular right shift

CLR P1.0 ; turn off parallel output

JMP OUT0 ; loop

Note: The DELAY delay subroutine can use the delay subroutine we used when talking about the P1 port running light, so it will not be given here.

2. Serial port asynchronous communication

org 0000H

AJMP START

ORG 30H

START:

mov SP,#5fh ;

mov TMOD,#20h ;T1: working mode 2

mov PCON,#80h ;SMOD=1

mov TH1,#0FDH ; Initialize baud rate (see table)

mov SCON,#50h ;Standard UART settings

MOV R0,#0AAH; the number to be sent

SETB REN ; Allow to receive

SETB TR1 ;T1 starts working

WAIT:

MOV A,R0

CPL A

MOV R0,A

MOV SBUF,A

LCALL DELAY

JBC TI, WAIT1; If TI is equal to 1, clear TI and switch to WAIT1

AJMP WAIT

WAIT1: JBC RI, READ; if RI is equal to 1, clear RI and switch to READ

AJMP WAIT1

READ:

MOV A,SBUF; send the obtained data to port P1

MOV P1,A

LJMP WAIT

DELAY: ; Delay subroutine

MOV R7,#0ffH

DJNZ R7,$

RET

END

Compile the program, write it into the chip, insert it into the experimental board, and use the through-read cable to connect the experimental board to the serial port of the host to conduct the experiment. The function of the above program is very simple, that is, to send the numbers 55H and AAH to the host in turn at regular intervals, and send the numbers sent by the host to the P1 port. You can use the serial port wizard on the PC to do experiments. The serial port wizard is available for download on my homepage. After running the serial port wizard, press the "Set Parameters" button on the main interface to enter the "Set Parameters" dialog box and set the parameters according to the following. Note that my machine uses serial port 2. If you are not using serial port 2, please change the serial port settings yourself.

After setting, press OK to return to the main interface. Note that there is a drop-down list on the right, and "According to Hexadecimal" should be selected. Then press "Start Sending" and "Start Receiving". According to this setting, two lights should be on and six lights should be off on the experimental board. You can change the sending characters in the setting parameters such as 55, 00, FF, etc., observe the lights on and off, and analyze the reasons. You can also change the "According to Hexadecimal" in the drop-down list on the main interface to "According to Decimal" or "According to ASCII Characters" to observe the phenomenon and analyze it carefully. This is also very helpful for everyone to understand hexadecimal, decimal, and ASCII characters. The program itself is very simple and has comments, so I won't explain it in detail here.

3. Interrupted version of the above program

org 0000H

AJMP START

org 0023h

AJMP SERIAL ;

ORG 30H

START:

mov SP,#5fh ;

mov TMOD,#20h ;T1: working mode 2

mov PCON,#80h ;SMOD=1

mov TH1,#0FDH ; Initialize baud rate (see table)

mov SCON,#50h ;Standard UART settings

MOV R0,#0AAH; the number to be sent

SETB REN ; Allow to receive

SETB TR1 ;T1 starts working

SETB EA ; open the total interrupt

SETB ES ; Open serial port interrupt

SJMP$

SERIAL:

MOV A,SBUF

MOV P1,A

CLR RI

RETI

END

This program does not include a sending program, you can add it yourself.