80C51 serial port structure and principle (1-concept)

1. Serial communication (transmitted one bit at a time): asynchronous communication (different clocks control data), synchronous communication;

  Parallel communication: (each bit of a data byte is transmitted simultaneously using multiple data lines);

2. 8N1, one start bit (one byte), 8 data bits, and one stop bit.

The length of each byte field in the common SCI or UART serial data format (8N1) is 10 bit times (BIT TIME). The start bit START BIT is a dominant bit, which marks the beginning of the byte field. 

The following are 8 data bits, with the least significant bit sent first. The stop bit STOP BIT is a recessive bit that marks the end of the byte field.

3.80C51 serial port structure

SBUF register: It is two physically independent receiving and transmitting buffers, which can send and receive data at the same time. The operation on the receiving buffer or the transmitting buffer can be distinguished by reading and writing instructions on SBUF. Thus, two independent external receiving and transmitting signal lines RXD and TXD can be controlled to send and receive data at the same time, realizing full-duplex.

SCON: A special function register used to set the serial port working mode, receive/send control and set status flags.

Four working modes:

Among them, fOSC is the clock frequency of the microcontroller; the baud rate refers to the number of bits sent (or received) per second by the serial port.

SM2: Multi-machine communication control bit. This is only used for multi-machine communication in mode 2 and mode 3. The transmitter SM2 = 1 (program control setting is required). The serial port of the receiver works in mode 2 or 3. When SM2 = 1, only when the 9th data bit (RB8) is 1, the first 8 bits of data received are sent to SBUF, and RI is set to issue an interrupt request to trigger a serial reception interrupt, otherwise the received data will be abandoned. When SM2 = 0, regardless of whether the data bit is 0 or 1, the data will be sent to SBUF, and RI will be set to issue an interrupt request. When working in mode 0, SM2 must be 0.

REN: Serial receive enable bit: When REN = 0, reception is prohibited; when REN = 1, reception is allowed.

TB8: In modes 2 and 3, TB8 is the 9th bit of data to be sent by the transmitter. In multi-machine communication, it represents the address or data to be transmitted. When TB8=0, it is data, and when TB8=1, it is address.

RB8: In methods 2 and 3, RB8 is the 9th bit of data received by the receiver, which happens to come from TB8 of the transmitter, thereby identifying the characteristics of the received data.

TI: Serial port sends interrupt request flag. When the CPU has sent a serial data, the SBUF register is empty, and the hardware sets TI to 1 to request an interrupt. After the CPU responds to the interrupt, the software clears TI.

RI: Serial port receive interrupt request flag. When the serial port receives a frame of serial data, the SBUF register is full, and the hardware sets RI to 1 to request an interrupt. After the CPU responds to the interrupt, the software clears RI.

PCON

Only SMOD (baud rate multiplication bit) is related to the serial port operation. When the serial port mode is 1, 2, or 3, SMOD = 1 doubles the baud rate, and SMOD = 0 when reset;

Baud rate = (2^SMOD/32) * Timer T1 overflow rate = (2^SMOD/32) * (fosc/12*(256-X))    

like:

9600   X = 253 0xfd

4600  X=250  0xfa  

2400 X=244 0xf4