(Notes Summary) Serial Port of 80C51

The structure of the serial port of 80C51

SBUF: Two physically independent receive and transmit buffers. They both occupy the same address 99H. The receiver is a double buffer structure, and the transmit buffer, because the CPU is active when sending, will not cause overlap errors.

80C51 serial port control register

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

SM0 and SM1 are the operating mode selection bits (same as before), and four operating modes can be selected, as follows

RI (SCON.0), receive interrupt flag. In mode 0, when the serial reception of the 8th bit of data is completed; or in other modes, when the serial reception is in the middle of the stop bit, the internal hardware sets RI to 1 and sends an interrupt request to the CPU. In the interrupt service program, it is cleared to 0 by software to cancel this interrupt request.

TI (SCON.1), the transmit interrupt flag. In mode 0, when the serial transmission of the 8th bit of data is completed; or in other modes, when the serial transmission of the stop bit is in the middle, the internal hardware sets TI to 1 and sends an interrupt request to the CPU. In the interrupt service program, it is cleared to 0 by software to cancel the interrupt request.

RB8 (SCON.2), in mode 2 or mode 3, is the ninth bit of the received data, which can be set by software and used as the parity bit of the data. In multi-machine communication, it is used as the flag bit of the address frame and data frame. In mode 1, if SM2=0, RB8 is the received stop bit.

TB8 (SCON.3), in mode 2 or mode 3, is the ninth bit of the transmitted data, which can be set by software and used as the parity bit of the data. In multi-machine communication, it is used as the flag bit of the address frame and data frame. In mode 0 and mode 1, this bit is not used.

REN (SCON.4), serial reception enable bit. If software sets REN=1, the serial port starts receiving data. If software sets REN=0, reception is disabled.

SM2 (SCON.5), multi-machine communication control bit. Mainly used in mode 2 and mode 3. When the receiver's SM2=1, the received RB8 can be used to control whether to activate RI. (When RB8=0, RI is not activated and the received information is discarded; when RB8=1, RI is activated, the received data enters SBUF, and then the data is read from SBUF in the interrupt service). When the receiver's SM2=0, regardless of whether the received RB8 is 0 or 1, the received data can enter SBUF and activate RI (that is, RB8 does not have the function of controlling RI activation at this time). By controlling SM2, multi-machine communication can be achieved. In mode 0, SM2 must be 0. In mode 1, if SM2=1, RI is set to 1 only when a valid stop bit is received.

2. Only one SMOD bit in PCON is related to the serial port operation, as follows

SMOD (PCON.7), baud rate multiplication bit. In serial port mode 1, mode 2, mode 3, the baud rate is related to SMOD. When SMOD=1, the bit rate is doubled. When reset, SMOD=0.