In this note, we are going to know about the SBUF Register in 8051 Microcontroller. Welcome to Poly Notes Hub, a leading destination for engineering notes for diploma and degree engineering notes.

Author Name: Arun Paul.

What is SBUF Register in 8051 Microcontroller?

The SBUF (Serial Buffer) register of the 8051 microcontroller is a special function register (SFR) at address 99H that allows for asynchronous serial communication via the microcontroller’s UART (Universal Asynchronous Receiver Transmitter). It functions as a dual-purpose register, transferring and receiving data via the serial port.

Function of SBUF Register in 8051 in Asynchronous Data Transfer

A. Data Transmission

• When the CPU wants to transfer a byte of data over the serial port, it writes it to the SBUF register.
• The value is then automatically moved to the transmit shift register, which initiates the transmission bit by bit (asynchronous).
• SBUF is only free for the next write once the transmission is completed, as indicated by the TI (Transmit Interrupt) signal.

B. Data Reception

• When data is received by the serial port, it is stored in the receive shift register.
• When a whole byte is received, it is automatically stored in the SBUF register.
• The CPU is able to obtain the data from the SBUF.
• The RI (Receive Interrupt) flag is set to notify that new data has arrived.

Structure of SBUF Register for Asynchronous Data Transfer (8051)

SBUF is a single register (address: 99H) in the memory map, however it behaves as two different registers internally:

• SBUF (Write): Writing to SBUF sends the byte to the Transmit Shift Register, which sends the data out through the TXD pin (asynchronous serial transmission).
• SBUF (Read): After receiving 8 bits through the RXD pin, the Receive Shift Register moves the byte into SBUF, which the CPU reads.
• TXD (Pin P3.1): Transmit Data pin.
• RXD (Pin P3.0): Receive Data pin.

SBUF Register Applications

Here are the applications of SBUF Register –

• Used for transmitting and receiving data between the 8051 microcontroller and a computer via RS232 (via a converter such as MAX232).
• It is used to transmit AT commands to GSM modules and receive replies (SMS, call status, etc.) via asynchronous serial transfer.
• Receives location data strings from a GPS receiver via serial input (RXD), which the microcontroller then processes.
• SBUF allows engineers to send debugging messages or variable values to a PC terminal for real-time monitoring while developing.
• SBUF transmits logged data (e.g., temperature, humidity) to a computer or storage device via serial connection.