2. 8051 Serial Ports
• One of the 8051’s many powerful features is its integrated
Universal Asynchronous Receiver Transmitter (UART), otherwise
known as a serial port. With integrated serial port of 8051, data
can be transmitted and received easily by reading and writing the
data to the serial port registers. The features of the 8051 serial
ports are
• Full duplex operation.
• Receive Buffered.
• Access using single double buffered Register SBUF.
• Four different modes of operation.
• Option to use fixed baud rate or programmable baud rate.
3. 8051 Serial Ports
• Full Duplex serial port means that it can transmit
and receive data simultaneously. It is also receive-
buffered, meaning it can commence reception of a
second byte before a previously received byte has
been read from the SBUF register.
• However, if the first byte still hasn’t been read by
the time reception of the second byte is complete,
one of the bytes will be lost. The serial port receive
and transmit registers are both accessed at Special
Function Register SBUF.
4. 8051 Serial Ports
• Data to be transmitted are written to the register
SBUF and the data received by the serial port are
read from the register SBUF. Physically reading
and writing SBUF actually accesses two separate
registers.
• This technique of having same address for two
different registers is called double buffering.
5. SERIAL PORT CONTROL SFRS
• The serial port of 8051 is controlled by two
registers in SFR area of 8051 as shown in Table
• The two registers are Serial Port control registers;
SCON and serial port buffer register SBUF.
SFR Name Description SFR Address
SCON
Serial port control
register
98h
SBUF
Serial port buffer
register
99h
6. SERIAL PORT CONTROL SFRS
• In addition to the above two registers, the MSB of PCON
register named as SMOD bit is used to double the baud
rate of serial transmission and reception.
• If SMOD bit is set to 1, then the baud rate is doubled.
• The individual bits of SCON have the functions as shown
in Table . As the SCON register has many individual status
bits, the individual bits of this register are bit addressable.
The bit address is also given in Table .
• The programmer can use these bit addresses to check the
status of the serial port and set the mode individually.
7. Bit patterns for SCON (98h) SFR
Bit Name Bit Address Explanation of Function
D7 SM0 9Fh
Serial port mode select bits
D6 SM1 9Eh
D5 SM2 9Dh Multiprocessor Communications Enable bit
D4 REN 9Ch
Receiver Enable. This bit must be set in order to receive
characters.
D3 TB8 9Bh Transmit bit 8. The 9th bit to transmit in mode 2 and 3.
D2 RB8 9Ah Receive bit 8. The 9th bit received in mode 2 and 3.
D1 TI 99h
Transmit Interrupt Flag. Set when a byte has been
completely transmitted.
D0 RI 98h
Receive Interrupt Flag. Set when a byte has been completely
received.
8. Bit Pattern - Description
• D7 and D6 bits of the SCON register define the operation modes of
the serial port and the basic operating modes are given in Table.
• The SM0 and SM1 bits can select any one of the four operating
modes described in the next section.
10. Bit Pattern - Description
• The next bit, SM2, is a flag used for enabling
"Multiprocessor communication" in modes 2 and 3.
If SM2 is set to 1 in modes 2 and 3, the “Receive
Interrupt” RI flag will not be activated if the
received 9th data bit is 0. If SM2 is set in Mode1,
then “Receive Interrupt” RI flag will not be
activated if valid stop bit is not received.
• This can be useful in certain advanced serial
applications. It can be now assumed that SM2 bit
has to be cleared so that RI flag will be set when
any character is received.
11. Bit Pattern - Description
• The next bit, REN, is "Receiver Enable." This bit is
set in order to receive the characters from the
receive data line of the serial port.
• The TB8 bit is used in modes 2 and 3. In modes 2
and 3, a total of nine data bits are transmitted.
The first 8 bits are the 8 bits of the data to be
transmitted, and the ninth bit is taken from TB8.
The RB8 also operates in modes 2 and 3 on the
reception side.
12. Bit Pattern - Description
• When a byte is received in modes 2 or 3, a total of
nine bits are received. In this case, the first eight bits
received are the data of the serial byte received and
the value of the ninth bit received will be placed in
RB8.
• TI means "Transmit Interrupt." When a program
writes a data to the serial port buffer SBUF, then the
serial port will start shifting this data in the serial
transmit line bit by bit at the predefined clock speed
or baud rate. 8051 will give TI signal to the
programmer after sending the data completely.
13. Bit Pattern - Description
• Upon sensing the TI bit set to 1, the programmer can
then write the next data for transmission. When the
TI bit is set, the programmer may assume that the
serial port is "free" and ready to send the next byte.
• Finally, the RI bit means "Receive Interrupt."
Whenever a data is received on the receive data line
of the serial port, this serial data will be shifted in to
a buffer and then stored in the SBUF register.
• Setting of RI bit indicates that a byte has been
received. Upon sensing the RI bit set to 1, the
programmer may read the data from the SBUF.
16. SBUF
• SBUF is physically two registers with the same
address.
• When data to be transmitted is written to the SBUF
register, then it will be shifted bit by bit into the TXD
line of 8051. The port 3 pin 3.1 acts as the TXD line.
The shifting is done by the transmit clock which
determines the baud rate.
• Similarly, when the data bits are received on the RXD
line (Pin 3.0 of port 3), the bits are shifted serially into
the shift register according to the Receive clock.
• After the reception is complete, the data received will
be placed on SBUF from where it can be read by the
programmer through the internal bus.
17. Reception / Transmission
• Mode 0: In this mode serial data is entering and exiting through
RxD pin. So, in mode 0, Full duplex is not possible – meaning that
both transmission and reception cannot take place
simultaneously. TxD pin outputs the shift clock. 8 bits are
transmitted/received (LSB first). The baud rate is fixed at 1/12 the
oscillator frequency. Transmission is started by writing a data
byte to the SBUF register and once the transmission is complete,
TI flag is set
• The reception is started by enabling REN in SCON register. Once
the data reception is complete, the RI flag is set
• The baud rate in Mode 0 is fixed at one twelfth of the clock
frequency. Baud rate= (Clock frequency/12)
19. Reception / Transmission
• Mode 1: In this mode, 10 bits are transmitted
through TxD and simultaneously 10 bits can be
received through RxD. The 10 bits are made up
of a start bit (0), 8 data bits (LSB first), and a stop
bit (1). On completion of reception, the stop bit
goes into RB8 in Special Function Register SCON.
The baud rate is variable and is set by the Timer
1 overflow rate. The baud rate for mode 1 is fixed
at the following rate.
20. Reception / Transmission
• Baud rate= (Timer 1 overflow rate /16) if SMOD
bit in PCON SFR is set to 1.
• Baud rate= (Timer 1 overflow rate /32) if SMOD
bit in PCON SFR is set to 0.
• The MSB of PCON register can be set or reset by
the programmer. The baud rate can be doubled
by setting the MSB of PCON.
21. Reception / Transmission
• In order to generate the baud rate clock from
timer 1, Timer 1 can be configured to act as
timer in auto reload mode with the timer 1
interrupt disabled.
• As in mode 0, the transmission is initiated by
writing a data to SBUF register. Reception is
initiated by a 1 to 0 transition that is the start bit
received and also when REN of SCON SFR is 1.
22. Reception / Transmission
• Mode 2: In this mode, 11 bits are transmitted through TxD or
received through RxD. The 11 bits are made up of one start bit
(always 0), 8 data bits (LSB first), a programmable 9th data bit, and a
stop bit (always 1). The 9th data bit transmitted is same as TB8 bit in
SCON special function register. It can be assigned the value of 0 or 1
by the programmer. Or, for example, the parity bit (P, in the PSW)
could be moved into TB8.
• On reception, the 9th data bit goes into RB8 in Special Function
Register SCON, while the stop bit is ignored. The baud rate is
programmable to either 1/32 or 1/64 of the oscillator clock
frequency.
• Baud rate= (Clock frequency /32) if SMOD bit in PCON SFR is set to 1.
• Baud rate= (Clock frequency /64) if SMOD bit in PCON SFR is set to 0.
23. Reception / Transmission
• Mode 3: In this mode, 11 bits are transmitted through TxD
and simultaneously 11 bits are received through RxD. The
11bits are made up of a start bit (0), 8 data bits (LSB first), a
programmable 9th data bit, and a stop bit (1). In fact, Mode 3
is the same as Mode 2 in all respects except baud rate. The
baud rate in Mode 3 is variable.
• The baud rate for mode 3 is fixed at the following rate similar
to mode1.
• Baud rate= (Timer 1 overflow rate /16) if SMOD bit in PCON
SFR is set to 1.
• Baud rate= (Timer 1 overflow rate /32) if SMOD bit in PCON
SFR is set to 0.
24. Reception / Transmission
• In all four modes, transmission is initiated by any
instruction that uses SBUF as a destination register.
Reception is initiated in Mode 0 by the condition RI =
0 and REN = 1. Reception is initiated in the other
modes by the incoming start bit when REN = 1.
• The flow chart 11.20 shows the steps in
programming the serial port of 8051. These steps are
detailed in the following sections and also in the
example programs.
