We make it 0 since we are not using the 8051 in a

•
We make it 0 since we are not using the 8051 in a
multiprocessor environment.
–
SM2=0 : Single processor environment
–
SM2=1 : multiprocessor environment

59
REN (Receive Enable)
•
SCON.4
•
Set/cleared by software to enable/disable
reception.
–
REN=1
•
It enable the 8051 to receive data on the RxD pin of the 8051 .
•
If we want the 8051 to both transfer and receive data, REN
must be set to 1.
•
SETB SCON.4
–
REN=0
•
The receiver is disabled.
•
The 8051 can not receive data.
•
CLR SCON.4

60
TB8 (Transfer Bit 8)
•
SCON.3
•
TB8 is used for serial modes 2 and 3.
•
The 9
th
bit that will be transmitted in mode 2 & 3.
•
Set/Cleared by software.

61
RB8 (Receive Bit 8)
•
SCON.2
•
In serial mode 1, RB8 gets a copy of the stop bit
when an 8-bit data is received.

62
TI (Transmit Interrupt Flag)
•
SCON.1
•
When the 8051 finishes the transfer of the 8-bit
character, it raises the TI flag.
•
TI is raised by hardware at the beginning of the
stop bit in mode 1.
•
Must be cleared by software.

63
RI (Receive Interrupt)
•
SCON.0
•
Receive interrupt flag. Set by hardware halfway
through the stop bit time in mode 1. Must be
cleared by software.
•
When the 8051 receives data serially via RxD, it
gets rid of the start and stop bits and place the byte
in the SBUF register.
•
Then 8051 rises RI to indicate that a byte.
•
RI is raised at the beginning of the stop bit.

64
Figure 10-9. SCON Serial Port Control
Register (Bit Addressable)
SM0 SCON.7 Serial port mode specifier
SM1 SCON.6 Serial port mode specifier
SM2 SCON.5 Used for multiprocessor communication. (Make it 0)
REN SCON.4 Set/cleared by software to enable/disable reception.
TB8 SCON.3 Not widely used.
RB8 SCON.2 Not widely used.
TI SCON.1 Transmit interrupt flag. Set by hardware at the beginning of
the stop bit in mode 1. Must be cleared by software.
RI SCON.0 Receive interrupt flag. Set by hardware halfway through the
stop bit time in mode 1. Must be cleared by software.
Note : Make SM2, TB8, and RB8 = 0.
SM0 SM1 SM2 REN TB8 RB8 TI RI

65
Transfer Data with the TI flag (1/2)
•
The following sequence is the steps that the 8051
goes through in transmitting a character via TxD:
1. The byte character to be transmitted is written into the
SBUF register.
2. It transfers the start bit .
3. The 8-bit character is transferred one bit at a time.
4. The stop bit is transferred.
SBUF
TxD
bit by bit
8-bit char
UART
TI

66
Transfer Data with the TI flag (2/2)
•
Sequence continuous:
5. During the transfer of the stop bit, the 8051 raises the
TI flag, indicating that the last character was
transmitted and it is ready to transfer the next
character.
6. By monitoring the TI flag, we know whether or not the
8051 is ready to transfer another byte.
–
We will not overloading the SBUF register.
–
If we write another byte into the SBUF before TI is raised, the
untransmitted portion of the previous byte will be lost.