FMA_Unit_4.4 8051 Serial Communications registers

BAUD RATE: The transmitter and receiver are configured to communicate at some data transfer rate before communication starts. This data transfer rate or number of bits transmitted per second is called baud rate for handling serial communication.

UART: It stands for Universal Asynchronous Receiver-Transmitter. A UART generates its internal data clock to the microcontroller. It synchronizes that clock with the data stream by using the start bit transition. The receiver needs the baud rate to know ahead of time to properly receive the data stream.

C. 8051 Serial Communications registers

1. SBUF Register (Serial Buffer register)

•  Buffer which is used for holding the data before sending / after receiving is called Serial Buffer.
•  In 8051 Micro-controller there are two 8 bits serial buffer register.
• One is write only and is used to hold data is to be transmitted from micro-controller via the TxD pin P3.1.
• While other is read only and used to hold data that is received from the external source via the RxD pin P3.0.

2. SCON Register (Serial Control register)

D7	D6	D5	D4	D3	D2	D1	D0
SM0	SM1	SM2	REN	TB8	RB8	TI	RI

Bit 7 & 6: SM0 & SM1 (Serial Mode control Bits)

These bits are serial mode bits and are used for selecting the operating modes of serial port.

SM0	SM1	Mode	Description	Baud Rate
0	0	0	8 bit Shift Register	Fixed Baud Rate (fosc / 12)
0	1	1	8 bit UART	Variable Baud Rate
1	0	2	9 bit UART	Fixed Baud Rate (fosc / 12 or focs/32)
1	1	3	9 bit UART	Variable Baud Rate

Mode 0 and Mode 1 used for one serial port

Mode 2 and Mode 3 used for one serial port

Bit 5: SM2 (Multiprocessor control bit)

In 8051, there is only one serial port. So this is don’t care condition.

SM2 = 1, Multiprocessor serial port (Two serial Port used)

SM2 = 0, Uniprocessor serial port (One serial Port used)

Bit 4: REN (Receiver enable bit)

This bit allows the data from external peripheral device to micro-controller.

                    REN = 1, receiver is enable

                    REN = 0, receiver is disable

Bit 3: TB8 (Consider when serial Mode 2 and serial Mode 3 is used)

In 8051, there is only one serial port. So this is don’t care condition.

It is the 9^th programmable transmitted bit along with data byte in multiprocessor mode.

                    TB8 = 1, the byte transmitted is data bit

                    TB8 = 0, the byte transmitted is address bit

Bit 2: RB8 (Consider when serial Mode 2 and serial Mode 3 is used)

In 8051, there is only one serial port. So this is don’t care condition.

It is the 9^th received programmable bit along with data byte in multiprocessor mode.

                    RB8 = 1, the byte received is data bit

                    RB8 = 0, the byte received is address bit

Bit 1: TI (Transmit Interrupt flag)

TI = 1, When one byte of data transmission is completed

TI = 0, When one byte of data transmission is not completed

Bit 0: RI (Receive Interrupt flag)

RI = 1, When one byte of data reception is completed

RI = 0, When one byte of data reception is not completed

3. PCON Register (Power Control register)

D7	D6	D5	D4	D3	D2	D1	D0
SMOD	--	--	--	GF1	GF0	PD	IDL

Bit 7: SMOD (Serial Mode selection bit)

Speed of serial communication is decided by this SMOD bit.

When SMOD is reset then it is indicated that data is transmitted or received by microcontroller at defined baud rate.  (Baud Rate = fosc/64)

When SMOD is set then it is indicated that data is transmitted or received by microcontroller at double the defined baud rate. (Baud Rate = fosc/32)

Bit 6, 5 & 4: Reserve bit

Bit 3 & 2: GF1 and GF0 (General purpose flags)

          These flags are used by the programmer to indicate the status of the various conditions

Bit 1: PD (Power Down)

          When PD is set, internal oscillator stops and power is reduced to 2V from 5V for power saving purpose.

Bit 0: IDL (Power saving mode)

When IDL is set, the clock signal is stopped to CPU, but is supplied to interrupt, timer and serial port blocks. It can be reset by hardware interrupt.