1. Source: http://www.edsim51.com/8051Notes/8051/serial.html
Serial Port
Source:

2. Serial communication Asynchronous serial communication
Data
Data
Clock
Internally generated clock synchronous to the data Tx
Gnd Rx Tx
Clk
Gnd Rx Tx Rx
Gnd Tx Rx
Clk
Gnd

3. Transmission modes Simplex One way all time Half-duplex Or
Either way all time
Full-duplex
Two ways all time
Receiver
Transmitter
Transmitter/Receiver
Transmitter/Receiver
Transmitter/Receiver

4. Serial port in 8051 8051 8051 serial port:
Q: Simplex? Half-duplex? Full duplex?
Q: Synchronous? Asynchronous?

5. Serial port in 8051 8051 8051 Properties of 8051 serial port:
*) Full duplex (i.e., it can transmit and receive data at the same time.)
*) Asynchronous
*) The data rate (baud rate, bit/sec) is controller by timer 1.

6. RelatedRegisters
SMOD

7. Serial Control Register (SCON)
Bit
Symbol
Address
Description 7
SM0
9FH
serial port mode bit 0 6
SM1
9EH
serial port mode bit 1 5
SM2
9DH
serial port mode bit 2 - will be dealt with at a later date 4
REN
9CH
receiver enable - this bit must be set to receive data 3
TB8
9BH
transmit bit 8 - this is the ninth bit transmitted in the 9-bit UART modes 2
RB8
9AH
receive bit 8 - this is the ninth bit received in the 9-bit UART modes 1 TI
99H
transmit interrupt flag - this bit is set by hardware when an entire byte has been transmitted - it must be cleared by software RI
98H
receive interrupt flag - this bit is set by hardware when an entire byte has been received - it must be cleared by software

8. Serial port modes SM0 SM1 Mode Description Baud Rate Mode 0
Mode 0
shift register
fixed - system clock frequency divided by 12 (machine cycle frequency) 1
Mode 1
8-bit UART
variable - set by timer 1
Mode 2
9-bit UART
fixed - system clock frequency divided by 12 or divided by 64
Mode 3
9-bit variable baud rate UART

9. Mode 1 --- 8-bit UART UART (Universal Asynchronous Receive/Transmit)
This is asynchronous communication, therefore no clock signal is sent with the data.
Instead, a single start bit (always logic 0) and a single stop bit (always logic 1) encapsulate the data (8 bits). Therefore, in total, ten bits are sent per byte of data.

10. Baud rate
If the designer of an 8051 system is required to transmit data from the serial port to a device (e. g., a monitor), he/she needs to discover what baud rate the device's serial port operates at and configure the 8051 serial port to operate at the same baud rate.
In mode 1, the baud rate is determined by the overflow of timer 1.
By default, the baud rate is equal to the timer 1 overflow frequency divided by 32.
Default baud rate = 12 MHz  NT  32
Timer
(NT)
 12
12 MHz
Baud rate
Default baud rate = 12 MHz  NT  32

11. How to control the baud rate?
To configure the port to a specific baud rate we therefore need to configure timer 1 to give us an overflow at an appropriate interval. This can be achieved by setting up timer 1 as an 8-bit auto-reload interval timer. Then, if we put the correct value into TH1 the overflow signal will have the appropriate frequency to give us the baud rate we desire.
The formula that determines the relationship between TH1 and baud rate:
TH1 = ((system frequency / (12 * 32)) / baud)
For example, to achieve a baud rate of 1200 using a system clock frequency of 12MHz and with SMOD = 0:
= ((12MHz / (12 * 32)) / 1200) =
= (rounding down with an error of 0.04)
= 230
Timer
(/26)
/12
12 MHz

12. The code below shows how to configure both timer 1 and the serial port to transmit data at 1200 baud.
CLR SM ;
SETB SM ; put the serial port in mode 1 (8-bit UART mode)
MOV TMOD, #20H ;put timer 1 in mode 2 (8-bit auto-reload mode) interval timing
MOV TH1, # ;put the reload value in TH1 - this results in a baud rate of 1200
SETB TR ;start timer 1
MOV SBUF, A ;send data in the accumulator down the serial line
Once the timer is started the programmer does not need to do anything else with it. The serial port itself monitors (through hardware) the overflow signal and every 32nd overflow (16 if SMOD is set) a bit is transmitted.

13. Various standard baud rates
The table shows some standard baud rates together with the required TH1 value to achieve this baud rate with the 8051 serial port in mode 1.

14. Calculation of system clock rate for desired baud rate
Counter cycle = (system frequency / (12 * 32)) / baud
Counter cycle = (system frequency / (12 * 16)) / baud
TH1 = 256 – counter cycle.
Real baud Rate = system clock/(12*32)/(rounded counter cycle)
Real baud Rate = system clock/(12*16)/(rounded counter cycle)
system clock (modified) = Baud Rate * (12*32) * (rounded counter cycle)
system clock (modified) = Baud Rate * (12*16) * (rounded counter cycle)
Baud rate (desired)
System frequency
SMOD
Counter cycle (exact)
Counter cycle (rounding)
TH1
Baud rate (real)
System clock (modified)
19200
12 MHz 2
254
15625 1 3
253
9600 6
250
4800 7
249 13
243
2400 26
230
1200 52
204

15. Why MHz
In Example 9, why do we need to choose MHz for the system clock when we select the 19,200 baud rate?
Ans.
Because
MHz / 12 / 3 / 16 = 20,
MHz / 12 / 3 / 16 = 19,  19,200
Timer
(3)
 12 M
19,

17. 8051 instructions for transmitting and receiving data
MOV SBUF, #45H
MOV A, SBUF
To transmit data along the serial line you simply write to the serial buffer and
to access data received on the serial port you simply read data from the serial buffer.
For example
MOV SBUF, #45H ; to send the byte 45H down the serial line
MOV A, SBUF ; to take data received by the serial port and puts it in the accumulator.

18. How do we know when the complete data byte has been sent?
MOV SBUF, #23H
MOV SBUF, #56H
If the second byte is sent to SBUF immediately after the first by is sent, the second byte may be overwritten before it was completely shifted down the line
MOV SBUF, #23H; JNB TI, $ CLR TI MOV SBUF, #56H;
“JNB TI, $” is to wait for the byte to be transmitted down the line.
The transmit interrupt flag (TI) is set by hardware once an entire byte has been transmitted.
The flag TI must be clear by software.

19. How do we know when data has been received?
JNB RI, $
CLR RI
MOV A, SBUF
“JNB RI, $” is to wait for an entire byte to be received
The receive interrupt flag (RI) is set by hardware when an entire byte is received by the serial port.
RI must be cleared by software.

20. Parity Even parity in 8051 accumulator:
Definition: The total number of 1’s in the accumulator and the parity bit (of the PSW) is even.
Example 1: If ACC contains the ASCII code of ‘Q’, then P = ?
Ans: ACC = B, P = (Acc has odd parity)
Example 2: If ACC contains the ASCII code of ‘S’
Ans: ACC = B, P = (Acc has even parity)
How can even parity be maintained while transmitting an ASCII code？
Method: Use the MSB for parity purpose, because it is not used for encoding the ASCII code.
Example:
Original
In the serial port
To transmit ‘Q’
ACC = B, P = 1 B
To transmit ‘S’
ACC = B, P = 0 B

21. Parity Instructions in program example 8 MOV C, P MOV ACC.7, C Purpose
To make the Acc even parity Q1
How does the p flag change? (What causes it to change? To what does it change?)
The 8051 maintains even parity with the accumulator, ie; the number of ones in the accumulator together with the parity bit (in the program status word) is always even. Q2
How can we make the Acc odd parity?
Acc = B (‘A’) P = Transmit B
Acc = (‘C’) P = Transmit B

22. Bit-addressability of SFR
SM0, SM1 bits in SCON register
SMOD bit in PCON register
Example instructions
CLR SM0 SETB SM1
MOV A, PCON SETB ACC.7 MOV PCON, A
Implication
The bits in SCON is bit-addressable.
The bits in PCON is NOT bit-addressable.

23. The ASCII codes go from 0 to 7FH
The ASCII　codes go from 0 to 7FH. Therefore, only seven bits are needed to code the basic ASCII characters. This leaves the 8th bit free - we can use this spare bit as a parity bit

24. Error detection
The receiver must be able to count the number of ones in the received data. If an 8051 was being used to receive even parity ASCII data on the serial line it could test the data validity as shown below:
waitForByte:
JNB RI, $ 　　　　　;wait for a byte to be received
CLR RI
MOV A, SBUF 　　　;move the received byte to the accumulator
JB P, waitForByte　　 ;if the parity bit is set, data transmission error => ignore data
CLR ACC.7 　　　　　;valid data - therefore clear the parity bit .

25. PCON Power Management Register (PCON)
Power management using a microcontroller  is something you see every day in mobile phones. Haven’t you noticed and got wondered by a mobile phone automatically going into stand by mode when not used for a couple of seconds or minutes ? This is achieved by power management feature of the controller used inside that phone.
As the name indicates, this register is used for efficient power management of 8051 micro controller. Commonly referred to as PCON register, this is a dedicated SFR for power management alone. From the figure below you can observe that there are 2 modes for this register :- Idle mode and Power down mode.
Setting bit 0  will move the micro controller to Idle mode and Setting bit 1 will move the micro controller to Power down mode.