This document discusses serial communication concepts and programming on 8051 microcontrollers. It explains the basics of serial vs parallel communication and asynchronous transmission using start and stop bits. It describes the SCON and SMOD registers used for serial communication and the various modes of operation including fixed and variable baud rates. The document provides code examples for transmitting and receiving data over serial in 8051 assembly language and demonstrates using Keil simulator.

1. Dr. Vikas Dongre
HOD Electronics
Government Polytechnic Washim
dongrevj1@gmail.com
High Resolution

2. Objectives
• Types of communication
• Baud rate setting
• Various SFRs used for serial communication
• Various modes of serial communication
• Programs for transmission and reception with
demonstration

3. UART
UART Universal Asynchronous Receiver Transmitter

4. Basics of serial communication
Parallel: expensive - short distance ( Within parts of a system) – fast
Serial :cheaper– long distance(Few Mtr. to many KM by modem)-slow

5. Basics of serial communication

6. Start and stop bits
When there is no transfer the signal is high Transmission
begins with a start (low) bit LSB first
Finally 1 stop bit (high) Data transfer rate (baud rate) is stated in
bps bps: bit per second
Framing ASCII “A” (41H)

7. Serial Port Block Diagram

8. In RS232, a 1 is represented
by -3 ~ -25 V, while a 0 bit is
+3 ~ +25 V, making -3 to +3
undefined

9. Micro
contol
ler 1
Max 232
Micro
contol
ler 2
Max 232
Tx
TxRx
Rx

10. How to communicate 8051 to PC
 Connect TXD to RXD and RXD to TXD from pc to 8051
 Use max232 to transform signal from TTL level to RS232 level
 The baud rate of the transmitting 8051 must matched the baud
rate of the receiving 8051 or pc
 PC standard baud rate
 2400-4800-9600-14400-19200-28800-33600-57600
 Timer 1 in mode 2 ( auto reload) is used for baud rate generation
 The 8051 UART divides the machine cycle frequency by 32
 Machine cycle is 1/12 XTAL frequency

11. TH1=FF+1- required Count

12. RxD and TxD pins in the 8051
• TxD pin 11 of the 8051 (P3.1)
• RxD pin 10 of the 8051 (P3.0)
SBUF register
MOV SBUF, #’D’ ;load SBUF=44H, ASCII for ‘D’
MOV SBUF, A ;copy accumulator into SBUF
MOV A, SBUF ;copy SBUF into accumulator
SFRs used
SBUF, SCON, PCON for actual communication
TCON,TMOD,TH1, TL1 for Baud Rate generation

13. Serial control (SCON) Register

14. Modes of operation
SM0 SM1 MODE operation transmit rate
0 0 0 shift register fixed (xtal/12)
0 1 1 8 bit UART variable (timer1)
1 0 2 9 bit UART fixed (xtal/32 or xtal/64)
1 1 3 9 bit UART variable (timer1)

15. Modes of operation
• Mode 0 :
• Serial data enters and exits through RxD
• TxD outputs the shift clock.
• 8 bits are transmitted/received(LSB first)
• The baud rate is fixed a 1/12 the oscillator frequency.
• Application
• Port expansion
8051
TXD
RXD Shift register
clk
data

16. Modes of operation
• Mode 1
• Ten bits are transmitted (through TxD) or received (through RxD)
• A start bit (0), 8 data bits (LSB first), and a stop bit (1)
• On receive, the stop bit goes into RB8 in SCON
• the baud rate is determined by the Timer 1 overflow rate.
• Transmission is initiated by any instruction that uses SBUF as a
destination register.

17. Modes of operation

18. Power control register
 Bit 7 of PCON register
 If SMOD=1 double baud rate
 PCON is not bit addressable
 How to set SMOD
Mov a, pcon
Setb acc.7
Mov pcon,a

19. • Mode 2 :
• Eleven bits are transmitted (through TxD), received (through RxD)
• A start bit (0)
• 8 data bits (LSB first)
• A programmable 9th data bit
• and a stop bit (1)
• On transmit, the 9th bit (TB8) can be assigned 0 or 1.
• On receive, the 9th data bit goes into RB8 in SCON.
• the 9th can be parity bit
• The baud rate is programmable to 1/32 or 1/64 the oscillator frequency in
Mode 2 by SMOD bit in PCON register
• Mode 3
• Same as mode 2
• But may have a variable baud rate generated from Timer 1.
Modes of operation

20. Steps for Serial Transmission programming
1. Set communication mode using SCON SFR
2. Set Baud rate using TMOD , TH1 SFR
3. Start timer to generate baud rate
4. Move data in SBUF which you wish to transmit
5. Wait till the data byte is transmitted by monitoring the
setting of TI flag
6. Clear TI flag
7. Repeat step 4 to 6 again and again to transmit more
bytes
Note: Serial transmission and serial reception should run on both transmitting
and receiving controllers at the same time , that too with same baud rate

21. Serial Transmit(1)
ORG 0000
MOV TMOD ,#20H ; set timer 1 in mode 2
MOV TH1, #-3 ; set baud rate as 9600
MOV SCON, #50H ; set communication in mode 1 with receive enable
SETB TR1 ; start timer to generate baud rate
AGAIN:
MOV SBUF, #Y’ ;Transmit Y
back: JNB TI, back
CLR TI
MOV SBUF, #'E‘ ;Transmit E
JNB TI,$
CLR TI
MOV SBUF, #'S‘ ; Transmit S
JNB TI,$
CLR TI
MOV SBUF, #' ‘ : Transmit space
JNB TI,$
CLR TI
SJMP AGAIN
END

23. ORG 0000
MOV TMOD ,#20H
MOV TH1, #-3
MOV SCON, #50H
SETB TR1
AGAIN:
MOV SBUF, #"M"
CALL TRANSMIT
MOV SBUF, #'S'
CALL TRANSMIT
MOV SBUF, #'B'
CALL TRANSMIT
MOV SBUF, #'T'
CALL TRANSMIT
MOV SBUF, #'E'
CALL TRANSMIT
MOV SBUF, #' '
CALL TRANSMIT
SJMP AGAIN
TRANSMIT: JNB TI, $
CLR TI
RET
Serial Transmit(2)

24. ORG 0000
MOV TMOD ,#20H
MOV TH1, #-3
MOV SCON, #50H
SETB TR1
SETB TI
AGAIN: MOV R0,#10
MOV DPTR,#100H
BACK: MOV A,#00H
MOVC A, @A+DPTR
MOV SBUF,A
CALL TRANSMIT
INC DPTR
DJNZ R0, BACK
SJMP AGAIN
TRANSMIT: JNB TI,$
CLR TI
RET
ORG 100H
DB 'G','P',' ', 'W', 'A', 'S', 'H', 'I', 'M',' '
END
Serial Transmit(3)

25. Steps for Serial Reception programming
1. Set communication mode using SCON SFR
2. Set Baud rate using TMOD , TH1 SFR
3. Start timer to generate baud rate
4. Wait till the data byte is transmitted by monitoring the
setting of RI flag
5. Move data received in SBUF to other place like
memory or use on any port
6. Clear RI flag
7. Repeat step 4 to 6 again and again to transmit more
bytes

26. ;PROGRAM FOR RECEIVING DATA SERIALY
ORG 0000
MOV TMOD ,#20H ;SET TIMER 1 IN MODE 2
MOV TH1, #-3 ; SET BAUD AS 9600
MOV SCON, #50H ; ENABLE RECRIVE
SETB TR1 ; START TIMER
BACK: JNB RI, $ ; CHECK IF BUFFER FULL
MOV A,SBUF ; IF YES READ THE SBUFFER
MOV P1,A ; SEND RECEIVED DATA TO PORT 1
CLR RI ; CLEAR RECEIVE FLAG
SJMP BACK
END
Serial Receive

27. Summery
• Concept of Serial and Parallel communication
• Asynchronous communication
• Concept of Start and Stop bits
• SCON, SMOD SFRs used for serial communication
• Various modes of serial communication
• Programs for transmission and reception with Keil Simulation

28. Thank you !!!
Dr. Vikas Dongre
HOD Electronics
Government Polytechnic Washim
dongrevj1@gmail.com