8051 chap5 serial VI XỬ LÝ

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The 8051 Microcontroller
Chapter 5
Serial Port Operation
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sites.google.com/site/chithong
Ref. I. Scott Mackenzie, The 8051 Microcontroller

Serial Port
•
•
•
•
•

RXD (P3.0) and TXD (P3.1) pins
Full Duplex: simultaneous transmission and reception
2 special function registers: SCON and SBUF
SCON: status bits and control bits
SBUF: same address but 2 buffers; 1 buffer for
transmission and 1 buffer for reception
• Baud rate (serial port frequency of operation) is
supplied and programmed by Timer1

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Serial port block diagram
P→
→S buffer

S→
→P buffer

Writing to SBUF loads data to be transmitted
Reading SBUF accesses received data
Ref. I. Scott Mackenzie

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SCON Register

Ref. I. Scott Mackenzie

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SCON Register

Ref. I. Scott Mackenzie

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Mode 0: 8-Bit Shift Register
• RXD is used for both data input and output
• Serial data enter and exit (LSB first) through RXD
• TXD line serves as the clock
• TXD outputs the shift clock
• Baud rate = 1/12 fOSC

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Mode 0: 8-Bit Shift Register
• Transmission is initiated by instruction that writes
data to SBUF (eg. MOV SBUF,A)

Ref. I. Scott Mackenzie

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Mode 0: 8-Bit Shift Register
• Reception is initiated when REN is 1 and RI is 0
• Set REN at the beginning of a program
• Clear RI to begin a data input operation

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Mode 0: 8-Bit Shift Register
• One application of shift register mode is to expand the
out capability of the 8051
• A serial-to-parallel shift register IC can be
connected to the 8051 TXD and RXD lines to
provide an extra output lines

Ref. I. Scott Mackenzie

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Mode 1: 8-Bit UART with Variable Baud Rate
• UART: Universal Asynchronous Receiver/Transmitter
• A data frame includes a start bit (low), data bits,

and a stop bit (high).
• A parity bit is sometimes inserted between the last
data bit and the stop bit.
• Mode 1: 10 bits are transmitted on TXD or received on
RXD, including a start bit (0), 8 data bits (LSB first),
and a stop bit (1).
• The stop bit goes into RB8 in SCON.
• Baud rate is set by the Timer 1 overflow rate.
Ref. I. Scott Mackenzie

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Mode 1: 8-Bit UART with Variable Baud Rate
• Transmission is initiated by writing to SBUF.
• TI is set at the end of character transmission and
indicates “transmit buffer empty”.
WAIT:JNB TI,WAIT ;Check TI until set
CLR TI
;Clear TI

MOV SBUF,A ;Send character
stop

synchronization event
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Mode 1: 8-Bit UART with Variable Baud Rate
• Reception is initiated by a 1-to-0 transition on RXD.
1. The stop bit goes into RB8 in SCON.
2. SBUF is loaded with 8 data bits.
3. RI is set at the end of character reception and
indicates “receiver buffer full”.
• Conditions for reception:
1. RI=0, and
2. SM2=0, or SM2=1 and the received stop bit = 1
WAIT:JNB RI,WAIT ;Check RI until set
CLR RI
;Clear RI
MOV A,SBUF ;Read character
Ref. I. Scott Mackenzie

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Serial Port Baud Rates
1MHz (12 MHz crystal)

375K/187.5K (12 MHz crystal)

Ref. I. Scott Mackenzie

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To set SMOD:
MOV
A,PCON
SETB
ACC.7
MOV
PCON,A

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Using Timer 1 as the Baud Rate Clock
•
•

•
•

Usually use Timer 1 Mode 2 to provide baud rate clock
Baud Rate = Timer 1 overflow rate / 32 (SMOD=0)
Baud Rate = Timer 1 overflow rate / 16 (SMOD=1)
Eg. Calculate Timer 1 overflow rate to provide 1200
baud operation (12 MHz crystal)
• fOSC = 12 MHz
fCLK = 1 MHz
TCLK = 1 μs
• Assume SMOD=0: Timer 1 overflow rate = 1200 x
32 = 38.4 KHz
Toverflow = 1/38.4 kHz = 26.04 μs
• An overflow requires Toverflow/TCLK ≈ 26 clocks

The reload value for Timer 1 is -26

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Error in Baud Rate

• Due to rounding, there is a slight error. Generally, a 5%
error is tolerable.
• Exact baud rates are possible using an 11.0592 MHz
crystal.
• Eg. Calculate Timer 1 overflow rate to provide 1200
baud operation (11.0592 MHz crystal)
• fOSC = 11.0592 MHz
TCLK = 12/11.0592 μs
• Assume SMOD=0: Timer 1 overflow rate = 1200 x
32 = 38.4 KHz
Toverflow = 1/0.0384 [μs]
• An overflow requires Toverflow/TCLK = 24 clocks

The initial value for Timer 1 is -24
Ref. I. Scott Mackenzie

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Baud Rate Summary

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Initialize the Serial Port
ORG 0000H
MOV SCON,#01010010B
MOV TMOD,#00100000B
MOV TH1,#-26
SETB TR1
…

Ref. I. Scott Mackenzie

;Serial port mode 1
;Timer 1 mode 2
;reload count for 1200 baud
;start Timer 1

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Initialize the Serial Port (SMOD=1)
ORG 0000H
MOV SCON,#01010010B
MOV A,PCON
SETB ACC.7
MOV PCON,A
MOV TMOD,#00100000B

MOV TH1,#-26
SETB TR1
…

Ref. I. Scott Mackenzie

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;Serial port mode 1
;SMOD=1
;Timer 1 mode 2
;reload count for 2400 baud
;start Timer 1

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Example 1: Transmission
Assume a 10-byte string of data is stored in the internal RAM from the location 30H.
Write a program that sends this string to the 8051 serial port (1200 baud, crystal
11.0592 MHz)
ORG 0000H

MOV SCON,#01010010B
MOV TMOD,#00100000B
MOV TH1,#-24
SETB TR1
MOV R2,#10
MOV R0,#30H
LOOP: MOV A,@R0
ACALL SEND
INC R0
DJNZ R2,LOOP
SJMP DONE
SEND: JNB TI,$
CLR TI
MOV SBUF,A
RET
DONE: NOP
END
Ref. I. Scott Mackenzie

;Serial port mode 1
;Timer 1 mode 2
;reload count for 1200 baud
;start Timer 1
;number of loops
;starting address
;get data
;send data
;increase pointer
;loop 10 times
;transmit buffer empty? No:check again

;yes: clear flag and
;
send data
;return
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Your Turn!
Assume that a string of data is stored in internal RAM at address
30H to 50H. Write a program that sends this string to serial port
using UART 8-bit, 2400 baud, 11.059-MHz crystal.

Ref. I. Scott Mackenzie

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Solution
Assume that a string of data is stored in internal RAM at address

30H to 50H. Write a program that sends this string to serial port
using UART 8-bit, 2400 baud, 11.059-MHz crystal.
ORG 0000H
MOV SCON,#01010010B
MOV TMOD,#00100000B
MOV TH1,#-12
SETB TR1
MOV R0,#30H
LOOP: MOV A,@R0
ACALL SEND
INC R0
CJNE R0,#51H,LOOP
SJMP DONE
SEND: JNB TI,$
CLR TI
MOV SBUF,A
RET
DONE: NOP
Ref. I. Scott Mackenzie
END

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Example 2: Reception
Write a program that receives a 20-byte string from the 8051 serial port (2400 baud,
crystal 11.0592 MHz) and then stores in the internal RAM from the location 40H.
ORG 0000H
MOV SCON,#01010010B

MOV TMOD,#00100000B
MOV TH1,#-12
SETB TR1
MOV R2,#20
MOV R0,#40H
LOOP: ACALL RECEIVE
MOV @R0,A
INC R0
DJNZ R2,LOOP
SJMP DONE
RECEIVE:
JNB RI,$
CLR RI
MOV A,SBUF
RET
DONE: NOP
END
Ref. I. Scott
Mackenzie

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;Serial port mode 1
;Timer 1 mode 2
;reload count for 2400 baud
;start Timer 1
;number of loops
;starting address
;receive data
;store data

;increase pointer
;loop 20 times

;receive buffer full? No: check again
;yes: clear flag and
;
receive data
;return
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Your Turn!
Write a program that receives a 20-byte string from serial port using
UART 8-bit, 4800 baud, 11.059-MHz crystal, and then writes data to
internal RAM from address 40H

Ref. I. Scott Mackenzie

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Problem 1

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Problem 2

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Mode 2: 9-Bit UART with Fixed Baud Rate
• Mode 2: 11 bits are transmitted on TXD or received on
RXD, including a start bit (0), 9 data bits (LSB first),
and a stop bit (1).
• On transmission, the 9th bit is whatever has been

put in TB8 in SCON.
• On reception, the 9th bit received is placed in RB8
in SCON.
• Baud rate is either fOSC/64 (SMOD=0)
or fOSC/32 (SMOD=1)

Ref. I. Scott Mackenzie

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Mode 3: 9-Bit UART with Variable Baud Rate
• 9-bit UART: same as mode 2
• Variable baud rate: same as mode 1

Ref. I. Scott Mackenzie

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Adding a Parity Bit
• A common use for the 9th bit is to add parity to a
character.
• The P bit in PSW register is set or cleared to establish
even parity with 8 bits in A register.
• Eg. Put even parity bit in TB8, which becomes the 9th
data bit to be transmitted:
MOV C,P

;put even parity bit in C flag

MOV TB8,C ;and move to the 9th data bit
MOV SBUF,A;move from A to SBUF to transmit

Ref. I. Scott Mackenzie

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Adding a Parity Bit
• Eg. Put odd parity bit in TB8, which becomes the 9th
data bit to be transmitted:
MOV C,P

;put even parity bit in C flag

CPL C
;convert to odd parity
MOV TB8,C ;and move to the 9th data bit
MOV SBUF,A;move from A to SBUF to transmit

Ref. I. Scott Mackenzie

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Example 3
Assume a 10-byte string of 8-bit ASCII codes is stored in internal RAM from the
location 30H. Write a program that transmits this string out the 8051 serial port (4800
baud, crystal 11.0592 MHz) with odd parity added as the 9th bit

Ref. I. Scott Mackenzie

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Example 3
ORG 0000H
MOV SCON,#11010010B
MOV TMOD,#00100000B
MOV TH1,#-6
SETB TR1
MOV R2,#10
MOV R0,#30H
LOOP: MOV A,@R0
MOV C,P
CPL C
MOV TB8,C
ACALL SEND
INC R0
DJNZ R2,LOOP
SJMP DONE
SEND: JNB TI,$
CLR TI
MOV SBUF,A
RET
DONE: NOP
END

Ref. I. Scott Mackenzie

;Serial port mode 3 (9-bit)
;Timer 1 mode 2
;reload count for 4800 baud
;start Timer 1
;number of loops
;starting address
;get data
;put even parity bit in C flag
;convert to odd parity
;and move to the 9th data bit
;send data
;increase pointer
;loop 10 times
;check TI empty? No: check again
;yes: clear flag and
;
send data
;return

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Example 4
Assume a 10-byte string of 7-bit ASCII codes is stored in internal RAM from the
location 30H. Write a program that transmits this string out the 8051 serial port (4800
baud, crystal 11.0592 MHz) with odd parity added as the 8th bit
ORG 0000H

MOV SCON,#01010010B
MOV TMOD,#00100000B
MOV TH1,#-6
SETB TR1
MOV R2,#10
MOV R0,#30H
LOOP: MOV A,@R0
CLR ACC.7
MOV C,P
CPL C
MOV ACC.7,C
ACALL SEND
INC R0
DJNZ R2,LOOP
SEND: JNB TI,$
CLR TI
MOV A,SBUF
RET
END
Ref. I. Scott Mackenzie

sites.google.com/site/chithong

;Serial port mode 1 (8-bit)
;Timer 1 mode 2
;reload count for 4800 baud
;start Timer 1
;number of loops
;starting address
;get data

;clear the 8th bit of A
;put even parity bit in C flag
;convert to odd parity
;and move to the 8th bit of A
;send data
;increase pointer
;loop 10 times
;check TI empty? No: check again
;yes: clear flag and
;
send data
;return
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Multiprocessor Communications
• When SM2=1, reception is done only if RB8=1.
• The master first sends out an address byte that has 1 in the 9th
bit. So all slave can receive the address byte and examine it to
test if it is being addressed.
• The addressed slave will clear its SM2 bit and prepare to receive
the data bytes that follow. The 9th bit in data byte is 0.

• The slaves that were not addressed leave their SM2 bits set and
ignore the incoming data bytes

Ref. I. Scott Mackenzie

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References
• I. Scott Mackenzie, The 8051 Microcontroller
• Các tài liệu trên Internet khơng trích dẫn hoặc khơng ghi tác
giả

Ref. I. Scott Mackenzie

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