Bài giảng Microprocessing Systems - Chapter 5: Serial port operation - Lê Chí Thông
ĐH Bách Khoa TP.HCM
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The 8051 Microcontroller
Chapter 5
Serial Port Operation
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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
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SCON Register
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SCON Register
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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)
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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
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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.
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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
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Serial Port Baud Rates
1MHz (12 MHz crystal)
375K/187.5K (12 MHz crystal)
To set SMOD:
MOV
A,PCON
ACC.7
SETB
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
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Baud Rate Summary
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Initialize the Serial Port
ORG 0000H
MOV SCON,#01010010B ;Serial port mode 1
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-26
SETB TR1
…
;reload count for 1200 baud
;start Timer 1
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Initialize the Serial Port (SMOD=1)
ORG 0000H
MOV SCON,#01010010B ;Serial port mode 1
MOV A,PCON
SETB ACC.7
MOV PCON,A
;SMOD=1
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-26
SETB TR1
…
;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 ;Serial port mode 1
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-24
SETB TR1
;reload count for 1200 baud
;start Timer 1
MOV R2,#10
MOV R0,#30H
;number of loops
;starting address
;get data
LOOP: MOV A,@R0
ACALL SEND
INC R0
;send data
;increase pointer
;loop 10 times
DJNZ R2,LOOP
SJMP DONE
SEND: JNB TI,$
;transmit buffer empty? No:check again
;yes: clear flag and
CLR TI
MOV SBUF,A
RET
;
send data
;return
DONE: NOP
END
Ref. I. Scott Mackenzie
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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.
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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 MEacNkeDnzie
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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 ;Serial port mode 1
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-12
SETB TR1
;reload count for 2400 baud
;start Timer 1
MOV R2,#20
MOV R0,#40H
LOOP: ACALL RECEIVE
MOV @R0,A
INC R0
;number of loops
;starting address
;receive data
;store data
;increase pointer
;loop 20 times
DJNZ R2,LOOP
SJMP DONE
RECEIVE:
JNB RI,$
;receive buffer full? No: check again
;yes: clear flag and
CLR RI
MOV A,SBUF
RET
;
receive data
;return
DONE: NOP
END
Ref. I. Scott Mackenzie
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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
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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)
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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
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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
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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
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Example 3
ORG 0000H
MOV SCON,#11010010B ;Serial port mode 3 (9-bit)
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-6
SETB TR1
;reload count for 4800 baud
;start Timer 1
MOV R2,#10
MOV R0,#30H
;number of loops
;starting address
;get data
LOOP: MOV A,@R0
MOV C,P
;put even parity bit in C flag
;convert to odd parity
;and move to the 9th data bit
;send data
CPL C
MOV TB8,C
ACALL SEND
INC R0
;increase pointer
;loop 10 times
DJNZ R2,LOOP
SJMP DONE
SEND: JNB TI,$
CLR TI
;check TI empty? No: check again
;yes: clear flag and
MOV SBUF,A
RET
;
send data
;return
DONE: NOP
END
Ref. I. Scott Mackenzie
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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 ;Serial port mode 1 (8-bit)
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-6
SETB TR1
;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
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
;
;return
send data
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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
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References
• I. Scott Mackenzie, The 8051 Microcontroller
• Các tài liệu trên Internet không trích dẫn hoặc không ghi tác
giả
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