Các vi điều khiển 8051 pps

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1
Chapter 5
Serial Port Operation
The 8051 Microcontroller
Lê Chí Thông
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
Ref. I. Scott Mackenzie 2Lê Chí Thông
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2
Serial port block diagram
Writing to SBUF loads data to be transmitted
Reading SBUF accesses received data
P→
→→
→S buffer S→
→→
→P buffer
Ref. I. Scott Mackenzie 3Lê Chí Thông
SCON Register

Ref. I. Scott Mackenzie 4Lê Chí Thông
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SCON Register
Ref. I. Scott Mackenzie 5Lê Chí Thông
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 f
OSC
Ref. I. Scott Mackenzie 6Lê Chí Thông
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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 7Lê Chí Thông
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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5
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 9Lê Chí Thông
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
synchronization event
stop

Ref. I. Scott Mackenzie 11Lê Chí Thông
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 12Lê Chí Thông
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7
Serial Port Baud Rates
1MHz (12 MHz crystal)
To set SMOD:
MOV A,PCON
SETB ACC.7
MOV PCON,A
375K/187.5K (12 MHz crystal)
Ref. I. Scott Mackenzie 13Lê Chí Thông
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)
• f
OSC
= 12 MHz  f
CLK
= 1 MHz  T
CLK
= 1 μs
• Assume SMOD=0: Timer 1 overflow rate = 1200 x
32 = 38.4 KHz  T
overflow
= 1/38.4 kHz = 26.04 μs
• An overflow requires T
overflow
/T
CLK
≈ 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)
• f

OSC
= 11.0592 MHz  T
CLK
= 12/11.0592 μs
• Assume SMOD=0: Timer 1 overflow rate = 1200 x
32 = 38.4 KHz  T
overflow
= 1/0.0384 [μs]
• An overflow requires T
overflow
/T
CLK
= 24 clocks
 The initial value for Timer 1 is -24
Ref. I. Scott Mackenzie 15Lê Chí Thông
Baud Rate Summary
Ref. I. Scott Mackenzie 16Lê Chí Thông
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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 ;reload count for 1200 baud
SETB TR1 ;start Timer 1
…
Ref. I. Scott Mackenzie 17Lê Chí Thông
Initialize the Serial Port (SMOD=1)
ORG 0000H

MOV SCON,#01010010B ;Serial port mode 1
MOV A,PCON
SETB ACC.7 ;SMOD=1
MOV PCON,A
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-26 ;reload count for 2400 baud
SETB TR1 ;start Timer 1
…
Ref. I. Scott Mackenzie 18Lê Chí Thông
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10
Example 1: Transmission
ORG 0000H
MOV SCON,#01010010B ;Serial port mode 1
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-24 ;reload count for 1200 baud
SETB TR1 ;start Timer 1
MOV R2,#10 ;number of loops
MOV R0,#30H ;starting address
LOOP: MOV A,@R0 ;get data
ACALL SEND ;send data
INC R0 ;increase pointer
DJNZ R2,LOOP ;loop 10 times
SJMP DONE
SEND: JNB TI,$ ;transmit buffer empty? No:check again
CLR TI ;yes: clear flag and
MOV A,SBUF ; send data
RET ;return
DONE: NOP

END
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)
19Lê Chí ThôngRef. I. Scott Mackenzie
Example 2: Reception
ORG 0000H
MOV SCON,#01010010B ;Serial port mode 1
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-12 ;reload count for 2400 baud
SETB TR1 ;start Timer 1
MOV R2,#20 ;number of loops
MOV R0,#40H ;starting address
LOOP: ACALL RECEIVE ;receive data
MOV @R0,a ;store data
INC R0 ;increase pointer
DJNZ R2,LOOP ;loop 10 times
SJMP DONE
RECEIVE:
JNB RI,$ ;receive buffer full? No: check again
CLR RI ;yes: clear flag and
MOV SBUF,A ; send data
RET ;return
DONE: NOP
END
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.
Ref. I. Scott Mackenzie 20Lê Chí Thông
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11
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 9
th
bit is whatever has been
put in TB8 in SCON.
• On reception, the 9
th
bit received is placed in RB8
in SCON.
• Baud rate is either f
OSC
/64 (SMOD=0)
or f
OSC
/32 (SMOD=1)
Ref. I. Scott Mackenzie 21Lê Chí Thông
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 22Lê Chí Thông
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12
Adding a Parity Bit
• A common use for the 9
th

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 9
th
data bit to be transmitted:
MOV C,P ;put even parity bit in C flag
MOV TB8,C ;and move to the 9
th
data bit
MOV SBUF,A;move from A to SBUF to transmit
Ref. I. Scott Mackenzie 23Lê Chí Thông
Adding a Parity Bit
• Eg. Put odd parity bit in TB8, which becomes the 9
th
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 9
th
data bit
MOV SBUF,A;move from A to SBUF to transmit
Ref. I. Scott Mackenzie 24Lê Chí Thông
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13
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 9
th
bit
Ref. I. Scott Mackenzie 25Lê Chí Thông
ORG 0000H
MOV SCON,#11010010B ;Serial port mode 3 (9-bit)
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-6 ;reload count for 4800 baud
SETB TR1 ;start Timer 1
MOV R2,#10 ;number of loops
MOV R0,#30H ;starting address
LOOP: MOV A,@R0 ;get data
MOV C,P ;put even parity bit in C flag
CPL C ;convert to odd parity
MOV TB8,C ;and move to the 9
th
data bit
ACALL SEND ;send data
INC R0 ;increase pointer
DJNZ R2,LOOP ;loop 10 times
SEND: JNB TI,$ ;check TI empty? No: check again
CLR TI ;yes: clear flag and
MOV A,SBUF ; send data
RET ;return
END
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 8
th

bit
Ref. I. Scott Mackenzie 26Lê Chí Thông
ORG 0000H
MOV SCON,#01010010B ;Serial port mode 1 (8-bit)
MOV TMOD,#00100000B ;Timer 1 mode 2
MOV TH1,#-6 ;reload count for 4800 baud
SETB TR1 ;start Timer 1
MOV R2,#10 ;number of loops
MOV R0,#30H ;starting address
LOOP: MOV A,@R0 ;get data
CLR ACC.7 ;clear the 8
th
bit of A
MOV C,P ;put even parity bit in C flag
CPL C ;convert to odd parity
MOV ACC.7,C ;and move to the 8
th
bit of A
ACALL SEND ;send data
INC R0 ;increase pointer
DJNZ R2,LOOP ;loop 10 times
SEND: JNB TI,$ ;check TI empty? No: check again
CLR TI ;yes: clear flag and
MOV A,SBUF ; send data
RET ;return
END
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14
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 9
th
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 9
th
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 27Lê Chí Thông
28
References
Lê Chí Thông
• I. Scott Mackenzie, The 8051 Microcontroller
• Các tài liệu trên Internet không trích dẫn hoặc không ghi tác
giả
Ref. I. Scott Mackenzie