8051 Chap5 Serial

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H Bch Khoa TP.HCM L Ch Thngwww.tinyurl.com/thongchile

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Chapter 5Serial Port Operation

The 8051 Microcontroller

L Ch Thng

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 fortransmission and 1 buffer for reception

Baud rate (serial port frequency of operation) issupplied and programmed by Timer1

Ref. I. Scott Mackenzie 2L Ch Thng

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Serial port block diagram

Writing to SBUF loads data to be transmittedReading SBUF accesses received data

P S buffer S P buffer


SCON Register


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SCON Register


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 TXDoutputs the shift clock

Baud rate = 1/12 f OSC


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Transmission is initiated by instruction that writesdata to SBUF (eg. MOV SBUF,A)



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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One application of shift register mode is to expand theout capability of the 8051

A serial-to-parallel shift register IC can beconnected to the 8051 TXD and RXD lines toprovide an extra output lines


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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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



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, and2. SM2=0, or SM2=1 and the received stop bit = 1

WAIT:JNB RI,WAIT ;Check RI until setCLR RI ;Clear RI

MOV A,SBUF ;Read characterRef. I. Scott Mackenzie 12L Ch Thng

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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)


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 TCLK= 1 s Assume SMOD=0: Timer 1 overflow rate = 1200 x

32 = 38.4 KHz Toverflow = 1/38.4 kHz = 26.04 s

An overflow requires T overflow /TCLK 26 clocksThe 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 MHzcrystal .

Eg. Calculate Timer 1 overflow rate to provide 1200baud operation (11.0592 MHz crystal)

f OSC = 11.0592 MHz TCLK= 12/11.0592 s

Assume SMOD=0: Timer 1 overflow rate = 1200 x32 = 38.4 KHz Toverflow = 1/0.0384 [s]

An overflow requires T overflow /TCLK= 24 clocksThe initial value for Timer 1 is -24


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 ;reload count for 1200 baud

SETB TR1 ;start Timer 1


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


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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 timesSJMP DONESEND: JNB TI,$ ;transmit buffer empty? No:check again

CLR TI ;yes: clear flag and MOV A,SBUF ; send data

RET ;returnDONE: 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 st ring to the 8051 serial port (1200 baud, crystal11.0592 MHz)

19L Ch ThngRef. 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


LOOP: ACALL RECEIVE ;receive data MOV @R0,a ;store data

INC R0 ;increase pointerDJNZ R2,LOOP ;loop 10 timesSJMP DONE

RECEIVE:JNB RI,$ ;receive buffer full? No: check againCLR RI ;yes: clear flag and

MOV SBUF,A ; send dataRET ;return

DONE: NOPEND

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.


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Mode 2: 9-Bit UART with Fixed Baud Rate

Mode 2: 11 bits are transmitted on TXD or received onRXD, including a start bit (0), 9 data bits (LSB first),and a stop bit (1).

On transmission , the 9 th bit is whatever has beenput in TB8 in SCON.

On reception , the 9 th bit received is placed in RB8in SCON.

Baud rate is either f OSC/64 (SMOD=0)or f OSC/32 (SMOD=1)



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 9 th bit is to add parity to acharacter.

The P bit in PSW register is set or cleared to establisheven parity with 8 bits in A register .

Eg. Put even parity bit in TB8, which becomes the 9 thdata 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


Adding a Parity Bit

Eg. Put odd parity bit in TB8, which becomes the 9 thdata 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


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


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


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 dataINC R0 ;increase pointerDJNZ R2,LOOP ;loop 10 times

SEND: JNB TI,$ ;check TI empty? No: check againCLR TI ;yes: clear flag and

MOV A,SBUF ; send dataRET ;returnEND

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


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


LOOP: MOV A,@R0 ;get dataCLR ACC.7 ;clear the 8 th bit of A

MOV C,P ;put even parity bit in C flagCPL 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 pointerDJNZ R2,LOOP ;loop 10 times

SEND: JNB TI,$ ;check TI empty? No: check againCLR TI ;yes: clear flag and

MOV A,SBUF ; send dataRET ;returnEND

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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 9 th

bit . So all slave can receive the address byte and examine it totest if it is being addressed.

The addressed slave will clear its SM2 bit and prepare to receivethe data bytes that follow. The 9 th bit in data byte is 0 .

The slaves that were not addressed leave their SM2 bits set andignore the incoming data bytes


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References

L Ch Thng

I. Scott Mackenzie, The 8051 Microcontroller Cc ti li u trn Internet khng trch d n hoc khng ghi tc

gi

Ref. I. Scott Mackenzie

8051 Chap5 Serial

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