8051 Serial Port - gmitWEB

8051 Serial Port 

• The 8051 contains a UART – Universal Asynchronous Receiver Transmitter 

– The serial port is full-duplex 

• It can transmit and receive simultaneously 

– 2 Port 3 pins are used to provide the serial interface 

• P3.0 is the receive pin (RXD) 

• P3.1 is the transmit pin (TXD) 

– Both synchronous and asynchronous transmission supported 

• A synchronous transmission sends a clock signal with the data 

– Adjustable baud rate and data frame size supported in asynchronous mode 

8051 

Crystal 

X1 

X2 

TXD 

RXD 

I/O 

Device 

Embedded Systems 1 5-1 8051 Peripherals

Asynchronous Data Transmission 

• Data is transmitted in a frame consisting of a start bit, 8 or 9 data bits and a stop bit 

– Start bit is always low 

– Data is transmitted LSB first 

– Stop bit is always high 

• Data transmission/reception rate is controlled by baud rate generator clock 

– Baud rate may be a division of the crystal frequency or programmed using a 

timer 

D0 D1 D2 D3 D4 D5 D6 D7 

Line Idle 

Start Bit 

Stop Bit 


Serial Port Modes 

• Mode 0 

– Only synchronous mode 

– Data transferred on RXD, clock on TXD 

– Clock is fixed at 1/12 of the oscillator frequency 

• Mode 1 

– Asynchronous mode 

– 10-bit data frame (start bit, 8 data bits and a stop bit) 

– Variable baud rate 

– Stop bit is placed in RB8 bit of SCON register on reception of a frame 


Serial Port Modes 

• Mode 2 

– Asynchronous mode 

– 11-bit data frame (start bit, 8 data bits, programmable 9 th bit and a stop bit) 

– On transmission 9 th bit is TB8 bit of SCON 

– On reception 9 th bit is placed in RB8 bit of SCON 

– 9 th bit may be used for data or as a parity bit 

– Baud rate may be 1/32 or 1/64 of oscillator frequency 

• Mode 3 

– Same as Mode 2 but with a programmable baud rate 


Data Transmission/Reception 

• The register SBUF is used to hold both the transmit and receive serial port data 

• To transmit data load the SBUF register 

– MOV SBUF, source 

– When transmission is complete the TI bit will be set in the SCON register 

• When a data frame is received the RI bit in SCON is set high 

– The received data may then be loaded from SBUF 

– MOV destination, SBUF 

– Data reception is double buffered 

• Reception of a second data frame may commence before the previous data 

frame has been read 

– The REN bit in SCON must be set to enable data reception 


SCON Register 


Configuring the Serial Port 

• Configure the serial port mode in SCON 

– Bits SM0 and SM1 determine the mode 

– Set the REN bit if serial data reception is required 

– Clear the RI and TI bits 

• The RI flag is set when a character has been received by the serial port 

• The TI flag is set after transmission of a character 

• Both flags must be cleared by software 

• Configure the serial port baud rates 

– The baud rate is fixed for modes 0 and 2 

• For mode 0 the baud rate = f osc 

/12 

• For mode 2 the baud rate = 2 SMOD * f osc 

/64 

– SMOD is a bit in the PCON register (not bit addressable) 

– For modes 1 and 3 the baud rate is configured by the timer 1 overflow rate 


Baud Rate for Modes 1 and 3 

• Configure timer 1 for 8-bit auto-reload mode (timer mode 2) 

– Upper 4 bits of TMOD register = 0010 

Baud Rate 

= 

2 

32 

SMOD 

x 

( Timer 1Overflow Rate) 

Baud Rate 

= 

2 

32 

SMOD 

x 

Oscillator Frequency 

12x 

[ 256 − TH1] 


Common Baud Rates 


Serial Program Example 1 

;Program to continually transmit the character A out the serial port 

ORG 0H 

MOV SCON, #40H ;serial port mode 1 

;configure timer 1 in auto-reload mode for 9600 baud 

MOV TMOD, #20H ;timer mode 2 

MOV TH1, #0FDH ;reload value for 9600 baud 

MOV TL1, #0FDH 

SETB TR1 

;start timer 

LOOP: MOV SBUF, #'A' ;transmit character 

WAIT4TI: JNB TI, WAIT4TI ;wait for end of transmission 

CLR TI 

JMP LOOP ;re-transmit 

END 



; Program to echo characters received on serial port 

ORG 0H 

MOV SCON, #50H ;serial port mode 1, receiver enabled 

ORL PCON, #80H ;SMOD = 1, double baud rate 



MOV TH1, #0FDH ;reload value for 19200 baud with SMOD = 1 


SETB TR1 

;start timer 

;wait for a character to be received 

WAIT4RI: JNB RI, WAIT4RI ;wait for receive flag to go high 

CLR RI 

MOV A, SBUF 

;read received character 

;transmit received character 

MOV SBUF, A 

;transmit character 


CLR TI 

JMP WAIT4RI 

;return and wait for next character 

END 



;Program to print the message "Hello" "Goodbye" on serial port 

;A new line is placed between the strings 

MESSAGE EQU 100H 

;address where string is stored 

;define characters to move cursor to next line 

CR EQU 0DH 

LF EQU 0AH 

ORG 0H 

MOV SCON, #40H ;serial port mode 1 



MOV TH1, #0FDH ;reload value for 9600 


SETB TR1 

;start timer 

;continued on next slide 


Serial Program Example 3 (cont) 

;loop to transmit message 

MOV DPTR, #MESSAGE 

TXMSG: CLR A 

MOVC A, @A+DPTR ;fetch character from string 

CJNE A, #'*', TRANSMIT ;check for end of message 

JMP DONE 

TRANSMIT: MOV SBUF, A ;transmit character 


CLR TI 

INC DPTR 

;point to next character 

JMP TXMSG 

DONE: 

ORG MESSAGE 

DB 'Hello' 

DB CR, LF 

DB 'Goodbye*' 

END 

;string to be printed 


Synchronous Serial Comms (Mode 0) 

• The TXD pin is used to carry the clock 

• Data is transmitted and received on the RXD pin (half duplex) 

• Data Transmission 

– Initiated by writing a character to the SBUF register 

– Data shifted out RXD pin, LSB first (8 bits in total) 

– 8 clocks shifted out TXD pin 

– TI flag set after byte of data has been transmitted 

• Data Reception 

– Initiated by REN = 1 and RI = 0 

– 8 clock pulses are outputted on TXD pin 

– 1 bit of data shifted into RXD pin for each clock 

– RI is set when 8 bits have been received into SBUF 


Serial I/O Expansion 

• A 74LS164 Serial In Parallel Out Shift Register may be used to expand the serial port 

to be used as 8 output pins 

– A number of shift registers may be cascaded to increase the number of output 

pins gained 

Data Input 

Clock 

Reset 

8 Parallel Outputs 


Serial Output Pin Expansion 

• P1.0 will clear the shift register outputs when brought low 

• 8 output pins generated from 3 8051 pins 

– Connect Q7 of 74LS164 to the data input of a second 74LS164 to generate 15 

outputs from 3 8051 pins 


74LS164 

P1.0 

RXD 

TXD 

MR 

DIN 

CLK 

Q0 

Q1 

Q2 

Q3 

Q4 

Q5 

Q6 

Q7 

8 Output 

Ports 


Serial Input Pin Expansion 

• A 74LS165 Parallel In Serial Out Shift Register may be used to allow the 8051 serial 

port to provide additional input port pins 

– A number of shift registers may be cascaded to increase the number of output 

pins gained 


Serial Input Pin Expansion 

• P1.0 will load the register parallel inputs when brought low 

• 8 input pins generated from 3 8051 pins 

– Connect data output pin of a second 74LS165 to the data input pin of another to 

generate 16 inputs from 3 8051 pins 

74LS165 

8 Input 

Ports 

QA 

QB 

QC 

QD 

QE 

QF 

QG 

QH 

INHIB 

OUT 

CLK 

LOAD 


RXD 

TXD 

P1.0 

INPUT 


Multi-Processor Communication 

• Modes 2 and 3 can be configured to allow serial communication between a master 

8051 and a number of slave 8051s 

– The TXD pin of the master connects to the RXD pin of the slave 8051s 

– In modes 2 and 3 a 9 th data bit is transmitted 

• This bit is received into the RB8 bit of the SCON register 

• If the SM2 bit of SCON is set, the RI flag will only be set when a serial 

frame is received with the 9 th bit = ‘1’ 

• This 9 th bit can be set/cleared to specify an address/data frame 

TXD 

Master 


RXD 

RXD 

RXD 

Slave 

8051 #1 

Slave 

8051 #2 

………….. 

Slave 

8051 #n 


Multi-Processor Communication 

• On initialisation all slaves set the SM2 bit in the SCON register 

– Each slave has a unique 8-bit address stored in non-volatile memory 

• Master 8051 sends an address frame – 9 th bit TB8 = ‘1’ 

– The address frame contains the 8-bit address of the destination slave 

• All slaves will see the received address frame 

– RI bit will be set because the 9 th bit RB8 = ‘1’ 

– Each slave checks the received address against it’s own address 

• The addressed slave will now clear it’s SM2 bit in SCON to ‘0’ 

• The master will now send some data frames with the 9 th bit = ‘0’ 

– Only the addressed slave will see the data frames 

– No RI flag will be set in the other slaves because the RB8 bit is ‘0’ 

– On completion of data reception the addressed slave will set the SM2 bit to await 

another address frame

8051 Serial Port - gmitWEB

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