8051 Serial Port

8051 Serial Port

The serial port of 8051 is full duplex, means it can transmit and receive simultaneously. It uses register SBUF to hold data. Register SCON controls data communication, register-PCON controls data rates and pin R×D (P3.0) and T×D (P3.1) do the data transfer.

SBUF is an 8-bit register dedicated for serial communication in 8051. Its address is 99H. It can be addressed like any other register in 8051. Writing to SBUF loads data to be transmitted and reading SBUF accesses received data. There are two separate and distinct registers, the transmit write-only register, and the receive read-only register. This is illustrated in Fig. 16.6.1.

The way in which SBUF is used for the transmission and reception of the data during serial communication is explained below.

• Transmission : When a byte of data is to be transmitted via the T×D pin, the SBUF is loaded with this data byte. As soon as a data byte is written into SBUF, it is framed with the start and stop bits and transmitted serially via the T×D pin.

• Reception : When 8051 receives data serially via R×D pin of it, the 8051 deframes it. The start and stop bits are separated out from a byte of data. This byte is placed in SBUF register.

Bit pattern of SCON register : The 8051 provides four programmable modes for serial data communication. A particular mode can be selected by setting the SMO and SMI bits in SCON. The mode selection also decides the baud rate. The Fig. 16.6.2 shows the bit patterns for SCON.

Bit pattern of PCON register :

1. Operating Modes for Serial Port

Mode 0 : In this mode, serial data enters and exits through RxD. TxD outputs the shift clock. 8 bits are transmitted/received : 8 data bits (LSB first). The baud rate is fixed at 1/12 the oscillator frequency. 

Mode 1 : In this mode, 10 bits are transmitted (through TxD) or received (through RxD): A start bit (0), 8 data bits (LSB first) and a stop bit (1). On receive, the stop bit goes into RB8 in Special Function Register SCON. The baud rate is variable.

Mode 2 : In this mode, 11 bits are transmitted (through TxD) or received (through RxD): A start bit (0), 8 data bits (LSB first), a programmable 9th data bit, and a stop bit (1). On Transmit, the 9th data bit (TB8 in SCON) can be assigned the value of 0 or 1. Or, for example, the parity bit (P, in the PSW) could be moved into TB8. On receive, the 9th data bit goes into RB8 in Special Function Register SCON, while the stop bit is ignored. The baud rate „ programmable to either 1 / 32 or 1 / 64 the oscillator frequency.

Mode 3 : In this mode, 11 bits are transmitted (through TxD) or received (through RxD): A start bit (0), 8 data bits (LSB first), a programmable 9th data bit and a stop bit (1). In fact, Mode 3 is the same as Mode 2 in all respects except the baud rate. The baud rate in Mode 3 is variable.

In all four modes, transmission is initiated by any instruction that uses SBUF as a destination register. Reception is initiated in Mode 0 by the condition RI = 0 and REN = 1. Reception is initiated in the other modes by the incoming start bit if REN = 1.

The Table 16.6.1 summarizes the four serial port modes provided by 8051.

2. Generating Baud Rates

Serial Port in Mode 0 Mode 0 has a fixed baud rate which is 1/12 of the oscillator frequency. To run the serial port in this mode none of the Timer/Counters need to be set up. Only the SCON register needs to be defined.

Baud rate = Oscillator frequency / 12

Serial Port in Mode 1: Mode 1 has a variable baud rate. The baud rate can be generated by either Timer 1 or Timer 2 (8052 only).

Using Timer/Counter 1 to Generate Baud Rates

For this purpose, Timer 1 is used in mode 2 (Auto-Reload).

Baud rate = K × Oscillator frequency / 32 × 12 × [(256-TH1)]

If SMOD = 0, then K = 1.

If SMOD = 1, then K = 2. (SMOD is the PCON register)

Most of the time the user knows the baud rate and needs to know the reload value for TH1. Therefore, the equation to calculate TH1 can be written as :

TH1 = 256 = K × Oscillator frequency / 384 × Baud rate

TH1 must be an integer value. Rounding off TH1 to the nearest integer may not produce the desired baud rate. In this case, the user may have to choose another crystal frequency.

Since the PCON register is not bit addressable, one way to set the bit is logical ORing the PCON register, (i.e. ORL PCON, #80H). The address of PCON is 87H.

The Table 16.6.2 shows the values to be loaded into TH1 to get the corresponding baud rate. It also shows that the baud rates are doubled when SMOD = 1.

Using Timer/Counter 2 to Generate Baud Rates

For this purpose, Timer 2 must be used in the baud rate generating mode. If Timer 2 is being clocked through pin T2 (P1.0) the baud rate is :

Baud rate = Timer 2 overflow rate / 16

And if it is being clocked internally the baud rate is :

Baud rate = Oscillator frequency / 32 × [65536 - (RCAP2H, RCAP2L)]

To obtain the reload value for RCAP2H and RCAP2L the above equation can be rewritten as :

RCAP2H, RCAP2L = 65536 - Oscillator frequency / 32 × Baud rate

Serial Port in Mode 2

The baud rate is fixed in this mode and is 1/32 or 1/64 of the Oscillator frequency depending on the value of the SMOD bit in the PCON register. In this mode none of the Timers are used and the clock comes from the internal phase 2 clock.

SMOD = 1, Baud rate = 1/32 Oscillator frequency

SMOD = 0, Baud rate = 1 / 64 Oscillator frequency

To set the SMOD bit : ORL PCON, #80H. The address of PCON is 87H.

Note By changing SMOD bit in PCON from 0 to 1 we can double the baud rate in 8051.

Serial Port in Mode 3

The baud rate in mode 3 is variable and sets up exactly the same as in mode 1.

3. Programming 8051 for Serial Data Transfer

To program 8051, to transfer data serially we have to perform following sequence of actions :

1. Load the TMOD register with the value 20H to use timer 1 in mode 2 (8-bit auto-reload) to set the baud rate.

2. Load TH1 to set the desire baud rate for serial data transfer.

3. Load SCON register with the value 50H, to use serial mode 1, where an 8-bit data is framed with start and stop bits.

4. Set TR1 to 1 to start timer 1.

5. Clear TI with CLR TI instruction.

6. Write a character to be sent in to the SBUF register.

7. Check the TI flag bit with instruction JNB TI, XXXX to see if the character has been transferred completely.

8. Go to step 5 to transfer the next character.

Example 17.6.1 8051 uses 11.0592 MHz crystal. To get 9600 hertz baud rate how will you program it for serial transmission ?

Solution: When 11.0592 MHz crystal is used and a standard baud rate of 9600 hertz is required then, the setting of TH1 can be found as,

TH1 = 256 - k × Oscillator frequency / 384 × Baud rate

= 256 - 1 × 11.0592 × 10⁶ / 384 × 9600 = 253 = FDH

Program: MOV TMOD, #020 ; Initialize timer 1 in mode 2

MOV SCON, #4CH ; Initialize serial mode 1

ORL PCON, #80H  ; Make SMOD = 1

MOV TH1, #FDH ; Load count

Example 16.6.2 Write an 8051 assembly language program to transfer letter "A" serially at 9600 baud rate, continuously.

Solution :

MOV TMOD, #20H ; timer 1, mode 2 (auto reload)

MOV TH1, #FDH         ; 9600 baud rate

MOV SCON, #50H ; 8-bit, 1 stop REN enabled

SETB TR1  ; start timer 1

START: MOV SBUF, #"A" ; Letter "A" to be transferred

HERE: JNB TI, HERE ; Wait for the last bit to transfer

CLR TI ; Clear TI for the next character

SJMP START ; Go to send the character again

Example 16.6.3 Write an 8051 assembly language program to transfer the message "HELLO "serially at 9600 baud, 8-bit data, 1 stop bit.

Solution :

MOV TMOD,#20H ; timer 1, mode 2

MOV TH1,#FDH ; 9600 baud rate

MOV SCON,#50H ; 8-bit, 1 stop bit, REN enabled

SETB TR1 ; start timer 1

START: MOV A, #"H" ; transfer "H"

ACALL TRANS

MOV A, #"E" ; transfer "E"

ACALL TRANS

MOV A, #"L" ; transfer "L"

ACALL TRANS

MOV A, #"L" ; transfer "L"

ACALL TRANS

MOV A,#"O" ; transfer "O"

ACALL TRANS ; Serial data transfer subroutine

TRANS: MOV SBUF, A         ; Load SBUF

HERE: JNB TI, HERE ; wait for the last bit to transfer

CLR TI ; Clear TI for the next character

RET

The importance of the TI flag bit :

When a data is to be transmitted via TxD pin, first a data byte is loaded into the SBUF register. A start bit, a data byte and then the stop bit are transmitted sequentially via TxD pin. During the transmission of the stop bit, 8051 sets the TI flag, i.e. TI = 1. This indicates the end of data byte transmission and 8051 is ready for the transmission and 8051 is ready for the transmission of next data. The programmer has to clear the TI flag, i.e. TI = 0, with the 'CLR TI' instruction to transmit next data. The TI flag bit should be monitored to make sure that the SBUF register is not overwritten. If we write the next byte to be transmitted into the SBUF register before setting the TI flag bit, the untransmitted portion of the previous byte will be lost. The programmer can check the TI flag bit by 'JNB TI, XX' instruction or by using an interrupt.

4. Programming 8051 for Receiving Data Serially

To program 8051, to receive data serially we have to perform following sequence of actions :

1. Load the TMOD register with the value 20H to use timer 1 in mode 2 (8-bit auto-reload) to set the baud rate.

2. Load TH1 to set the desire baud rate for serial data transfer.

3. Load SCON register with the value 50H, to use serial mode 1, where an 8-bit data is framed with start and stop bits.

4. Set TRI to 1 to start timer 1.

5. Clear RI with CLR RI instruction.

6. Check the RI flag bit with instruction JNB RI, XXXX to see if an entire character has been received yet.

7. If RI is set, SBUF has the byte. Save this byte.

8. Go to step 5 to receive the next character.

Example 16.6.4 Write an 8051 assembly language program to receive bytes serially with baud rate 9600, 8-bit data and 1 stop bit. Simultaneously send received bytes to port 2.

Solution :

MOV TMOD, #20H ; timer 1, mode 2 (auto reload)

MOV TH1, #FDH ; 9600 baud rate

MOV SCON, #50H ; 8-bit, 1 stop, REN enabled

SETB TR1 ; start timer 1

HERE: JNB RI, HERE ; wait for character receive completely

MOV A, SBUF ; save the received character

MOV P2, A ; send character to port 2

CLR RI ; Get ready to receive next byte

SJMP HERE ; Go to receive next character

The importance of the RI flag bit : When a data is to be received via RxD pin, first the start bit, and a data byte with one bit at time are received sequentially via RxD pin. When the last bit of a data byte is received, a byte is formed and the SBUF register is loaded with this byte. Then the stop bit is received. During the reception of the stop bit, 8051 sets the RI flag, i.e. RI = 1. This indicates that the entire data byte has been received. This data byte which is loaded in the SBUF register, should be placed to a safe place such as any register or memory before it is lost. The programmer has to clear the RI flag, i.e. RI = 0, with the 'CLR RI' instruction to receive next data and place it in SBUF register. The RI flag bit should be monitored to make sure that the entire byte has been received. This monitoring of the RI flag bit till it becomes one to ensure the reception of complete data place it in SBUF register, copy it to the safe place and then make RI zero with 'CLR RI' instruction are the necessary steps to avoid any loss of received data.

5. Doubling the Baud Rate in the 8051

We can double the baud rate in 8051 using two way,

• By doubling the crystal frequency.

• By making SMOD bit in the PCON register from 0 to 1.

Example 16.6.5 Write a program to receive message from PC to 8051. Message string is "Hello". After this microcontroller sends message to PC "Fine".

Solution : The Fig. 16.6.4 shows the connections between 8051 and PC.

MOV TMOD, #20H ; Initialize timer 1 in mode 2

MOV TH1, #0FDH ; Load count to get 9600     

; baud rate

MOV SCON, #50H ; 8-bit, 1 stop, REN enabled

SETB TR1 ; Start timer 1

MOV DPTR, #2000H ; Initialize memory pointer to

; save received data                

MOV R0,#05H ; Initialize counter to read

; 5 characters       

RECV: JNB RI,RECV ; wait for character

MOV A,SBUF ; Read the character

MOVX @DPTR,A ; Save it in memory

INC DPTR ; increment memory pointer

CLR RI ; Get ready for next character

DJNZ R0, RECV ; If not last character repeat

MOV DPTR, #MYDATA ; Initialize pointer for message

CLR A

MOV R0, #4H ; Initialize counter to send 4 characters

MOVC A, @A+DPTR ; Get the character

MOV SBUF, A ; Load the data

HERE: JNB TI,HERE ; Wait for complete byte transfer

CLR TI ; get ready for next character        

6. 8051 Connection to RS 232C

In RS 232C the voltage level + 3 V to +15 V is defined as logic 0; from - 3 V to -15 V is defined as logic 1. The control and timing signals are compatible with the TTL level. Because of the incompatibility of the data lines with the TTL logic, voltage translators, called line drivers and line receivers, are required to interface TTL logic with the RS 232C signals. The Fig. 16.6.5 shows the connection between RS 232C and 8051. Here, MAX 232 chip is used as a line driver and line receiver. We know that 8051 assigns two pins RxD (P 3.0) and TxD (P 3.1) for reception and transmission of serial data, respectively. These pins are TTL compatible; therefore, they require line driver and line receiver to make them RS 232C compatible. The MAX 232 has two sets of line drivers and line receivers for transmitting and receiving data. Only one set is required for one serial communication.

7. Serial Communication Programming in C

The SFR registers of the 8051 are accessible directly in 8051 C compilers by inclusion of the reg51.h file. However, to use second serial port we have to declare the byte addresses of the new SFR registers, i.e. SBUF1 and SCON1 using SFR data type. In addition, we have to declare bit addresses for Til and RI1 using bit data type.

Example 16.6.6 Write a C program for the 8051 to transfer the letter "C" serially at 9600 baud continuously. Use 8-bit data and 1 stop bit.

Solution :

#include < reg51 .h >

void main (void)

{

TM0D= 0×20;

TH1 = 0×FD;

SCON = 0×50;

TRI = 1;

while(1)

{

SBUF=’C’;

while(TI= =0);

TI=0;

}

}

Example 16.6.7 Write a C program that continuously receives a single bit of data from P1.0 and sends if to P2.0, while simultaneously mating a square wave of 400 ,us period on pin P2.5. Use timer 0 to create the square wave. Assume that XTAL = 11.0592 MHz.

Solution : We will use timer 0 in mode 2 (auto-reload). One half of the period is 200 µs 200/1.085 µs = 184 and TH0 = 256 - 184 = 72 or 48H.

#include <reg51.h>

sbit IBit = P1 ^ 0;

sbit Obit = P2 ^ 0;

sbit SWAVE = P2 ^ 5;

 void timer0(void) interrupt 1

{

SWAVE = ~ SWAVE; /* toggle pin P2.5*/

}

Void main(void)

{

Obit = 1; /* make P1.0 input */

TMOD = 0×02;

TH0 = 0×82; /* TH0 = -184 */

IE = 0×82; /* enable interrupts for timer 0 */

While(1)

{

Obit = Ibit; /* send received bit to P2.0 */

}

}

Example 16.6.8 Write a C program that continuously gets a single bit of data from P1.0 and sends if to P2.0 in the main, while simultaneously (a) mating a square wave of 400 ,µs period on pin P2.5, and (b) sending letter 'A' through 'Z' to the serial port. Use timer 0 to create the square wave. Assume that XTAL = 11.0592 MHz. Use the 9600 baud rate.

Solution : We will use timer 0 in mode 2 (auto-reload). TH0 = 200/1.085 µs = - 184, which is 72H.

#include <reg51.h>

sbit Ibit = P1 ^ 0;

sbit Obit = P2 ^ 0;

sbit SWAVE = P2 ^ 5;

unsigned char ch = ‘A';

void timer0(void) interrupt 1

{

SWAVE =~SWAVE; /* toggle pin 2.5 */

}

void seria10(void) interrupt 4

{

if(TI == 1)

{

SBUF = ch; /* send character to serial port */

TI = 0; /* clear interrupt */

ch+ + ;

if(ch >= 'Z')

ch ='A';

}

else

{

RI = 0; /* clear interrupt */

}

}

void main(void)

{

Ibit = 1; /* make switch input */

TH1 = -3; /* 9600 baud */

TMOD = 0x22; /* set mode 2 for both timers */

TH0 = 0x72; /*load 72H as a timer count */

SCON = 0x50;

TR0 = 1;

TR1 = 1; /* start timer *

IE = 0x92; /* enable interrupt for T0*/

while(1) /* stay here */

{

Obit = Ibit; /* send received bit to bit P2.0 */

}

}

Example 16.6.9 Write a C program using interrupts to do the following :

a) Receive data serially and send it to P0,

b) Read port Pl, transmit data serially, and give a copy to P2,

c) Set timer 0, generate a square wave of 2.5 kHz frequency on P0.1. Assume that XTAL - 11.0592 MHz. Set the baud rate at 9600.

Solution :

#include <reg51.h>

Sbit SWAVE = P0^1;

void timer0 () interrupt 1

{

SWAVE = - SWAVE; /* toggle pin */

}

void serial0()interrupt 4

{

if(TI == 1)

{

TI = 0; /* clear interrupt */

}

else

{

P0 = SBUF; /* put value on pins */

RI = 0; /* clear interrupt */

}

}

void main()

{

unsigned char c;

P1 = 0xFF; /* make 7P1 an input */

TMOD = 0x22;

TH1 = 0xF0; /* 9600 baud rate */

SCON = 0x50;

TH0 = 0x72; /* 2.5 kHz has T = 400 µs */

IE = 0x92; /* enable interrupts */

TR1 = 1; /* start timer 1 */

TR0 = 1; /* start timer 0 */

while(l)

{

c = P1; /* read data byte from port Pl */

SBUF = c; /* put data byte in buffer */

P2 = c; /* send data byte to port P2 */

}

}

Example 16.6.10 Write a 8051 'C program that continuously gets a single bit of data from Pl.7 and sends it to P1.0, which creates a square wave of 200 ,us period on pin P2.5. XT AL frequency = 11.0592 MHz.

Solution : We will use timer 0 in mode 2 (auto-reload). One half of the period is 100 ,µs. 100/1.085 µs = 92 and TH0 = 256 - 92 = 164 or A4H.

#include <reg51.h>

shit IBit = Pl ^ 7;

shit Obit = P1 ^ 0;

shit SWAVE = P2 ^ 5;

void timer0(void) interrupt 1

{

SWAVE = ~ SWAVE;  /* toggle pin P2.5 */

}

void main(void)

{

Obit = 1; /* make Pl.7 input */

TMOD = 0x02;

TH0 = 0xA4; /* TH0 = - 92 */

IE = 0x82; /* enable interrupts for timer 0 */

while(1)

{

Obit = Ibit; /* send received bit to P1.0 */

}

}

Example 16.6.11 Write an 8051 C program to i) Continuously read the status of switch connected to pin Pl.2 and send it to pin P2.1 in the main program and ii) Generate a square wave of100 µsec period on P2.3 and send character continuously serially using timer and serial interrupt routines, respectively. Use XTAL frequency as 11.0592 MHz and 8 bits data, one stop bit, 4800 baud rate format.

Solution : We will use timer 0 in mode 2 (auto-reload).

For square wave   T_ON = T/2 = 50 µsec

#include <reg51.h>

sbit SW = P1^2;

sbit ST = P2^1;

sbit SWAVE =P2^3;

void timer 0 (void) interrupt 1

{

swave = ~ swave ;          /* toggle pin */

}

void serial 0() interrupt 4

{

if (TI = = 1)

{

SBUF = '*';  /*send '*' to serial port*/

TI = 0; /* clear interrupt */

}

else

{

RI = 0; /* clear interrupt */

}

}

void main ()

{

SW = 1; /* Make switch input */

TH1 = - 6; /* 4800 baud rate */

TMOD = 0X22; /* Mode 2 for both timers */

TH0 = 0XD2; /* Load count D2H for timer 0 */

SCON = 0X50; /* 8-bit data, 1-stop bit */

TR0 = 1; /* start timer 0 */

TR1 = 1; /* start timer 1 */

IE = 0X92; /* Enable interrupt for T0 */

While (1) /* wait for interrupt */

{

ST = SW; /* send status of SW to pin P2.1 */

}

}

Review Questions

1. Discuss the serial interface of 8051. AU : May-05, Marks 8

2. Discuss in detail, the hardware and software support provided by 8051 for serial communication. AU : Dec.-09,17, Marks 16

3. Describe how the serial communication is performed in 8051. AU : May-10, Dec.-08,11, Marks 8

4. Draw the flowchart for programming of serial port of 8051. AU : Dec.-11, Marks 2

5. Explain the different serial communication modes in 8051. AU : June-07, Marks 8

6. Write 8051 ALP to transmit "Hello World" to PC at 9600 baud for external crystal frequency of AU : June-07, Marks 8