Serial Communication with 8051

Serial Communication:

BASICS OF SERIAL COMMUNICATION:

·        serial communication uses single data line making it much cheaper

·        enables two computers in different cities to communicate over the telephone

·        byte of data must be converted to serial bits using a parallel-in-serial-out shift register and transmitted over a single data line

·        receiving end there must be a serial-in-parallel-out shift register

·        if transferred on the telephone line, it must be converted to audio tones by modem

·        for short distance the signal can be transferred using wire

·        how PC keyboards transfer data to the motherboard

·        2 methods, asynchronous and synchronous

·        synchronous method transfers a block of data (characters) at a time

·        asynchronous method transfers a single byte at a time

·        Uses special IC chips called UART (universal asynchronous receiver-transmitter) and USART (universal synchronous asynchronous receiver-transmitter)

·        8051 chip has a built-in UART

·        Half- and full-duplex transmission

·        if the data can be transmitted and received, it is a duplex transmission

·        simplex transmissions the computer only sends data

·        duplex transmissions can be half or full duplex

·        depends on whether or not the data transfer can be simultaneous

·        If one way at a time, it is half duplex

·        If can go both ways at the same time, it is full duplex

·        full duplex requires two wire conductors for the data lines (in addition to the signal ground)

·        Asynchronous serial communication and data framing

·        data coming in 0s and 1s

·        to make sense of the data sender and receiver agree on a set of rules

·        Protocol

·        how the data is packed

·        how many bits/character

·        when the data begins and ends

·        Start and stop bits

·        asynchronous method, each character is placed between start and stop bits

·        called framing

·        start bit is always one bit

·        stop bit can be one or two bits

·        start bit is always a 0 (low)

·        stop bit(s) is 1 (high)

·        LSB is sent out first

·        in modern PCs one stop bit is standard

·        when transferring a text file of ASCII characters using 1 stop bit there is total of 10 bits for each character

·        8 bits for the ASCII code (1 parity bit), 1 bit each for the start and stop bits

·        for each 8-bit character there are an extra 2 bits, which gives 20% overhead

·        Data transfer rate

·        rate of data transfer bps (bits per second)

·        widely used terminology for bps is baud rate

·        baud and bps rates are not necessarily equal

·        baud rate is defined as the number of signal changes per second

·        RS232 standards

·        most widely used serial I/O interfacing standard

·        input and output voltage levels are not TTL compatible

·        1 bit is represented by -3 to -25 V

·        0 bit is +3 to +25 V

·        -3 to +3 is undefined

·        to connect RS232 to a microcontroller system must use voltage converters such as MAX232 to convert the TTL logic levels to the RS232 voltage levels, and vice versa

·        MAX232 IC chips are commonly referred to as line drivers

·       

Data communication classification

·        DTE (data terminal equipment)

·        DCE (data communication equipment)

·        DTE - terminals and computers that send and receive data

·        DCE - communication equipment responsible for transferring the data

·        simplest connection between a PC and microcontroller requires a minimum of three pins, TxD, RxD, and ground

·        Examining RS232 hand­shaking signals

·        many of the pins of the RS-232 connector are used for handshaking signals

·        they are not supported by the 8051 UART chip

·        PC/compatible COM ports

·        PC/compatible computers (Pentium) microprocessors normally have two COM ports

·        both ports have RS232-type connectors

·        COM ports are designated as COM 1 and COM 2 (replaced by USB ports)

·        can connect the 8051 serial port to the COM 2 port

8051 CONNECTION TO RS232:

·        RxD and TxD pins in the 8051

·        8051 has two pins used for transferring and receiving data serially

·        TxD and RxD are part of the port 3 group

·        pin 11 (P3.1) is assigned to TxD

·        pin 10 (P3.0) is designated as RxD

·        these pins are TTL compatible

·        require a line driver to make them RS232 compatible

·        driver is the MAX232 chip

·        MAX232

·        converts from RS232 voltage levels to TTL voltage levels

·        uses a +5 V power source

·        MAX232 has two sets of line drivers for transferring and receiving data

·        line drivers used for TxD are called T1 and T2

·        line drivers for RxD are designated as R1 and R2

·        T1 and R1 are used together for TxD and RxD of the 8051

·        second set is left unused

·        MAX233

·        MAX233 performs the same job as the MAX232

·        eliminates the need for capacitors

·        much more expensive than the MAX232

8051 SERIAL PORT PROGRAMMING IN ASSEMBLY:

·        Baud rate in the 8051

·        serial communications of the 8051 with the COM port of the PC

·        must make sure that the baud rate of the 8051 system matches the baud rate of the PC's COM port

·        can use Windows HyperTerminal program

·        Baud rate in the 8051

·        baud rate in the 8051 is programmable

·        done with the help of Timer 1

·        relationship between the crystal frequency and the baud rate in the 8051

·        8051 divides the crystal frequency by 12 to get the machine cycle frequency

·        XTAL = 11.0592 MHz, the machine cycle frequency is 921.6 kHz

·        8051's UART divides the machine cycle frequency of 921.6 kHz by 32 once more before it is used by Timer 1 to set the baud rate

·        921.6 kHz divided by 32 gives 28,800 Hz

·        Timer 1 must be programmed in mode 2, that is 8-bit, auto-reload

Ex-1 (a):
With XTAL = 11.0592 MHz, find the TH1 value needed to have the following baud rates.          (a) 9600      (b) 2400      (c) 1200

·        machine cycle frequency

= 11.0592 MHz / 12 = 921.6 kHz

·        Timer 1 frequency provided by 8051 UART

= 921.6 kHz / 32 = 28,800 Hz

(a) 28,800 / 3 = 9600     where -3      = FD (hex)

(b) 28,800 / 12 = 2400   where -12    = F4 (hex)

(c) 28,800 / 24 = 1200   where -24    = E8 (hex)

·        SBUF (serial buffer) register

·        a byte of data to be transferred via the TxD line must be placed in the SBUF register

·        SBUF holds the byte of data when it is received by the RxD line

·        can be accessed like any other register

                          MOV SBUF,#'D'    ;load SBUF=44H, ASCII for 'D‘

                          MOV SBUF,A                  ;copy accumulator into SBUF

                          MOV A,SBUF                  ;copy SBUF into accumulator

·        when a byte is written, it is framed with the start and stop bits and transferred serially via the TxD pin

·        when the bits are received serially via RxD, it is deframe by eliminating the stop and start bits, making a byte out of the data received, and then placing it in the SBUF

·        SCON (serial control) register

·        to program the start bit, stop bit, and data bits

·        SM0 and SM1 determine the mode

·        only mode 1 is important

·        when mode 1 is chosen, the data framing is 8 bits, 1 stop bit, and 1 start bit

·        compatible with the COM port of PCs

·        mode 1 allows the baud rate to be variable and is set by Timer 1 of the 8051

·        for each character a total of 10 bits are transferred, where the first bit is the start bit, followed by 8 bits of data, and finally 1 stop bit.

·        REN (receive enable)

·        REN=1, allows 8051 to receive data on the RxD

·        if 8051 is to both transfer and receive data, REN must be set to 1

·        REN=0, the receiver is disabled

·        SETB SCON.4 and CLR SCON.4,

·        TI (transmit interrupt)

·        when 8051 finishes the transfer of the 8-bit character, it raises the TI flag to indicate that it is ready to transfer another byte

·        RI (receive interrupt)

·        when the 8051 receives data serially via RxD, it places the byte in the SBUF register

·        then raises the RI flag bit to indicate that a byte has been received and should be picked up before it is lost

·        Program to transfer data serially

·        TMOD register is loaded with the value 20H

·        TH1 is loaded with value to set the baud rate

·        SCON register is loaded with the value 50H

·        TR1 is set to 1 to start Timer1

·        TI is cleared by the "CLR TI" instruction

·        transmit character byte is written into the SBUF register

·        TI flag bit is monitored to see if the character has been transferred completely.


Serial Data Transmission Procedure:

Ex1(b):
Write a program to transfer letter "A" serially at 4800 baud, continuously.

#include<reg51.h>

unsigned char i;

int j;         

void main()

{                

          while(1)

          {

           TMOD=0X20;

          SCON=0X50;

          TH1=0XFD;

                             TR1=1;

                             SBUF=A;

                             while(TI==0);

                   TR1=0;

                   TI=0;

          }

}

Ex 1(c):
Write a program to transfer the message "Hello" serially at 9600 baud, 8-bit data, 1 stop bit. Do this continuously.

#include<reg51.h>

unsigned char i;

int j;

unsigned char arr[]={"Welcome To All"};

void main()

{                

          while(1)

          {

                    int z;

          TMOD=0X20;

          SCON=0X50;

          TH1=0XFD;

                             for(z=0;z<=16; z++)

                             {

                             TR1=1;

                             SBUF=arr[i];

                             while(TI!=1);

                    TR1=0;

                       TI=0;

                             }

                  

          }

}

Ex 1(d):

Program the 8051  to receive bytes of data serially and put them on PORTB. Set the baud rate at 9600, 8-bit data, and 1 stop bit.

#include<reg51.h>

void dis();

void com();

int i;

sbit rs = P3^7 ;

sbit rw = P3^6 ;

sbit en = P3^5 ;

int c[]={0X01,0X0e,0X38,0X80,0X06} ;

void main()

{                

          for(i=0;i<5;i++)

          {

                             P1=c[i];

                             com();

          }

         

          while(1)

          {

           

          TMOD=0X20;

          SCON=0X50;

          TH1=0XFD;

          TR1=1;

          while(RI!=1);

          P1=SBUF;

          dis();          

          TR1=0;

          RI=0;

                  

          }

}

void delay(int u)

{

int l,o;

for(l=0;l<u;l++)

{

          for(o=0;o<1250;o++)

          {

          }

}

}

void com()

{

          rs=0;

          rw=0;

          en=1;

          delay(10);

          en=0;

}

void dis()

{

         

          rs=1;

          rw=0;

          en=1;

          delay(10);

          en=0;

}

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