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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interfacing of matrix keypad with 8051

Matrix Keypad: Matrix Keypads are commonly used in calculators, telephones etc where a number of input switches are required. We know that matrix keypad is made by arranging push button switches in row and columns. In the straight forward way to connect a 4×4 keypad (16 switches) to a microcontroller we need 16 inputs pins. But by connecting switches in the following way we can read the status of each switch using 8 pins of the microcontroller.

Interfacing of matrix keypad with 8051:

1)     

a)    Circuit:

b)    Code:

·        ASM Code:

$mod51

org 00h

mov P1,#01h

acall command

mov P1,#06h

acall command

mov P1,#38h

acall command

mov P1,#0eh

acall command

mov P1,#80h

acall command

back:

mov P2,#11111110b

Jnb P2.4,S1

JNB P2.5,S2

JNB P2.6,S3

JNB P2.7,S4

mov P2,#11111101b

Jnb P2.4,S5

JNB P2.5,S6

JNB P2.6,S7

JNB P2.7,S8

mov P2,#11111011b

Jnb P2.4,S9

JNB P2.5,S10

JNB P2.6,S11

JNB P2.7,S12

mov P2,#11110111b

Jnb P2.4,S13

JNB P2.5,S14

JNB P2.6,S15

JNB P2.7,S16

sjmp back

S1: mov P1,#'A'

    Acall display

    Ljmp back

S2: mov P1,#'B'

    Acall display

    Ljmp back

S3: mov P1,#'C'

    Acall display

    Ljmp back

S4: mov P1,#'D'

    Acall display

    Ljmp back

S5: mov P1,#'E'

    Acall display

    Ljmp back

S6: mov P1,#'F'

    Acall display

    Ljmp back

S7: mov P1,#'G'

    Acall display

    Ljmp back

S8: mov P1,#'H'

    Acall display

    Ljmp back

S9: mov P1,#'I'

    Acall display

    Ljmp back

S10: mov P1,#'J'

    Acall display

    Ljmp back

S11: mov P1,#'K'

    Acall display

    Ljmp back

S12: mov P1,#'L'

    Acall display

    Ljmp back

S13: mov P1,#'M'

    Acall display

    Ljmp back

S14: mov P1,#'N'

    Acall display

    Ljmp back

S15: mov P1,#'O'

    Acall display

    Ljmp back

S16: mov P1,#'P'

    Acall display

    Ljmp back

command:

clr P3.0

clr P3.1

setb P3.2

acall delay

clr P3.2

ret

                        display:

setb P3.0

clr P3.1

setb P3.2

acall delay

clr P3.2

ret

delay:

mov R1,#100

  L1: mov R2,#100

   L2: mov R3,#50

        L3: Djnz R3,L3

      DjnZ R2,L2

     Djnz R1,L1

Ret

end

·        C Code:

#include<reg51.h>

sbit RS=P3^0;

sbit RW=P3^1;

sbit En=P3^2;

 sbit col1=P2^4;

 sbit col2=P2^5;

 sbit col3=P2^6;

 sbit col4=P2^7;

 int k;

 unsigned char b[]={0x01,0x06,0x80,0x0e,0x38};

  void delay()

 

  {

  for(k=0;k<35;k++)

  {

   TMOD=0x01;

   TH1=0x00;

   TL1=0x00;

   TR1=1;

   while(TF1==0);

   TF1=0;

   TR1=0;

   }

  }

 void command(unsigned char x)

 {

   P1=x;

   RS=0;

   RW=0;

   En=1;

   delay();

   En=0;

  }

   void display(unsigned char x)

 {

   P1=x;

   RS=1;

   RW=0;

   En=1;

   delay();

   En=0;

  }

  int j;

  void main()

  {

    For (j=0;j<5;j++)

       {

        command(b[j]);

       }

   while(1)

   {  P2=0xff;

      P2=0xfe;

         delay();

       if(col1==0)

       {

        command(0x01);

        display('A');

       }

       else if(col2==0)

       { command(0x01);

        display('B');

       }

       else if(col3==0)

       { command(0x01);

        display('C');

       }

       else if(col4==0)

       { command(0x01);

        display('D');

       }

       P2=0xfd;

       if(col1==0)

       {

        command(0x01);

        display('E');

       }

       else if(col2==0)

       { command(0x01);

        display('F');

       }

       else if(col3==0)

       { command(0x01);

        display('G');

       }

       else if(col4==0)

       { command(0x01);

        display('H');

       }

   P2=0xfb;

   if(col1==0)

       {

        command(0x01);

        display('I');

       }

       else if(col2==0)

       { command(0x01);

        display('J');

       }

       else if(col3==0)

       { command(0x01);

        display('K');

       }

       else if(col4==0)

       {

        command(0x01);

        display('L');

       }

       P2=0xf7;

       if(col1==0)

       {

        command(0x01);

        display('M');

       }

       else if(col2==0)

       { command(0x01);

        display('N');

       }

       else if(col3==0)

       { command(0x01);

        display('O');

       }

       else if(col4==0)

       { command(0x01);

        display('P');

       }

   }

  }

c)     Simulation:

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Interfacing of led with 8051

Led (  Light emitting diode) : Light Emitting Diodes are the mostly commonly used components in many applications. They are made of semiconducting material. This article describes about basic interfacing of LEDs to the 8051 family microcontrollers.

Circuit diagram for blinking a led:

How to Operate LED Interfacing with 8051 Microcontroller Circuit:

• Initially, burn the code into the microcontroller.
• Now, connect the LEDs to the port of the microcontroller.
• Switch on the circuit.
• Thus one can observe LEDs glowing.
• Now, switch off the circuit.

Program for blinking a led:

1) ASM code:

$mod51
org 00h
back:
setb p1.0
clr p1.1
sjmp back
end

2) C code:

#include<reg51.h>
sbit  led =P1^0;

void main()
{
      while(1)
         {
              led=1;
              led=0;
          }
}


3) Proteus Simulation:

Program to switch on and off an 8 Leds connected at P1

1) ASM code:

$mod51

org 00h

back:

mov p1,#0ffh

mov p1,#00h

sjmp back

end

2) C code:

#include<reg51.h>

void main()
{
      while(1)
         {
            P1=0XFF;
            P1=0X00;
          }
}


3) Proteus Simulation:

Program to switch on and off an 8 leds connected at port1 with 1 sec delay:

1) ASM code:

$mod51

org 00h

back:

mov p1,#0ffh

acall delay

mov p1,#00h

acall delay

sjmp back

delay:
mov r4, #255
L4: mov r5,#255
L5: mov r6,#10
L6: djnz r6,L6
    djnz r5,L5
    djnz r4,L4
ret

end

2) C code:

#include<reg51.h>

void delay()
{   int x,y;
     for(x=0;x<=1000;x++)
       {
             for(y=0;y<=1275;y++);
       }
}

void main()
{
      while(1)
         {
            P1=0XFF;
            delay();
            P1=0X00;
            delay();
          }
}

3) Proteus simulation:

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