Serial Communication with AVR

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)

·        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 AVR 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 AVR serial port to the COM 2 port

AVR CONNECTION TO RS232:

·        RxD and TxD pins in the AVR

·        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 AVR

·        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

AVR SERIAL PORT PROGRAMMING IN C:

·        Baud rate in the AVR

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

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

·        can use Windows HyperTerminal program

Ex1(a):

Write a program to transfer letter "A" serially at 9600 baud, continuously.

#include<avr/io.h>

#include<util/delay.h>

int main()

{

UBRRL=51;

UCSRB=0x08;

UCSRC=0x86;

while(1)

{

UDR='A';

_delay_ms(100);

while((UCSRA&0x40)==0);

}

}

Simulation:

Ex 1(b):

Write a program to transfer the message " Welcome To All " serially at 9600 baud, 8-bit data, 1 stop bit. Do this continuously.

#include<avr/io.h>

#include<util/delay.h>

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

int main()

{ int i;

UBRRL=51;

UCSRB=0x08;

UCSRC=0x86;

while(1)

{

for(i=0;i<=15;i++)

{

UDR=arr[i];

_delay_ms(100);

while((UCSRA&0x40)==0);

}

}

}

Simulation:

Ex 1(c):

Program the AVR 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<avr/io.h>

#include<util/delay.h>

#include"lcd_header.h"

void main()

{

DDRC=0xFF;

DDRB=0xFF;

UBRRL=51;

UCSRB=0x10;

UCSRC=0x86;

init();

unsigned char x;

while(1)

{

while((UCSRA&0x80)==0);

x=UDR;

datta(x);

}

}

lcd_header.h

#include<avr/io.h>

#include<util/delay.h>

void datta(unsigned char a)

{

PORTB=a;

PORTC=0x05;

_delay_ms(30);

PORTC=0x01;

}

void command(unsigned char s)

{

PORTB=s;

PORTC=0x04;

_delay_ms(30);

PORTC=0x00;

}

void init()

{

command(0x38);

_delay_ms(100);

command(0x01);

_delay_ms(100);

command(0x0E);

_delay_ms(100);

command(0x80);

_delay_ms(100);

}

void stringg(unsigned char *p)

{

while(*p!='\0')

{

datta(*p);

p++;

}

}

void num(unsigned int i)

{

int j=1,k,l;

k=i;

while(k>=9)

{

j=j*10;

k=k/10;

}

while(j>=1)

{

l=i/j;

i=i%j;

j=j/10;

datta(l+48);

}

}

Simulation: