Welcome to the 3rd part of RF Communication tutorial. In the last two parts I have introduced the basics of RF Communication.

Part III will be covering mostly the practical part, i.e. we will build a complete & working data transfer system. Here you will get circuit and program to implement the solution. The application is very simple in this case, just to transfer a byte of data from Tx station to the Rx station. Once you implement it and get it working you will have enough information and experience to make other RF based projects.

I request all users to follows the instruction exactly as given (unless they are smart enough to know what they are doing). The most important thing in this article is timing of the MCU, so

  • Use the exact frequency crystals as used in the designs.
  • Write High Fuse = C9 (HEX Value) and Low Fuse FF (HEX Value) to enable external crystal.

Hardware

We will have two units. One is Tx (Transmitter) and Other is Rx (Receiver). Both units are based around ATmega16 MCU(you can use ATmega32 also) on external 16MHz crystal. On the Tx unit PORTC will act as input. While in Rx unit it will act as output. The value at PORTC of TX unit is constantly sent over the air to the RX unit where it is latched on its PORTC. That means whatever value you put in the PORTC of Tx station is available on PORTC of Rx station (8bits or 1 byte). We will connect 8 micro switches on the PORTC of Tx station and 8 leds on the PORTC of Rx station. For testing you can press keys on the Tx side and corresponding LED on the Rx side will glow. Simple!

You can then use the same techniques of sending/receiving data in any other application, like SWARM robotics.

RF Module Interface with AVR ATmega

RF Module Tx + AVR ATmega16

 

AVR Atmega interface with RF Module

RF Module Rx + AVR ATmega16

The above two schematic gives detailed connection of AVR ATmega16 (or ATmega32) MCU with RF Modules.

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Note About the Schematic

  1. Power PINs of MCU are not shown but must be connected properly.
  2. BC548(NPN) transistor and two 4.7K resistor forms a very simple NOT gate(inverter). This is because in RS232 communication the "idle" state is HIGH to make idle state LOW we have use the inverter.
  3. You can use a low cost ready made avr development board for quick and easy experimentation.(like this one). It has inbuilt power supply, crystal, reset circuit and ISP port(and much more) Only you have to connect the transistor and RF modules that's it! You can easily program the MCU by using USB AVR Programmer.
AVR ATmega16 RF module circuit

RF Communication Test

Software

The software is written in C language and compiled using the open source compiler avr-gcc. For project management AVR Studio was used. I have used my fully buffered, interrupt driven USART library for usart related job. The library comes in two files.

  • USART.c
  • USART.h

These files must be copied to the current project folder and added to AVR Studio Project. How to add a file to avr studio project is given here.

Some important functions of USART library are as follows

USARTInit();

Initializes the USART of AVR MCU. The parameter is the UBRR value. What is UBRR? Its not the topic of this article! I assume that you know about the Basics of RS232 communication and have knowledge of USART. Don't worry much if you don't know that. Just leave this article and read the following articles

UWriteData();

This function sends a byte of data over the Tx channel.

UDataAvailable()

Returns the number of byte of data currently waiting in the fifo queue.

UReadData()

Reads and returns a byte of data from the buffer.

Complete Listing of Tx.c


#include <avr/io.h>
#include <util/delay.h>

#include "usart.h"

void main()
{
   //Initialize the USART with Baud rate = 2400bps
   USARTInit(416);

   //Enable Internal Pullups on PORTC
   PORTC=0xFF;

   /* 

   Keep transmitting the Value of Local PORTC
   to the Remote Station.

   On Remote RX station the Value of PORTC
   sent on AIR will be latched on its local PORTC
   */

   uint8_t data;
   while(1)
   {
      data=PINC;

      /* 
      Now send a Packet
      Packet Format is AA<data><data inverse>Z

      total Packet size if 5 bytes.
      */

      //Stabilize the Tx Module By Sending JUNK data
      UWriteData('J');  //J for junk

      //Send 'A'
      UWriteData('A');

      //Send Another 'A'
      UWriteData('A');

      //Send the data;
      UWriteData(data);

      //Send inverse of data for error detection purpose

      UWriteData(~data);

      //End the packet by writing 'Z'
      UWriteData('Z');

      //Wait for some time
      _delay_loop_2(0);
      _delay_loop_2(0);
      _delay_loop_2(0);
      _delay_loop_2(0);
   }
}

Complete Listing of Rx.c 


#include <avr/io.h>

#include "usart.h"

void main()
{

   uint8_t i; //Clasical loop varriable

   uint8_t packet[5],data=0;

   DDRC|=0xFF; //All Output

   //Initialize the USART with Baud rate = 2400bps
   USARTInit(416);

   /*
   Get data from the remote Tx Station
   The data is the value of PORTC on Remote Tx Board
   So we will copy it to the PORTC of this board.

   */

   while(1)
   {
      //Wait for a packet
      while(!UDataAvailable());
      if(UReadData()!='A') continue;
      while(!UDataAvailable());
      if(UReadData()!='A') continue;

      while(UDataAvailable()!=3);

      //Get the packet

      for(i=2;i<5;i++)
      {
         packet[i]=UReadData();
      }

      //Is it ok?
      if(packet[2]!=((uint8_t)~packet[3])) continue;

      if(packet[4]!='Z') continue;

      //The packet is ok

      data=packet[2];

      //Now we have data put it to PORTC
      PORTC=data;
   }


}

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By

Avinash Gupta

me@avinashgupta.com
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