How to interface RFID with AVR microcontroller (ATmega16)

       Knowingly or unknowingly the RFID technology is used by us in our day to day life. The most familiar example is seen in MNCs, schools and offices for daily attendance or automatic door opening system. The RFID contains two parts, namely, tag and receiver modem. When an RFID tag comes in the range of receiver, the tag gets activated and transmits its unique identification code to the receiver module.

       The output of the RFID receiver is the unique ID in either serial (RS232) or wiegand format. Most of the receivers are equipped with additional hardware to send the extracted code in the above format, which can then be used by digital signal processors. This article shows the interfacing of ATmega16 with RFID.

The RFID module used here gives a 12 byte unique ID of a particular tag in serial RS232 logic level format. Hence a level converter MAX232 is used in between RFID receiver module and microcontroller. The connections of RFID module and ATmega16 are shown in thecircuit diagram. The ground pin of MAX232 and serial output of RFID module is made common.  A cross cable connection is set up between the RFID module and the MAX232 by connecting transmitter pin of one to the receiver pin of the other and vice versa as shown in the circuit diagram.

Note: In case the output of the RFID module is in TTL format, there is no need of MAX232. In such a case the output of the RFID module can be directly given to the microcontroller.
 Pin 2 of max 232    Pin 3 of RFID modem
Pin 3 of max 232    Pin 2 of RFID modem
Pin 5 ground pin of max 232    Pin 5 ground of RFID modem


Code description
In order to understand the code for RFID (given below) which is interfaced with ATmega16, one must have a basic knowledge of serial communication and LCD. The serial data from RFID module can be taken by microcontroller either by polling or by using serial interrupt concepts. (To understand the difference between them, refer to tutorial on Interrupts) This article explores the interfacing of RFID module with AVR microcontroller (ATmega16) using the polling technique. The code described here keeps monitoring the serial input till it receives all the twelve bytes from the RFID module.

Receiving 12 byte serial interrupt data by polling method:
Steps to receive twelve byte serial data
        i.            Initialize USART in read mode.
       ii.            Get a 12 byte string (RFID card no.)
void getcard_id(void)    // Function to get 12 byte ID no. from rfid card
{  
    for(i=0;i<12;i++)
    {
        card[i]= usart_getch();    // receive card value byte by byte
    }
    return;
}
      iii.            Display that 12 byte data on LCD.
void LCD_display(void)    // Function for displaying ID no. on LCD
{
    for(i=0;i<12;i++)
    {
        LCD_write(card[i]);    // display card value byte by byte
    }
    return;
}

// Program to get the 12 byte string and display it on LCD by Polling method:
/*
The RFID unique code is been displayed on LCE
LCD DATA port----PORT B
ctrl port------PORT D
    rs-------PD0
    rw-------PD1
    en-------PD2
*/

#define F_CPU 12000000UL

#define USART_BAUDRATE 9600
#define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1)

#include
#include

#define LCD_DATA PORTA        // LCD data port
#define ctrl PORTB
#define en PB2        // enable signal
#define rw PB1        // read/write signal
#define rs PB0        // register select signal

void LCD_cmd(unsigned char cmd);
void init_LCD(void);
void LCD_write(unsigned char data);

void usart_init();
unsigned int usart_getch();

unsigned char i, card[12];
void getcard_id(void);
void LCD_display(void);

int main(void)
{
    DDRA=0xff;        //LCD_DATA port as output port
    DDRB=0x07;        //ctrl as out put
    init_LCD();        //initialization of LCD
    delay_ms(50);        // delay of 50 milliseconds
    usart_init();        // initiailztion of USART
    LCD_write_string("Unique ID No.");    //Function to display string on LCD
    while(1)
    {
        getcard_id();    // Function to get RFID card no. from serial port
        LCD_cmd(0xC0);    // to go in second line and zeroth position on LCD
        LCD_display();        // a function to write RFID card no. on LCD
    }
    return 0;
}

void getcard_id(void)    //Function to get 12 byte ID no. from rfid card
{   
    for(i=0;i<12;i++)
    {
        card[i]= usart_getch();    // receive card value byte by byte
    }
    return;
}

void LCD_display(void)    //Function for displaying ID no. on LCD
{
    for(i=0;i<12;i++)
    {
        LCD_write(card[i]);    // display card value byte by byte
    }
    return;
}

void init_LCD(void)
{
    LCD_cmd(0x38);        //initializtion of 16x2 LCD in 8bit mode
    _delay_ms(1);

    LCD_cmd(0x01);        //clear LCD
    _delay_ms(1);

    LCD_cmd(0x0E);        //cursor ON
    _delay_ms(1);

    LCD_cmd(0x80);        // ---8 go to first line and --0 is for 0th position
    _delay_ms(1);
    return;
}

void LCD_cmd(unsigned char cmd)
{
    LCD_DATA=cmd;
    ctrl =(0<
    _delay_ms(1);
    ctrl =(0<
    _delay_ms(50);
    return;
}

void LCD_write(unsigned char data)
{
    LCD_DATA= data;
    ctrl = (1<
    _delay_ms(1);
    ctrl = (1<
    _delay_ms(50);           
    return ;
}

void usart_init()
{
    UCSRB |= (1 << RXEN) | (1 << TXEN);   // Turn on the transmission and reception circuitry
    UCSRC |= (1 << URSEL) | (1<
                                                            // Use 8-bit character sizes

    UBRRL = BAUD_PRESCALE;     // Load lower 8-bits of the baud rate value..
                            // into the low byte of the UBRR register
    UBRRH = (BAUD_PRESCALE >> 8); // Load upper 8-bits of the baud rate value..
                                  // into the high byte of the UBRR register
}

unsigned int usart_getch()
{
    while ((UCSRA & (1 << RXC)) == 0); // Do nothing until data have been received..
                       // and is ready to be read from UDR
    return(UDR); // return the byte
}

void LCD_write_string(unsigned char *str)    // take address value of the string in pointer *str
{
    int i=0;
    while(str[i]!='\0')        // loop will go on till the NULL characters is soon in string
     {
        LCD_write(str[i]);    // sending data on LCD byte by byte
        i++;
    }
    return;
}