Interface a GSM (SIM 300) Modem with 8051 Microcontroller

Aim:

GSM (Global System for Mobile Communication) technology lets user to communicate with others across mobile networks hence it offers a vast area of coverage. Interfacing GSM technology with microcontroller will enable us to extend the communication to cover large area. This tutorial will teach you about interfacing GSM module with 8051  microcontroller and program it to communicate it with another user.

Description:

STEP 1: MODEM TESTING:

The Modem consists of two indicating LED’s Green and Red to indicate the availability of the network. Green indicates the availability of the network whereas red indicates its absence. Turn the modem ON and wait for sometime to register itself in GSM network.

STEP 2: INTERFACING WITH AVR MICROCONTROLLER:

The Communication between AVR and modem takes place through USART protocol. GSM Modem SIM900 works on TTL level hence we can interface directly to RXD and TXD pins of 8051 microcontroller. There is no need of using Voltage or level converter between them. However if you ever happen to buy a SIM300 or other module which operates above TTL level, you may want to use MAX232 level converter IC to make the communication possible.

STEP 3: INITIALIZING MODEM:

The modem must be Initialized using the commands and then the process you are about to carry out must be selected. In this tutorial am going to initialize the modem and them make it to send a message “Hi Raghav” to my mobile using the GSM modem.

Block Diagram

Schematic

Code

// *********************************************************
// Project: Interfacing GSM to 8051 microcontroller
// Author: Code Bloges
// Module description: Operate GSM
// *********************************************************
#include <reg52.h>
//DEFINE CONSTANT
#define Baud_rate 0xFD  // BAUD RATE 9600                    
//DEFINE PROTOTYPES
void USART_Init(void);
void USART_Transmit(unsigned char);  
void cct_init(void);
void delay(unsigned int t);
void Tx_String( char *str);
void Number( char *str);
void main()
{
                cct_init();
                USART_Init();   
                Number("9999999999");
                while(1);
}
void Number( char *str)
{
                Tx_String("AT");
                USART_Transmit('\r');
                delay(1000);
                Tx_String("AT+CMGF=1");
                USART_Transmit('\r');
                delay(1000);
                Tx_String("AT+CMGS=");
                USART_Transmit('"');
                while(*str)
                {
                                USART_Transmit(*str);
                                str++;
                                delay(200);
                }
                USART_Transmit('"'); 
                USART_Transmit('\r');
                delay(1000);
                Tx_String("Hi Raghav"); 
                delay(1000);
                USART_Transmit(0x1a);
}
void delay(unsigned int t)
{
unsigned int i,j;
for(i=0;i<t;i++)
{
for(j=0;j<50;j++);
}
}
void cct_init(void)   //initialize cct
{
                P0 = 0x00; //not used
                P1 = 0x00; //Used for Appliances
                P2 = 0x00; //not used
                P3 = 0x03; //used for serial
}
void USART_Init(void)                              // INITIALIZE SERIAL PORT
{
                TMOD = 0x20;                           // Timer 1 IN MODE 2 -AUTO RELOAD TO GENERATE BAUD RATE
                SCON = 0x50;                                                    // SERIAL MODE 1, 8-DATA BIT 1-START BIT, 1-STOP BIT, REN ENABLED
                TH1 = Baud_rate;                                            // LOAD BAUDRATE TO TIMER REGISTER
                TR1 = 1;                                                                   // START TIMER
}
void USART_Transmit(unsigned char serialdata)
{
                SBUF = serialdata;                                                        // LOAD DATA TO SERIAL BUFFER REGISTER
                while(TI == 0);                                                   // WAIT UNTIL TRANSMISSION TO COMPLETE
                TI = 0;                                                                                    // CLEAR TRANSMISSION INTERRUPT FLAG
}
void Tx_String( char *str)
{
                while(*str)
                {
                                USART_Transmit(*str);
                                str++;
                                delay(200);
                }
}

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