SIM900A GSM module Interfacing with AVR ATmega16/ATmega32

Introduction

Global System for Mobile communication (GSM) is a digital cellular system used for mobile devices. It is an international standard for mobile which is widely used for long distance communication.

There are various GSM modules available in the market like SIM900, SIM700, SIM800, SIM808, SIM5320 etc.

SIM900A module allows users to send/receive data over GPRS, send/receive SMS and make/receive voice calls.

It communicates serially with the devices like a microcontroller, PC using AT commands.

To interface SIM900A module with the cellular network, it requires SIM card provided by a network operator.

For more information about Sim900A and how to use it, refer the topic SIM900A GSM/GPRS Module in the sensors and modules section.

The GSM/GPRS module uses USART communication to communicate with microcontroller or PC terminal.

For information about USART in AVR ATmega16/ ATmega32 and how to use it, refer the topic USART in AVR ATmega16/ATmega32 in the ATmega inside section.

Sim900A Module

Interfacing Diagram

Interfacing Sim900A GSM Module with ATmega 16

Application

Let’s design a little application in which user will receive SMS “call me” and immediately user will call that message sender.

Program

/*
    SIM900 GSM Interfacing with ATmega16/32
    http://www.electronicwings.com 
 */ 
#define F_CPU 8000000UL  /* define Clock Frequency */
#include <avr/io.h>     
#include <avr/interrupt.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#include "LCD_16x2_H_file.h" /* include 16x2 LCD Header file */
#include "USART_Interrupt.h" /* include USART Header file */

#define SREG   _SFR_IO8(0x3F)

void GSM_Begin();       
void GSM_Calling(char *);
void GSM_HangCall();
void GSM_Response();
void GSM_Response_Display();
void GSM_Msg_Read(int);
bool GSM_Wait_for_Msg();
void GSM_Msg_Display();
void GSM_Msg_Delete(unsigned int);
void GSM_Send_Msg(char* , char*);
void GSM_Delete_All_Msg();

char buff[160];   /* buffer to store responses and messages */
char status_flag = 0;  /* for checking any new message */
volatile int buffer_pointer;            
char Mobile_no[14];  /* store mobile no. of received message */
char message_received[60]; /* save received message */
int position = 0;  /* save location of current message */


int main(void)
{
 buffer_pointer = 0;
 int is_msg_arrived;
 memset(message_received, 0, 60);
        LCD_Init();
 USART_Init(9600); /* initialize USART */
 sei();
 LCD_String_xy(1,0,"GSM Initializing");
 _delay_ms(3000);
 LCD_Clear();
 LCD_String_xy(1,0,"AT");
 GSM_Begin();  /* Start GSM with AT*/
 LCD_Clear();
 
 while (1){
  
  /*check if any new message received */
  if(status_flag==1){     
   is_msg_arrived = GSM_Wait_for_Msg(); /*check for message arrival*/
   if(is_msg_arrived== true)
   {
      LCD_Clear();
      LCD_String_xy(1,0,"new message"); /* new message arrived */
      _delay_ms(1000);
      LCD_Clear();
      GSM_Msg_Read(position); /* read arrived message */  
      _delay_ms(3000);
     
   /*check if received message is "call me" */
    if(strstr( message_received,"call me")){
     
        GSM_Calling(Mobile_no); /* call sender of message */
               LCD_Clear();
        LCD_String_xy(1,0,"Calling...");
        _delay_ms(15000);
        GSM_HangCall(); /* hang call */
        LCD_String_xy(1,0,"Hang Call");
        _delay_ms(1000);
    }
 
   LCD_Clear(); 
   GSM_Msg_Delete(position); /* to save SIM memory delete current message */
   LCD_String_xy(1,0,"Clear msg");
   GSM_Response();
   _delay_ms(1000);
   
   }
    
        
   is_msg_arrived=0;
   status_flag=0;
   LCD_Clear(); 
   }
  LCD_String_xy(1,0,"waiting for msg");
  memset(Mobile_no, 0, 14);
  memset(message_received, 0, 60);
  
  
 }
}



void GSM_Begin()
{

 while(1)
 {
  LCD_Command(0xc0);
  USART_SendString("ATE0\r"); /* send ATE0 to check module is ready or not */
  _delay_ms(500);
  if(strstr(buff,"OK"))
  {
   GSM_Response();
   memset(buff,0,160);
   break;
  }
  else
  {
   LCD_String("Error");
  }
 }
 _delay_ms(1000);

 LCD_Clear();
 LCD_String_xy(1,0,"Text Mode");
 LCD_Command(0xc0);
 USART_SendString("AT+CMGF=1\r"); /* select message format as text */
 GSM_Response();
 _delay_ms(1000);
}

void GSM_Msg_Delete(unsigned int position)
{
 buffer_pointer=0;
 char delete_cmd[20];

      /* delete message at specified position */
 sprintf(delete_cmd,"AT+CMGD=%d\r",position);  
 USART_SendString(delete_cmd);
}

void GSM_Delete_All_Msg()
{
   USART_SendString("AT+CMGDA=\"DEL ALL\"\r"); /* delete all messages of SIM */ 
}

bool GSM_Wait_for_Msg()
{
 char msg_location[4];
 int i;
 _delay_ms(500);
 buffer_pointer=0;

 while(1)
 {
           /*eliminate "\r \n" which is start of string */

    if(buff[buffer_pointer]=='\r' || buff[buffer_pointer]== '\n'){
   buffer_pointer++;
  }
  else
   break;
 }

 /* "CMTI:" to check if any new message received */
 
 if(strstr(buff,"CMTI:")){
  while(buff[buffer_pointer]!= ',')
  {
   buffer_pointer++;
  }
  buffer_pointer++;
  
  i=0;
  while(buff[buffer_pointer]!= '\r')
  {
   msg_location[i]=buff[buffer_pointer];          /* copy location of received message where it is stored */
   buffer_pointer++;
   i++;
  }

  /* convert string of position to integer value */
  position = atoi(msg_location);
  
  memset(buff,0,strlen(buff));
  buffer_pointer=0;

  return true;
 }
 else
 {
  return false;
 }
}

/* ISR routine to save responses/new message */
ISR(USART_RXC_vect)
{
   buff[buffer_pointer] = UDR; /* copy UDR (received value) to buffer */
   buffer_pointer++;
   status_flag = 1;  /* flag for new message arrival */
}


void GSM_Send_Msg(char *num,char *sms)
{
 char sms_buffer[35];
 buffer_pointer=0;
 sprintf(sms_buffer,"AT+CMGS=\"%s\"\r",num);
 USART_SendString(sms_buffer); /*send command AT+CMGS="Mobile No."\r */
 _delay_ms(200);
 while(1)
 {
  if(buff[buffer_pointer]==0x3e) /* wait for '>' character*/
  {
     buffer_pointer = 0;
     memset(buff,0,strlen(buff));
     USART_SendString(sms); /* send msg to given no. */
     USART_TxChar(0x1a); /* send Ctrl+Z */
     break;
  }
  buffer_pointer++;
 }
 _delay_ms(300);
 buffer_pointer = 0;
 memset(buff,0,strlen(buff));
 memset(sms_buffer,0,strlen(sms_buffer));
}

void GSM_Calling(char *Mob_no)
{
   char call[20];
   sprintf(call,"ATD%s;\r",Mob_no);
   USART_SendString(call); /* send command ATD<Mobile_No>; for calling*/
}

void GSM_HangCall()
{
   LCD_Clear();
   USART_SendString("ATH\r"); /*send command ATH\r to hang call*/ 
}

void GSM_Response()
{
 unsigned int timeout=0;
 int CRLF_Found=0;
 char CRLF_buff[2];
 int Response_Length=0;
 while(1)
 {
  if(timeout>=60000)        /*if timeout occur then return */
  return;
  Response_Length = strlen(buff);
  if(Response_Length)
  {
   _delay_ms(2);
   timeout++;
   if(Response_Length==strlen(buff))
   {
    for(int i=0;i<Response_Length;i++)
    {
     memmove(CRLF_buff,CRLF_buff+1,1);
     CRLF_buff[1]=buff[i];
     if(strncmp(CRLF_buff,"\r\n",2))
     {
         if(CRLF_Found++==2)                                        /* search for \r\n in string */
          {
      GSM_Response_Display();
      return;
          }
     }

    }
    CRLF_Found = 0;

   }
   
  }
  _delay_ms(1);
  timeout++;
 }
 status_flag=0;
}

void GSM_Response_Display()
{
 buffer_pointer = 0;
 int lcd_pointer = 0;
 while(1)
 {
          /* search for \r\n in string */
   if(buff[buffer_pointer]== '\r' || buff[buffer_pointer]== '\n')  
  {
   buffer_pointer++;
  }
  else
   break;
 }
 

 LCD_Command(0xc0);
 while(buff[buffer_pointer]!='\r')           /* display response till "\r" */
 {
  LCD_Char(buff[buffer_pointer]);        
  buffer_pointer++;
  lcd_pointer++;
  if(lcd_pointer==15) /* check for end of LCD line */
    LCD_Command(0x80);
 }
 buffer_pointer = 0;
 memset(buff,0,strlen(buff));
}

void GSM_Msg_Read(int position)
{
 char read_cmd[10];
 sprintf(read_cmd,"AT+CMGR=%d\r",position);
 USART_SendString(read_cmd); /* read message at specified location/position */
 GSM_Msg_Display(); /* display message */
}

void GSM_Msg_Display()
{
 _delay_ms(500);
 if(!(strstr(buff,"+CMGR"))) /*check for +CMGR response */
 {
  LCD_String_xy(1,0,"No message");
 }
 else
 {
  buffer_pointer = 0;
  
  while(1)
  {
                        /*wait till \r\n not over*/

   if(buff[buffer_pointer]=='\r' || buff[buffer_pointer]== 'n')                    {
    buffer_pointer++;
   }
   else
           break;
  }
  
  /* search for 1st ',' to get mobile no.*/
  while(buff[buffer_pointer]!=',')
  {
   buffer_pointer++;
  }
  buffer_pointer = buffer_pointer+2;

  /* extract mobile no. of message sender */
  for(int i=0;i<=12;i++)
  {
   Mobile_no[i] = buff[buffer_pointer];
   buffer_pointer++;
  }
  
  do
  {
   buffer_pointer++;
  }while(buff[buffer_pointer-1]!= '\n');
  
  LCD_Command(0xC0);
  int i=0;

  /* display and save message */
  while(buff[buffer_pointer]!= '\r' && i<31)
  {
    LCD_Char(buff[buffer_pointer]);
    message_received[i]=buff[buffer_pointer];
    
    buffer_pointer++;
    i++;
    if(i==16)
     LCD_Command(0x80); /* display on 1st line */
  }
  
  buffer_pointer = 0;
  memset(buff,0,strlen(buff));
 }
 status_flag = 0;
}