GSM SIM900

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

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

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

For information about UART in 8051 and how to use it, refer to the topic UART in 8051 in the 8051 Inside section.

GSM SIM900 Connection with 8051 

Application

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

GSM SIM900 Message and Calling using 8051

/*
    SIM900 GSM Interfacing with 8051
    http://www.electronicwings.com 
 */ 
#include<reg52.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "LCD_8_bit.h"			/* include 16x2 LCD Header file */
#include "UART_Interrupt.h"		/* include UART Header file */

void GSM_Begin();				           			
void GSM_Calling(char *);          
void GSM_HangCall();
void GSM_Response();
void GSM_Response_Display();
void GSM_Msg_Read(int);
int GSM_Wait_for_Msg();				/* check whether new msg arrival */
void GSM_Msg_Display();
void GSM_Msg_Delete(unsigned int);	/* delete msg at specific location*/
void GSM_Send_Msg(char* , char*);	/* send messages*/
void GSM_Delete_All_Msg();			/* delete all msgs */

idata char buff[50];				/* buffer to store responses and messages */
char status_flag = 0;				/* for checking any new message */
volatile int buffer_pointer;
idata char Mobile_no[14];			/* store mobile no. of received message */
idata char message_received[50];	/* save received message */
int position = 0;					/* save location of current message */

int main(void)
{
	int is_msg_arrived;
	buffer_pointer = 0;	
	memset(message_received, 0, 35);
	LCD_Init();
	UART_Init();					/* initialize UART communication */
	Ext_int_Init();
	LCD_String_xy(1,0,"GSM Initializing");
	delay(3000);
	LCD_Clear();
	LCD_String_xy(1,0,"AT");    
	GSM_Begin();					/* initialize GSM */
	LCD_Clear();	
	while (1){
		
		/*check if any new message received */
		if(status_flag == 1){					
			is_msg_arrived = GSM_Wait_for_Msg();	/*check for new message arrival*/
			if(is_msg_arrived)
			{
				LCD_Clear();
				LCD_String_xy(1,0,"new message");	/* new message arrived */
				delay(1000);
				LCD_Clear();
				GSM_Msg_Read(position);				/* read arrived message */		
				delay(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(20000);
					GSM_HangCall();   				/* hang call */
					LCD_String_xy(1,0,"Hang Call");
					delay(1000);
				}
	
				LCD_Clear();
				GSM_Msg_Delete(position);		/* to save SIM memory delete current message */
				LCD_String_xy(1,0,"Clear msg");
				GSM_Response();
				delay(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);
		UART_SendString("ATE0\r");	/* send ATE0 to check module is ready or not */
		delay(500);
		if(strstr(buff,"OK"))
		{
			GSM_Response();			/* get Response */
			memset(buff,0,50);
			break;
		}
		else
		{
			LCD_String("Error");
		}
	}
	delay(1000);

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

void GSM_Msg_Delete(unsigned int position)
{
	idata char delete_cmd[15];
	buffer_pointer=0;
	sprintf(delete_cmd,"AT+CMGD=%d\r",position); /* delete message at specified position */
	UART_SendString(delete_cmd);
}

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

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

	while(1)
	{
		if(buff[buffer_pointer]=='\r' || buff[buffer_pointer]== '\n') /*eliminate "\r \n" which is start of string */
		{
			buffer_pointer++;
		}
		else
			break;
	}
		
	if(strstr(buff,"CMTI:"))				/* "CMTI:" to check if any new message received */
	{
		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 1;
	}
	else
	{
		return 0;
	}
}

void GSM_Send_Msg(char *num,char *sms)
{
	idata char sms_buffer[30];
	buffer_pointer=0;
	sprintf(sms_buffer,"AT+CMGS=\"%s\"\r",num);
	UART_SendString(sms_buffer);		/*send command AT+CMGS="Mobile No."\r */
	delay(200);
	while(1)
	{
		if(buff[buffer_pointer]==0x3e)	/* wait for '>' character*/
		{
			buffer_pointer = 0;
			memset(buff,0,strlen(buff));
			UART_SendString(sms);		/* send msg to given no. */
			UART_TxChar(0x1a);			/* send Ctrl+Z then only message will transmit*/
			break;
		}
		buffer_pointer++;
	}
	delay(300);
	buffer_pointer = 0;
	memset(buff,0,strlen(buff));
	memset(sms_buffer,0,strlen(sms_buffer));
}

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

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

void GSM_Response()
{
	int i;
	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(2);
			timeout++;
			if(Response_Length==strlen(buff))
			{
				for(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();	/* display response */
							status_flag=0;
							return;
						}
					}

				}
				CRLF_Found = 0;

			}
		}
		delay(1);
		timeout++;
	}
	
}

void GSM_Response_Display()
{
	int lcd_pointer = 0;
	buffer_pointer = 0;	
	while(1)
	{
		if(buff[buffer_pointer]== '\r' || buff[buffer_pointer]== '\n')  /* search for \r\n in string */
		{
			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)
{
	idata char read_cmd[12];
	sprintf(read_cmd,"AT+CMGR=%d\r",position);
	UART_SendString(read_cmd);					/* read message at specified location/position */
	GSM_Msg_Display();							/* display message */
}

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

/* ISR routine to save responses/new message */

void Serial_ISR() interrupt 4
{
	if(RI){
		EA = 0;
		buff[buffer_pointer] = SBUF;	/* copy SBUF(received value) to buffer */
		buffer_pointer++;
		status_flag = 1;				/* flag for new message arrival */
		RI = 0;
		EA = 1;
	}
}

GSM SIM900 Message and Calling using 8051 Video