Overview of GSM
The Global System for Mobile Communication (GSM) is a digital cellular network standard widely used for mobile communication and long-distance data transmission.
Various GSM modules are available on the market, including models like SIM900, SIM700, SIM800, SIM808, and SIM5320. The SIM900A module, specifically, enables users to send and receive data over GPRS, send and receive SMS, and make and receive voice calls.
This module communicates serially with devices such as microcontrollers or PCs using AT commands. To connect the SIM900A module to a cellular network, it requires a SIM card provided by a network operator.
For more detailed information on the SIM900A and its functions, refer to the “SIM900A GSM/GPRS Module” section in sensors and modules.
The GSM/GPRS module uses USART communication for interaction with a controller or PC terminal. For further details on USART usage with the PIC18F4550, see the “USART in PIC18F4550” topic in the PIC inside section.
SIM900A GSM Module
Connection Diagram of SIM900A GSM Module to PIC18F4550
GSM Module Interfacing with PIC microcontroller
SIM900A GSM Code for PIC18F4550
/*
* SIM900A Interfacing with PIC18F4550
* http://www.electronicwings.com
*/
#include <pic18f4550.h>
#include "Configuration_Header_File.h"
#include "LCD_16x2_8-bit_Header_File.h"
#include "USART_Header_File.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void GSM_Init();
void GSM_Calling(char *);
void GSM_HangCall();
void GSM_Response();
void GSM_Response_Display();
void GSM_Msg_Read(int);
void GSM_Wait_for_Msg();
void GSM_Msg_Display();
void GSM_Msg_Delete(unsigned int);
void GSM_Send_Msg(const char* , const char*);
char buff[160]; /* buffer to store responses and messages */
volatile char status_flag; /* monitor to check for any new message */
volatile int a;
void main(void)
{
a=0;
OSCCON =0x72; /* set internal oscillator Freq = 8 MHz*/
LCD_Init(); /* initialize LCD */
INTCONbits.GIE=1; /* enable Global Interrupt */
INTCONbits.PEIE=1; /* enable Peripheral Interrupt */
PIE1bits.RCIE=1; /* enable Receive Interrupt */
MSdelay(100);
USART_Init(9600); /* initialize USART communication */
GSM_Init(); /* check GSM responses and initialize GSM */
LCD_Clear();
LCD_String("Sending SMS...");
MSdelay(3000);
GSM_Send_Msg("8983923089","TEST"); /*send sms on "mobile no."*/
MSdelay(3000);
LCD_Clear();
while(1)
{
/* waiting for message to receive */
status_flag=0;
if(status_flag)
{
LCD_Clear();
LCD_String_xy(0,0,"New msg arrive");
MSdelay(1000);
LCD_Clear();
GSM_Wait_for_Msg(); /*wait for new message to read */
status_flag = 0;
}
}
}
void GSM_Init()
{
while(1)
{
LCD_Command(0xc0);
USART_SendString("ATE0\r"); /* send AT to check module is ready or not*/
MSdelay(500);
if(strstr(buff,"OK"))
{
GSM_Response(); /* find response and display it on LCD16x2 */
memset(buff,0,160);
break;
}
else
{
LCD_String("Error");
}
}
MSdelay(2000);
LCD_Clear();
LCD_String_xy(0,0,"Text Mode");
LCD_Command(0xc0);
USART_SendString("AT+CMGF=1\r"); /* select message format as text */
GSM_Response();
MSdelay(3000);
LCD_Clear();
LCD_String_xy(0,0,"Mfd name");
LCD_Command(0xc0);
USART_SendString("AT+GMI\r"); /* identify manufacturer */
GSM_Response();
MSdelay(3000);
LCD_Clear();
LCD_String_xy(0,0," Model No.");
LCD_Command(0xc0);
USART_SendString("AT+GMM\r"); /* find model no. */
GSM_Response();
MSdelay(3000);
LCD_Clear();
LCD_String_xy(0,0," IMEI No. ");
LCD_Command(0xc0);
USART_SendString("AT+GSN\r"); /* find IMEI no. of module */
GSM_Response();
MSdelay(3000);
LCD_Clear();
LCD_String_xy(0,0,"Service Provider");
LCD_Command(0xc0);
USART_SendString("AT+CSPN?\r"); /* find service provider name */
GSM_Response();
MSdelay(3000);
}
void GSM_Msg_Delete(unsigned int position)
{
a=0;
char delete_cmd[20];
sprintf(delete_cmd,"AT+CMGD=%d\r",position); /* delete message at specified position */
USART_SendString(delete_cmd);
MSdelay(100);
memset(buff,0,strlen(buff));
}
void GSM_Wait_for_Msg()
{
char i,val[4];
int position;
LCD_Clear();
a=0;
while(1)
{
if(buff[a]==0x0d || buff[a]==0x0a) /*eliminate "\r \n" which is start of string */
{
a++;
}
else
break;
}
if(strstr(buff,"CMTI:")) /* check if any new message received */
{
while(buff[a]!=',')
{
a++;
}
a++;
i=0;
while(buff[a]!=0x0d)
{
val[i]=buff[a];
a++;
i++;
}
position = atoi(val);
if(position>20)
{
LCD_String_xy(0,0,"Msg mem full");
memset(buff,0,strlen(buff));
return;
}
memset(buff,0,strlen(buff));
a=0;
GSM_Msg_Read(position); /* read message which is recently arrived from position */
}
}
void interrupt ISR()
{
if(RCIF)
{
buff[a] = RCREG; /* read received byte from serial buffer */
a++;
if(RCSTAbits.OERR) /* check if any overrun occur due to continuous reception */
{
CREN = 0;
NOP();
CREN=1;
}
status_flag=1; /* use for new message arrival */
}
}
void GSM_Send_Msg(const char *num,const char *sms)
{
int i;
char sms_buffer[35];
a=0;
sprintf(sms_buffer,"AT+CMGS=\"%s\"\r",num);
USART_SendString(sms_buffer); /*send command AT+CMGS="Mobile No."\r */
MSdelay(200);
while(1)
{
if(buff[a]==0x3e) /* wait for '>' character*/
{
a=0;
memset(buff,0,strlen(buff));
USART_SendString(sms); /* send msg to given no. */
USART_TxChar(0x1a); /* send Ctrl+Z then only message will transmit*/
break;
}
a++;
}
MSdelay(300);
a=0;
memset(buff,0,strlen(buff));
memset(sms_buffer,0,strlen(sms_buffer));
}
void GSM_Calling(char *mobile)
{
char call[20];
sprintf(call,"ATD%s;\r",mobile); /* send command ATD8007xxxxxx; for calling*/
USART_SendString(call);
}
void GSM_HangCall()
{
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)
{
MSdelay(1);
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;
}
}
MSdelay(1);
timeout++;
}
}
void GSM_Response_Display()
{
a=0;
while(1)
{
if(buff[a]==0x0d || buff[a]==0x0a)
{
a++;
}
else
break;
}
LCD_String_xy(1,0," ");
LCD_Command(0xc0);
while(buff[a]!=0x0d)
{
LCD_Char(buff[a]);
a++;
if(a==15)
LCD_Command(0x80);
}
a=0;
memset(buff,0,strlen(buff));
}
void GSM_Msg_Read(int position)
{
int i,k;
char flag,read_cmd[10];
i=0;
sprintf(read_cmd,"AT+CMGR=%d\r",position);
USART_SendString(read_cmd);
MSdelay(1000);
GSM_Msg_Display();
}
void GSM_Msg_Display()
{
if(!(strstr(buff,"+CMGR"))) /*check for +CMGR response */
{
LCD_String_xy(0,0,"No message");
}
else
{
a=0;
while(1)
{
if(buff[a]==0x0d || buff[a]==0x0a) /*wait till \r\n not over*/
{
a++;
}
else
break;
}
while(buff[a]!=0x3a) /*wait till string not equal to ':' */
{
a++;
}
do
{
a++;
}while(buff[a-1]!=0x0a);
LCD_Command(0xC0);
int i=0;
while(buff[a]!=0x0d && i<31)
{
LCD_Char(buff[a]);
a++;
i++;
if(i==16) /* if received message is greater than 16(for display message)*/
LCD_Command(0x80); /* resume display of message from 1st line */
}
a=0;
memset(buff,0,strlen(buff));
}
status_flag = 0;
}