Electronic – the best way to receive unsolicited response from GSM SIM900 in PIC18

cmicrocontrollerpicsim900

I am using PIC18F2520 and trying to communicate with GSM SIM900. I am using UART interrupt to receive data from the GSM. I am saving each byte in rxData. Now most of the commands ends with response OK. But there are few commands which does not respond with OK like when we receive sms notification from GSM, GSM send following notification

 +CMTI: "SM",2

Now this response doesn't have any OK. How to receive data in this case. In my application I need to check for the received sms. I am using following code:

void rx_handler(void)
{
  rxData[index] = ReadUSART();

  if(<some condition>)    //condition to check for sms notification
  {
    rxFlag = 1;        //set flag to process it in main loop

  }
  index++;

PIR1bits.RCIF==0;

}

I am getting confused on how to check for the sms notification. Till now for other commands, I was setting condition for OK but this doesnt have any OK response. Please help.

Best Answer

Thanks to @Bence Kaulics & @m.Alin. I have solved the issue by double checking the \r \n. Following is the code I used:

volatile int flag = -1;   
volatile int sFlag = 0; 
volatile int uFlag = 0;

void rx_handler(void)
{
    rxData[index] = ReadUSART();

    if(rxData[index]=='\n' && rxData[index-1]== '\r' )
    {
      flag = flag + 1;

      if(flag == 1)    
      {
         sFlag = 1;
      }
    }

}

index++;

PIR1bits.RCIF==0;

}

I have used a flag = -1. So when I first got \r \n value of flag is 0. And again when I got \r \n that means we have detected a line end and this is what we are expecting so set sFlag = 1. When sFlag = 1 that means we have received the data and now we can process it.

Related Solutions

Electronic – SIM900 Error on AT Command AT+SAPBR=1,1

I'm actually also working on a SIM900 and had the same problem.

One possible cause of the problem is that the SIM900 already has made a connection. Try re-starting the sim900 before issuing the commands.

Also I set the APN settings with the AT+CSTT= command

Below the in/output of my serial connection:

^{For testing purposes use this. Be sure to have the right APN settings, this will differ for other countries and ISPs.}

StopSim900;
[SIM] Stopping
[SIM] Module disabled.
StartSim900;
[SIM] Starting...
[SIM] Module started
-----------DEBUG-----------//Pressed button 1 here. Initiates the HTTP get
AT+CSTT="internet","tmobile","tmobile"
AT Issued, response:
OK

AT+SAPBR=1,1
AT Issued, response:
OK

AT+HTTPINIT
AT Issued, response:
OK

AT+HTTPPARA="CID",1
AT Issued, response:
OK

AT+HTTPPARA="URL","http://www.minimalsites.com/"
AT Issued, response:
OK

AT+HTTPACTION=0
AT Issued, response:
OK

-----------DEBUG-----------//Pressed button 2 here, reads the http message
AT+HTTPREAD=20,10
AT Issued, response:
+HTTPREAD:10
>Minimal S
OK

AT+HTTPTERM
AT Issued, response:
OK

I have left my answer here for further reference.

Error getting data from GSM SIM900

In your code the following function call is wrong:

serial_Tx(data1[i]);

Because data1[i] has a type of char but void serial_Tx(char *str) expects a char*, this function is for sending strings, not single characters. This way it causes runtime error, hence you won't get the expected output.

Though, it won't generate a compilation error, but the compiler will give a warning like this:

It is recommended to check and fix these warnings, because as you could see they could cause troubles in runtime.

So, replace the function above with the void uart_put(char data) function, like:

uart_put(data[i]);

and it will work.

Best Answer

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Electronic – SIM900 Error on AT Command AT+SAPBR=1,1

Error getting data from GSM SIM900

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