[PIC] Unable to read SMS and turn on device

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ericparggah

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Hello All,
I am working on a GSM based device control system based on the PIC16F876 and a SIM900 module from Rhydozlabs. The compiler i am using is the FED WIZ-C version 17.02.
I need to be able to control 4 devices but in the meant time just trying to make sure one works before i tackle the rest. The problem is that I want to turn on port RB4 using the command: TV ON with the code as shown below:

Code C - [expand]
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void UserLoop()
{   
    if(RxFlag)
    {
        ReadGsmLine(&GsmResponse);
        if(memcmp("+CMTI: \"SM\",", &GsmResponse, 12) == 0)
        {
            ReadSmsCommand[8] = GsmResponse[12];
            DeleteSmsCommand[8] = GsmResponse[12];
            SendGsmCommand(&ReadSmsCommand);
            Wait(1000);
            ReadGsmLine(&GsmResponse); //Read modem response
            ReadGsmLine(&ControlInstruction); // Read Instruction
            ReadGsmLine(&GsmResponse); //Read modem response
            ReadGsmLine(&GsmResponse); //Read modem response OK
            
                        if(memcmp("TV ON", &ControlInstruction, 5) == 0) // ------(Remove)
            {
                Device_A = ON;
            }
            SendGsmCommand(&DeleteSmsCommand);
                        Wait(1000);
            ReadGsmLine(&GsmResponse);//Read OK
        }
    }
}


When I comment out the line:

Code C - [expand]
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if(memcmp("TV ON", &ControlInstruction, 5) == 0) // ------(Remove)


then the Device_A turns on. Without commenting out that line then it doesn't turn on. I need to use that line to test the various command strings for the four different devices.

Any help will be most appreciated.

Thanks in advance.
 
Last edited by a moderator:

You'll want to check which answers are actually received from the modem and why the comparison fails. This can be either done by operating the PIC16 under an in-circuit debugger or by sniffing the serial interface between processor and modem with dual channel RS232 interface and a suitable tool, e.g. docklight.
 

I have already done that with tools like proteus and Hterm. I see the communication between the uC and the modem. However there are some blank spaces in the responses from the modem and I don't know how to handle such responses. Any help will be very much appreciated.
Thanks a lot FvM for the swift reply to my problem.
 

In other words, you are stumbling upon wrong assumptions about the format of the modem response.

I presume that the SMS content itself is transmitted correctly. The problem is about the format of the "ReadSMS" (AT+CMGR) response. The correct solution is decode the response according to the at commands specification. At places where a variable number of blanks or other format variations can occur, you should be prepared for it. Instead of expecting the answer at a fixed string position, decode the delimiters.
 

Hi,
These are the read and send functions that I am using:
Code C - [expand]
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void ReadGsmLine (char *Buffer)
{
   unsigned char receivedData;
   do
   {
       receivedData = WaitRx ();// WaitRx() for serial 
       *Buffer++ = receivedData;
   }while (receivedData != '\n');
}
 
Last edited by a moderator:

ericparggah said:
Hi,
These are the read and send functions that I am using:
Click to expand...

This is the USART read function that i am using:

Code C - [expand]
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void ReadGsmLine (char *Buffer)
{
   unsigned char receivedData;
   do
   {
       receivedData = WaitRx ();// WaitRx() for serial 
       *Buffer++ = receivedData;
   }while (receivedData != '\n');
}


When I used the Hterms to sniff the responses from the GSM modem, I then noticed that all the responses are sandwiched by the <CR><LF><Response><CR><LF> delimiters. I really need help on decoding the delimiters.
Any help please will be very much appreciated.
Thanks in advance.
 

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