# What is the best way to receive unsolicited response from GSM SIM900 in PIC18

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)
{

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.

• Maybe all commands (both the OK responses and unsolicited ones) really end up with a newline ('\n' or '\r' or "\r\n").. Then it will be easy to know when either message ends.. – m.Alin Aug 2 '16 at 11:12
• yes I have tried checking for \r \n but what happens is lets say I send AT\r so it stops after this command and doesnt wait for response. I checked into the buffer while debugging and found out that it stores  A T \r \r \n so due to last \r \n it stops after sending command and doesnt wait for response from sim900 – S Andrew Aug 2 '16 at 11:16
• From the AT command manual: "Commands are usually followed by a response that includes <CR><LF><response><CR><LF>" – m.Alin Aug 2 '16 at 11:17
• So just ignore the first \r\n and look for the second pair of \r\n you receive.. – m.Alin Aug 2 '16 at 11:23
• The SIM900 echos back the command it received, this feature can be disabled. I found it pretty annoying in some cases. But all the same, there will be two \r\n sequences, acknowledge the first and stop and the second as Alin has told you. – Bence Kaulics Aug 2 '16 at 11:43

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)
{

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.

• Code looks okay. But i think unsolicited reponse have onlt one \r\n. You will also have to add a condition for it. – Aircraft Aug 2 '16 at 14:00