Help needed regarding AT commands Response

zeeshan ahmed · Joined: 02 Dec 2010 Posts: 22

Hi all,
I am working on a project , in which i have to use GSM MODEM, i have successfully tested GSM MODEM with PIC18f252 and its working OK. Now WHAT I WANT TO DO IS TO COMPARE AT COMMAND'S RESPONSE.
e.g. if i issue AT+cmgf=1, then GSM modem will repond with OK. same as if i issue AT+CMGR=3, then GSM modem will repond with "+cmgr: message , location" etc.
I tried to do it by comparing OK with the characters 'o' and 'k'. But i need some more flexible method to do this comparision, such as by comparing strings etc.

I m using sim900D gsm modem.

Any sort of help will be highly appreciated.
Thanks.

Douglas Kennedy · Joined: 07 Sep 2003 Posts: 755 Location: Florida

You don't describe the interface to the PIC but I suspect it might be RS232.
First, it is very hard to have a successful RS232 outcome unless you use a circular buffer fed from an ISR. Since RS232 is asynchronous there must be no expectation that the buffer contains OK. It may just have O or nothing at all. The ISR should count in consecutive O and K chars and set a one byte global flag. In your main routine you would write a parser that would find and extract the AT response from the circular buffer providing the flag is set. The time the parser takes to do this may overlap the receipt of a new char. The ISR with interrupt the parser and place the new char in the circular buffer. In this way your main routine is uncoupled from the asynchronous receipt of chars.
Second, look at the string functions in the compiler manual Ex strcmp

Ttelmah · Joined: 11 Mar 2010 Posts: 19520

Or, consider a state machine.
Depends massively on how many things you are looking for, the likely lengths etc.. String comparison, becomes 'hard work', and slow, if there are more than a few options to look for;

Best Wishes

bkamen · Posted: Sat Jun 04, 2011 8:12 pm

Douglas Kennedy · Joined: 07 Sep 2003 Posts: 755 Location: Florida

Well rambling a bit also
For NMEA a GPS will squawk several sentences ( most do this but some allow for a master to request a certain sentence). I would in the isr use a state machine to gate a specific sentences into the circular buffer and set a flag pointer to the beginning of the sentence in the circular buffer. The main routine would parse the specific sentences and allowing for the case that if an incoming new sentence overlaps the parsing it still will be captured.

bkamen · Posted: Sun Jun 05, 2011 12:35 pm

In my state machine, I immediately make a copy of the string and then release the ISR to continue capturing.

While I haven't tested, it is possible the next sentence is missed during the copy of the current string into the "temp" string before the release..

A ring buffer would certainly solve that but I usually run the PIC doing the processing at 40MHz and so far, i think it's been able to keep up.

but a ring buffer would definitely solve that.

-Ben
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Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

...i tackled this issue some time ago..

check for my post on "Recieving text messages"

http://www.ccsinfo.com/forum/viewtopic.php?t=42527&highlight=gabriel

i use a circular buffer.. and look for strings in the responses from the modem...

its old code... not very clean... but might help...
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