Are you seeing four dots on your terminal when SIM900/SIM908 GSM module starts up? . That is because when SIM900/SIM908 is powered on for first time, it will be in auto baud mode. This feature makes the GSM module smarter by which it can detect at which baud rate your microcontroller/terminal is sending the data.

SIM300 GSM module had the default baud rate set to 9600 bps, and as soon as you power on the modem you will be able to communicate with the module only at a fixed baud rate of 9600 bps (Ofcourse baud rate could be changed later).

SIM900/SIM908 GSM modules on the other hand have autobaud mode enabled, So when it is powered ON it will give out some data, which will look as four dots like this  " . . . ."   .

If you enter AT commands, chances are you might not get any response from the module (Dont panic now !, its not dead :)) Its just trying to figure out at which speed/baud rate you are talking to it. 

If you enter AT commands like this,

at      //you will not get any response, because the module doesnt know at which baud rate it should respond to you

One thing you should know about auto baud detection mechanism is, the receiver is able to detect the baud rate  of the transmitter only when it detects a specific character, Characters such as ? or >  are normally used. In our case SIM900/SIM908 is looking for the letter A (Capital A)

If you enter AT commands like this,

AT      // you should get  response as OK

But i suggest that when you key in your first command from your microcontroller/terminal, stuff more before your AT command like,

AAAAT     //This has worked well for me, and hopefully it works for you too :)

Once you start getting the response, you can now fix the baud rate of the module by using the command, AT+IPR, like

AT+IPR=0    // Enable auto bauding, this is enabled by default

AT+IPR=1200 // Set baud rate to 1200 bps

AT+IPR=2400 // Set baud rate to 2400 bps

AT+IPR=9600 // Set baud rate to 9600 bps

AT+IPR=19200 // Set baud rate to 19200 bps

AT+IPR=38400 // Set baud rate to 38400 bps

AT+IPR=57600 // Set baud rate to 57600 bps

AT+IPR=115200 // Set baud rate to 115200 bps

Once you set the baud rate, from next startup of the module you need to communicate in the baud rate you fixed in the above command. 

This is all mentioned in the SIM900/SIM908 AT command manual on page number 13 and 14 . Hope it helps some one :)

Also see: 

 




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is the author of this blog and is an Embedded systems design engineer by profession & Founder @ Valetron Systems Pvt Ltd


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