This article shows how to use the inbuilt DTMF decoder present in SIM900/SIM800/SIM908 GSM modules. Earlier we used some external DTMF decoder chips like MT8870 to decode the DTMF tones present in the incoming audio stream. But life is simpler now.

Since i have tested the below commands in the SIM900 GSM modem, I will only explain the DTMF decoding functionality with regards to SIM900. When i set out to test the inbuilt DTMF decoder in SIM900, the first thing i came to know was that the DTMF commands are not supported in all firmwares, SIM900 has released separate firmwares for implementing each specific features. Likewise, the default firmware came on SIM900 didnt have support for DTMF AT commands, I had to download a new firmware which supports DTMF commands and upload it to SIM900 using the software tool provided by SIMCOM via Serial port interface.

Click here to download the DTMF enabled firmware for SIM900 

Click here to download the SIM900 firmware update tool for Windows

Once you upload the new DTMF firmware you can enable the DTMF decoding functionality using the command AT+DDET=1 followed by \r\n. This will enable the reporting whenever a button is pressed on the other users end on dial pad during a call. The SIM900 will automatically decode the DTMF tone coming from the other end and report it as +DTMF:1 or +DTMF:2 for number 1 or number 2 respectively as shown in the image below. 

Hoping it will help someone :)

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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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