What is DTMF? Dual Tone Multi Frequency is a method of sending 1 of 16 characters (0 -9 * # A -D) over an audio path (telephone, radio etc.) for dialing, selective calling or control purposes.

The characters are sent using two simultaneous audio frequencies per the following table:-

The duration of each character is from a minimum of 40 ms to as long as you want, and each character must be separated by a pause of at least 40 ms.

Generation of the DTMF tone pairs is achieved using an encoder chip such as the HT9200A from Holtek. This IC will interface with a microcontroller via a simple serial protocol, and generates all 16 DTMF tone pairs. Other DTMF encoder IC's such as the TCM5087N can be connected directly to a keypad, but these are getting difficult to obtain.

Detection of the tone pairs is achieved using a decoder chip such as the HT9170 from Holtek or MT8870 for Mitel which output a 4 bit binary code to represent the decoded character and a strobe when the binary code has been updated with a new character. You can use this output directly for simple applications, but more usually it would be fed to a microcontroller.

The following list shows the output codes from the HT9170 dtmf decoder IC.  Note that a dialed ' 0' is coded as binary '1010' or '10' in decimal, this is presumably a throwback from telephone dials which produced 10 pulses when a ' 0' was dialed.