Oleg,

Generating DTMF is pretty straight forward. The DTMF tone is the combination of two sine frequencies corresponding to the Digit you intend to encode.

The technique is often refered to a "phase-increment oscillator" and is the standard technique used by Numerically Controlled Oscillators.

With a Sample Rate of 8KHz, Phase Accum/Freq. Increment of 24 bits) the task is fine for an 8051. My 1st implementation was with a 6.144 Mhz 8085.

Reliable software only detection of DTMF is far trickier requirement FFT techniques which even with "tricks" tasks a '51 type processor.

It isn't uncommon for folks to choose to generate the DTMF digits and to use a DTMF receiver/decoder for detection. At least saving the cost of a transceiver.

Of course their are analog ways of detecting the DTMF (or atleast "front-ending a simpler micro detection algorithm) but they are only cheaper than an IC detector in very large quantity, very large form factor, and unit by unit manual adjustment applications.

regards,
p