It always pays to check other posts for similar problems. I can't give you code but I can suggest you goto the Atmel web site and look for an app note for generating dtmf using their AVR micros. Whilst the code is of little use to you the description of the operation should point you in the right direction. The technique used is just as valid on the aduc812 - just use the DAC rather than the pwm as used in the app note.