My personal 2 cents for this matter:

The effort to make things like that, using a microcontroller and
software to decode DTMF is 100% valid upon the point of view of learning
things, but for the sake of functionality just use dedicated microchips
to do the job, they were done with much more knowledge and it will saves
your time and money.  It is the same to build up a microcontroler using
TTL discrete chips... imagine the board size and problems...
educationally, or school projects speaking it is fantastic... but that's
all.  There are several DTMF decoders available for less than $2.  Your
school project could be the one to read the chip, display the numbers,
store them, and so on, not the decoding itself...

Wagner

Scott Dattalo wrote:
>
> Harry Febianto wrote:
> >
> > Hi,
> >
> > I have a final project for my college. I plan to detect dtmf signal from
> >
> > telephone to be decoded. I use PIC 16F84. And I have some
> > possibilities/theorys. Using interrupt and counter together or counter
> > only
> > or ADC 1bit (or 8 bit ADC).
> >
> > I think I need some literatures, and many thanks if you can give me some
> >
> > references that I can download. Which one is better for above theorys ?
> > Or
> > do you have any advise or some example program ?
> >
> > Thank you very much for your help.
> >
> > Best Regards,
> >
> > Harry Febianto
>
> As others have already said, using a dedicated DTMF decoder IC is the
> preferred way to decode DTMF. But..., if you're somewhat demented like
> myself then take a look at:
>
> http://www.interstice.com/~sdattalo/technical/theory/dtmf.html
>
> As I've said in previous posts on this subject: this algorithm works
> well, but additional analog signal processing is required to make it
> robust.
>
> Scott