On Fri, 26 Feb 1999, Andrew Russell Morris wrote:

> Some time ago there was a thread about DTMF decoders and one gentleman
> stated that he had developed a fully functional DTMF
> decoder that used an incredibly small amount of PIC processor resources. He
> could not share the details with us because he
> had done the work for his employer. I'm sure I know how he did it. I
> stumbled on the trick quite by accident while testing
> software I wrote for my employer. Since this knowledge is too important to

That somebody was probably me. But then again it may've been John Payson
too.

All of the details on HOW to decode dtmf signals is discussed from a
somewhat theoretically perspective on my web page:

http://www.interstice.com/~sdattalo/technical/theory/dtmf.html

But as I like to say, "in theory there's not much difference in theory and
practice, but in practice there is." Believe me, there are several tricks
necessary to implement this theory. And in fact, a robust design even
needs a little bit of extra (low-cost) hardware that's not discussed on
the web page.

I can't release the stuff I've written. However, I know that vertical
counters could be utilized to improve my algorithm. Right now, the
algorithm samples all 8 frequencies in an isochronous 84 instruction cycle
loop. I think the vertical counters can cut this down to 70 cycles (or
less).

Scott