> I've always been curious whether or not you could use a square wave and
have
> it recognized by a DTMF receiver.  Would make things a lot simpler,
> obviously.  Anyone know?

I don't know, but since the first harmonics of a square wave start at 3
times the fundamental frequency you might just get away with it.

That brings up another idea, though.  Generate the square waves in software,
then apply a little low pass filtering to each one and add the results.  A
simple one or two pole low pass filter should suffice.  These are very easy
to do in software and take few cycles, especially when the filter fraction
is 1 / 2**N so you can use bit shifting for the divide.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.