At 08:26 PM 10/10/96 -0700, Walter Banks  wrote:
>I've seen a lot of DTMF decoding  scheme's over the last 10 years or so
>it basically boils down to measuring energy at a 12 or 16 frequencies and
>picking out the most likely pairs. Every other scheme that I have seen
>has a fatal flaw.
>
  Some DTMF decoder parts (hardware, not software) separate the "high" and
"low" groups, often with switched capacitor filters, then count cycles to
decode the DTMF.  This is how some of the more "modern" parts, such as the
Mitel/Teltone M8870 work.  Older parts used 8 single-frequency bandpass filters.

  For whatever application somebody wanted to make a PIC decode touch-tone,
let me suggest investigating the Motorola 68HC05F5, which has an on-chip
DTMF decoder, in a 40 pin DIP.  Thie big problem with this device is that it
has ROM as opposed to PROM, making programming hobbiest quantities a bit of
a problem.

  I have had good results interfacing the M8870 into a PIC.  You can even
slave the PIC clock off of the 3.58 MHz oscillator in the M8870.  use about
30pf from the osc out of the m8870 right into the osc in of the pic.

Jeff
Jeff Otterson
-------------
otterson@mindspring.com
Maker and user of tools
PGP key available at http://www.mindspring.com/~otterson/pgp.htm