Tom & Listers,

Thankyou for your interest in my DTMF dialling and V23 routines.

My initial brief was to get a 16C54 to work in an autodialler application,
and so the code is perhaps not as efficient as it could be - having to cope
with the lack of page 2 table reads on the C54, and other C54 limitations.
I only had a $3 component budget to work with, so it had to be kept simple.

I use the whole of the 8 bit B port to drive an R-2R resistor ladder network
as this costs about 2 cents. It allows a simple connection to the telephone
line using a single NPN transistor acting as an amplifying gyrator, and 2
high voltage transistors to form a hook switch. The whole lot was put on a
board and built for about $5.

The gyrator is key to the design, it is a single transistor circuit which
simulates an inductor ie it blocks ac from flowing but passes dc. They are
very useful in telecoms for powering equipment from the line - just 1
transistor and 4 passive components.

The routine for DTMF and tone generation started out as the one published by
Microchip in their application note. However I found that this was very poor
at the higher tone frequencies and just would not get through the telecom
approvals harmonics tests.

I went back to basics and rewrote it using aan algorithm with 2 moving
pointers which integer step their way through a 128 sample sinusoidal half
wave look-up table.  By varying the step size of the pointers it is possible
to generate the frequencies very accurately - within the +/- 1.5% required
for telecom DTMF.

The V23 routine also arose from this work and will send a V23 message to
line having dialled out using the DTMF routine.

I am now working on another routine to demodulate V23 tones  - again just
using a PIC16F84A  (minimum platform) and its meagre resources.  If I can
get this to work, a 16F84A could be used as a complete telecom modem for
simple projects requiring a telephone line connection.

This offers the possibilities of getting a WWW micro web server - based on
Dennis Petrov's implementation  and modem onto a cheap device like the
16F628.

If you use a 16F628 you can send the DTMF samples to the PWM stage and save
all the B port lines for driving additional hardware.

You also have the means of putting a real time clock on a F628, and allowing
512 bytes for the telecoms routines, you have 3 times the application
codespace than was available on the F84.

If anyone has any good idea on V23 demodulation - I would be pleased to hear
them.


regards,


Ken Boak


You can follow my work, and access all the code listings and schematics by
subscribing to the Rat_Ring discusssion group.

www.geocities.com/kenboak/rat_ring.html

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