> I noticed the PIC DTMF generator circuit (via link) suggested by Scott
uses
> a 16F84. =


It probably dates from a time when the 16F84 was the best available chip
for the job.  Nowadays, there is basically nothing for which the '84 is t=
he
ideal chip: anything it could do, can be done cheaper with some more rece=
nt
PIC.

> However, other respondents mentioned the 18xxx series. Am I
> correct in assuming that the differing amplitudes of each SW necessitat=
e
> the hardware multiplier of the latter micro? =


Doing the amplitude adjustment in software implies a multiplication or
division operation.  This can certainly be done in software, but that wou=
ld
be a lot slower than the 1-cycle 8x8 bit hardware multiply of the 18's. =

With a 100 Hz max frequency, doing a software multiply is probably
feasable; if you needed 10 KHz it proabably wouldn't be...

> I am a bit concerned as have
> heard these are more complicated to program. =


Hmm... 18F's have less available sample code, and lots more configuration=

options to set (although MPLAB comes with templates for each chip, so you=

don't have to figure out the options from scratch).  On the other hand, y=
ou
can largely ignore some of the main problem areas of lesser PICs, such as=

RAM banking and ROM paging.  The main strike against 18F's, as I see it, =
is
that after getting used to them you may not be able to stand the thought =
of
programming for lesser PICs - and there are still some attributes that ar=
e
only available in 12x or 16x parts (such as low cost, and packages smalle=
r
than 18 pins).

> Is there a way to do this with
> the 16F84? =


The main problem with the '84 for your application is that it doesn't hav=
e
a PWM module.  Any software PWM inherently has less resolution, and takes=

more time away from the rest of your code.

> An off-list reply suggested a special PIC designed for motor control
which
> has 3 PWM outputs. The differing amplitudes could then be trimmed
> externally. Is this a much simpler approach for a beginner?

This is certainly a workable approach to your problem - I believe the
18Fxx31 motor control PICs are the only ones with 3 hardware PWMs in a le=
ss
than 68 pin package.  You'll probably find much less available sample cod=
e
for the motor control PWMs (which are quite different from the standard P=
IC
PWM), and you'll have to wade through quite a bit of motor control-relate=
d
features that are completely irrelevant to your problem (fault detection,=

deadbands, etc.).

Note that you could do external adjustment of individual amplitudes even =
if
using a single PWM for output: connect your trimmer potentiometers to
analog inputs of the PIC, and use the ADC readings as the amplitude
multipliers.
        Jason Harper

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