Hi James,

Interesting project. I had a quick look at the 16F1503 for an audio
project myself, which would have included sweeping, or slewing /
portamento from one note to another

Note that as the NCO uses a timer, the method to use it will work
for any timer. And being digital (the base unit is Tcy), there are
certain limitations with resolution, which put me off it a little for my
application.

I agree that interrupts and look-up tables could be used to simplify
the coding. There is some groundwork needed to generate the data
but after that it's fairly straightforward. Even without NCO (such as
the F84) the pins can be alternated with BSF/BCF at the end of
each interrupt half-cycle

So, you'd have an interrupt for when to reload the timer, one table
for the reloads, one for the sweep period, (the number of interrupts),
possibly one for the initial load but that could be done via a goto
and the DIP switch

If you choose a short interrupt period (eg 100us) you'll have finer
control at the Hz level but it does mean a lot of changes (big tables)

A coarse interrupt time, (eg 20ms) may cause the tone sweep to
be noticeably made of individual notes. At each interrupt, the reload
value for the timer used to toggle the outputs pins is decremented.
That is, the timer period becomes shorter -> higher frequency

Say you decide on 5ms, which will be increments of 72Hz from
500Hz to 2660Hz in 150ms, assuming you want a linear output
curve rather than perhaps exponential

So if the PIC is running with Fosc of 16MHz, Tcy =3D 250ns, the initial
load would be 4000000/500/2 =3D 4000 for 500Hz.

ie 4000 Tcy on (1000us) and 4000 Tcy off (1000us) -> 2ms period

To change from 500Hz to 572Hz at the first interrupt, the load changes
from 4000 to 3496. The on/off time is now 1144us each

For the next change, to 644Hz, the reload is 3105. Notice that the
difference between reloads has gone from 4000-3496 (504) to
3496-3105 (391). The last change, 2588Hz to 2660Hz, is from 772
to 751. So doing it this way you'd need to work out each reload and
put it in a table. You could do it by calculation

((Fop / frequency1) - (Fop / frequency2)) /2

There's something for you to think about for now, particularly what
frequency stepping incements are noticeable. I think that is going to
be an important factor. We'd all be glad to help you out with the
assembly code

Joe


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