> On Fri, 22 Mar 2002, Bob Ammerman wrote:
>
> > If you only need to generate the fixed frequency of 625Hz, you could
easily
> > do it with a 12C509A by using a single bit highly oversampled output.
> >
> > Use a 4MHz crystal to get 1,000,000 instructions per second. This
> > corresponds to 1600 instructions for each cycle of the 625Hz output.
> >
> > You would compute the ideal oversampled output waveform as a long string
of
> > zeros and ones using a program on a PC. (Anybody have any pointers to
how to
> > do this part?).
>
> I discuss it briefly here. Maybe you didn't see the post the other day.
>
> http://www.dattalo.com/technical/theory/sqwave.html

Yeah, I took a look at that, but didn't see how I got the 'best' pulse
train.

How about this idea: Do a simple delta encoding by treating the output as a
simple 1=up, 0=down kind of thing and comparing the computed value of the
integral to the correct value of the sign at each step to determine the next
output bit. The maximum slope occurs at the zero crossings, so at those
points we would expect to see a sequence of repeated ones or zeros.

> > Here is a puzzle/challenge:
> >
> > How can you reduce the program memory requirements by a factor of
> > approximately 2?

My thought here is that the waveform is symetrical. So if we replace all the
BCF GPIO,0 and BSF GPIO,0 instructions with XORWF GPIO,F and make sure that
W always contains a one, then our loop can encode only half the waveform, as
long as it contains an odd number of XORWF GPIO,F instructions.

Bob Ammerman
RAm Systems

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