Dear PICers,

Combining Ray Gardiner's ideas with Bob Ammerman's clever ideas, we have a
frequency-locked-loop (FLL).

Basic idea: Count both input pulses and output pulses. Try to force both to be
equal.

[tangential rambling]
I've been told there used to be a guy at electric power stations who watched 2
clocks.
The "reference" clock always told the exact time ( traceable to the National
Bureau of Standards, which has since been renamed NIST).
The other was a simple, cheap electric clock running off the power lines from
his station. It would run fast or slow when the power grid ran above or below
the precisely 60 Hz.
The guy would tweak the generators at his plant to try to get both clocks to
read the same time (more or less).
[/tangential rambling]

 * Output pulse generator, generate a pulse every P (variable period) cycles.
 * Input pulse counter: every input pulse, add Ki (constant) to a 24 bit error
counter.
 * Output pulse counter: every output pulse, subtract Ko (constant) from that 24
bit error counter.
 * Hope the counter doesn't overflow :-).

Then periodically tweak P:
 * When error counter is negative (too many output pulses), make P bigger (slow
down output frequency).
 * When error counter is positive (too many input pulses), make P shorter (speed
up output frequency).

You can have some complicated PID method of adjusting P if you want. Or do
something simple like increment/decrement P once every 8 input pulses.

This forces the output frequency (at steady state) to converge on
  output frequency = Ki/Ko * input frequency.
.

Advantages:
* no need to measure periods with any kind of accuracy. Just count pulses.
* Jitter in input pulses is completely ignored.
* If the output frequency uses a hardware timer, it has very low jitter, and can
change frequency very smoothly.
Disadvantages:
* Initial output frequency when you first turn it on might be *way* off.
* After a step change in input frequency, might take a long time to settle.
* After a step change in input frequency, might cause output frequency to
overshoot, maybe even oscillate, if the "tweak P" code isn't properly tuned.

If you *know* a step change in input frequency just occurred, you might be able
jump immediately to almost exactly the correct output frequency by directly
forcing period P to the (estimated) new frequency. Then let it slowly converge.

--
David Cary

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