On 14/07/2011 11:30, RussellMc wrote:
> Determine period of input waveform.
>
> 1. You can gross-synchronise by just starting your sine wave on its
> rising edge at the negative to positive zero crossing. I think you
> already know that.
>
> 2. If you have a synthesised  sinewave already being output and you
> want to bring it into synchronisation with an external waveform of the
> same frequency then you MUST slip frequency either + or - in order to
> change phase. eg if you are say zero crossing 30 degrees too late you
> need to progressively advance  each of your zero crossings by a
> suitably small amount until you catch up. eg you could decide that an
> OK rate was 1 degree per cycle. By effectively speeding up to 360/359
> x 50 Hz =3D 19,944 uS/cycle instead of 20,000 uS you will catch up in 30
> cycles +` 0.6 seconds. How fast is acceptable is up to you.
>
> 3. Synchronising with phase offset is just a matter [tm] of
> determining where the desired phase point is wrt the input zero
> crossings and steering for that as above.
>
> 4.  To maintain running synchronisation you can monitor input zero
> crossings wrt output zero crossings and make decisions about changing
> output sampling rate as above. Simplest is just add or subtract an
> extra delta count to compensate for observed error. Nicer is to
> introduce correction which takes several cycles to correct. You could
> end up at something like full PID and also introduce input frequency
> filtering in a particularly obnoxious environment but this is unlikely
> to be needed.
>
>
>            Russell McMahon

Excellent stuff. Better than I can say it.

And you only need 1/4 of a sinewave table (90 degrees)  for 360 degrees,=20
and any frequency and phase, even a number output at once is generated=20
from the one table.

The PIC16F887 is fine to start will. If using JAL or C to program, you=20
can move trivially to cheaper and better 18F4550 which has option of USB=20
slave port and is up to 48MHz internal clock from a wide range of=20
external crystals, instead of clock =3D crystal on 16F887 and max of 20MHz.

Unless you are (a) already conversant with C, or (b) absolutely want to=20
use C, try JAL. It's specially designed for 10F/12F/16F/18F family and=20
embedded Programming.


--=20
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