On Fri, 13 Mar 1998 01:05:43 -0500 Gennady Palitsky
<gennadyp@MAINLINK.NET> writes:
>I wonder if anybody tried to synchronize PIC to video signal.
[...]
> Nevertheless vertical line on a screen have jitters of
>about one
>cycle size (~280 nS). According to datasheet  (I am using 16C71)
>interrupt
>latency = 3-4 cycles depending on where interrupt occurred during an
>instruction cycle time. It looks like this +/- 1 cycle gives jitters
>observed.

Yes indeed it does.  The problem is that the PIC can't resolve events of
smaller than one instruction clock (xtal/4), and that your crystal (even
if it's a very good one) is never exactly the same frequency as the
incoming video.  14.318 MHz is a bad choice anyway, as it makes the PIC
instruction rate 3.579545 MHz, or 227.5 PIC instructions per NTSC line.
So the PIC will always be 1/2 cycle off of the video.

Several people have used RC or LC oscillators in circuits that stop them
before each line.  The horizontal sync pulse from the video signal is
used to restart the oscillator, always in consistent phase with the
video.

I've had good results with a continuously running LC VCO as the PIC
oscillator, and phase locked it to the video.  You should be able to find
a detailed post about that in the archive.


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