[time-nuts] time-nuts Digest, Vol 56, Issue 71
magnus at rubidium.dyndns.org
Mon Mar 30 23:51:41 UTC 2009
Tom Van Baak skrev:
>> Probably the higher jitter and periodic phase modulation due to
>> simultaneous switching of multiple outputs at different frequencies.
>> The magnitude of the latter will depend on the loads driven by each output.
>> The cure is to use an external flipflop to resynchronise the outputs to
>> the 10Mhz clock.
> Kit, Bruce,
> There was no phase modulation effect that I could measure.
> Note that in that design all pins (a single 8-but IO port) are
> re-written each time through the loop; not just ones that change.
> See the source code for details.
> My understanding of the PIC architecture is that all outputs
> are essentially "resynchronized" to the clock by design. So
> that's why the PIC divider works so well. I can't see how an
> external off-chip flip-flop would be better than the existing
> internal on-chip flip-flop. Might even make things worse?
The PIC as any other modern synchronous design has a clock distribution
for which all DFFs is tied. The output DFFs is not different in this
context. The delay of the clock distribution tree changes with it's
environmentals just as anything else. Temperature and power voltage is
obvious, but more importantly in this case is also neighbouring signals
and their levels and shifts will shift delays. Just as with any other
synchronous CMOS design.
You can make things worse, but you could most probably also make things
better. It can just be hard to measure it. If it is hard to measure it,
then maybe it is not so relevant or at least not the first ugly fish to
The PIC is a handy little tool to step state with at least. Beats a
bunch of 74xx90 and an output 74xx74 in size at least.
> But I don't know for sure and should not guess. In cases like
> this I'd take an actual test over a random guess.
> As for jitter, I tested the PIC divider when I wrote it ten years
> ago and if I recall correctly the jitter was just over what I could
> measure with a SR620; about 25 ps. With better equipment
> these days, one could measure how much of that is input jitter,
> or output jitter, or measurement system jitter. But I don't have
> anything better than a 5370 or 620 for 1PPS measurements.
> I know the PIC divider was an order of magnitude better than
> other discrete 1PPS dividers that I had at the time, and it was
> 100x better than the reference 1PPS out of any GPS boards
> that I had, so I was very pleased with the performance (and
> the simplicity, and the cost) of the one-chip divider concept.
Performance per buck was certainly rewarding, just as performance per
square cm board.
> But it would be very interesting to me if someone with a working
> Wavecrest could make measurements of various PIC dividers
> and refine this old data; to find out just how low the noise floor is.
I just lack the PIC dividers... I have the toys... :)
Infact my latest toy sitting here on the desk would get its first real
assignment that way. :)
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