[time-nuts] Metastability (was Brooks Shera)

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 25 19:05:25 EDT 2013

Tom Van Baak wrote:
>> Both edges of the 24MHz clock gating pulse are asynchronous with respect
>> to the signal being gated.
>> Metastability can result with clock pulse widths that lie within a
>> critical range.
>> Bruce
> I don't disagree with your statement above, but my question was -- does it matter in a GPSDO; does it matter in this GPSDO?
> Occasionally missing a 24 MHz tick is a not a worry (all gated frequency counters share this "feature"). A one-count ambiguity is normal and expected, even welcome. Note also that the PIC will see only 0 or 1; there is no metastability in software. So where exactly is the problem?
Agilent go to a lot of trouble to add synchronisers (and sometimes clock 
jitter eg HP5345A) to their gated frequency counters to ensure that the 
average measured frequency is unbiased.
For more detail see:
http://www.hpmemory.org/an/pdf/an_200.pdf                            pp 
pp 12-15

The average value of the counter output when sampled is important in a 
Metastability can result in an output that oscillates or dwells at a 
level that is neither 1 nor 0.
How the PIC reacts to such inputs is unpredictable although on an 
interrupt input, at least, its likely to use a synchroniser.

> For educational purposes if nothing else, I'm looking for a precise description of the scenario (at the picosecond level if necessary) that reveals the flaw in his board. I'm not saying there isn't; I'm just saying I'd like to see it explained. Either his design was accidentally or intentionally clever, or there is in fact a minor fault. However, if there is a flaw, we also need to explain why in 15 years no one has reported glitches in their Shera boards.
Probably because those that built them either didn't have the necessary 
equipment or were unaware of the potential problem.
> I sort of understand metastability, but just adding more hardware to reduce it doesn't seem to be the only way to deal with the issue in a GPSDO.
> Said -- how do you handle this in your Fury design?
> /tvb
> ____________________________

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