[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 
27-28
<http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1974-06.pdf>http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1974-06.pdf  
pp 12-15

The average value of the counter output when sampled is important in a 
GPSDO.
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
>
>
> ____________________________
Bruce



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