I had a quick look at the IEEE-1355 HIC bus on which spacewire is based and it seems that although the clock is not on the wire, it can be reconstructed as an XOR of the strobe and data. So a passthrough connector sampling those lines (differential) with RS-644 receivers and a quad NAND may be all you need to recover it.

Le 06/02/2011 07:14, Hal Murray a écrit :

I've got a system at work with an internal clock oscillator that I want  to
get some statistics on, but there's no direct visibility for the
oscillator, nor do I have a convenient test point that I can probe.
...


Fun problem.  Thanks for tossing it out.

I see two approaches.  Are there others?

One is to generate something like a PPS pulse and capture timestamps.  Then
crunch the data and hope you see N buckets so you can ignore anything that
isn't in bucket 0.  (or correct them by shifting by N ticks)

The other approach would be to measure the time between pairs of pulses.  You
can probably do that much faster than once per second.  This should give you
2*N buckets.

I can't quite figure out how far apart the pulses should be for best results.
  (It will probably be simple after I see it.)  I expect it will depend on the
ADEV of the measuring system and the ADEV of the clock you are trying to
measure.

I assume you can get a rough ADEV of the clock you want to measure by
measuring a similar part on a typical lab setup.

It's probably worth sanity checking the divide step to make sure it's
dividing by M rather than M-1 or M+1.  (Digital geeks are often off by one,
especially if nobody has checked carefully.)  I'm not sure how to do that.
Probably something like divide by 2*M and see if it matches.  Or divide by a
small M and measure the frequency.

-----------

Plan B would be to use an inconvenient test point. (or make one)

Years ago, my boss gave a neat talk about how to prototype hardware.  Step 0
was hire a good technician.  :)






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