[ntp:questions] Is 24PPM an Excessive Real-Time Clock Correction?

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Thu Jul 12 19:08:37 UTC 2007


>On a related note, I've read the following:
>http://en.wikipedia.org/wiki/Crystal_oscillator
>
>"Care must be taken to use only one crystal oscillator source when 
>designing circuits to avoid subtle failure modes of metastability in 
>electronics. If this is not possible, the number of distinct crystal 
>oscillators, PLLs, and their associated clock domains should be 
>rigorously minimized, through techniques such as using a subdivision of 
>an existing clock instead of a new crystal source. Each new distinct 
>crystal source needs to be rigorously justified, since each one 
>introduces new, difficult to debug probabilistic failure modes, due to 
>multiple crystal interactions, into equipment."

It's a standard problem in digital design.

It happens all over the place, mostly in the I side of IO devices.
The RTC is an IO device, just like the keyboard or an ethernet.


-- 
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