[time-nuts] Metastability (was: Frequency Stability of Trimble Mini-T)

[BriMDavis at aol.com]

Hal wrote:
>
>Back in those days, there was a lot of activity in designing kludgey 
circuits 
>to "fix" metastability.  I could usually find the flaw.  It got boring after 
>a while.  The classic was a circuit to detect metastability and reset the 
FF. 
>That reset signal would sometimes have runt pulses.
>

My favorite online metastability page:

_http://www.fpga-faq.com/FAQ_Pages/0017_Tell_me_about_metastables.htm_ 
(http://www.fpga-faq.com/FAQ_Pages/0017_Tell_me_about_metastables.htm) 

This page discusses a few of those "fixes" and why they don't work,
along with some pointers to the relevant literature.

There are also some nice real-time scope shots showing
some of the various runt pulses that can occur:
"
"  _http://www.fpga-faq.com/Images/meta_pic_1.jpg_ 
(http://www.fpga-faq.com/Images/meta_pic_1.jpg)  
"  _http://www.fpga-faq.com/Images/meta_pic_2.jpg_ 
(http://www.fpga-faq.com/Images/meta_pic_2.jpg) 
"  _http://www.fpga-faq.com/Images/meta_pic_3.jpg_ 
(http://www.fpga-faq.com/Images/meta_pic_3.jpg) 
"
"These are far from just delayed outputs! The end result though
"is still the same, systems that fail. But seeing these scope
"pictures of the actual Q output might make you think about how
"you measure metastability.
"

Brian
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