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Larry wrote
> Barry and all,
>    In all this discussion of multiple clocks and 
> unclocked logic, I think this is the first mention
> of metastability.
>    That has always worried me, in all computer 
> design including occam and CSP. Transforming a
> smooth (time) function into a step function, and
> deciding any race... Is it true there is always
> a non-vanishing chance of failure?
>    Larry

I have worked with asynchronous circuit for many years and operated them in
a mode where there is a high probability of repeated occurrence of a
metastable state.  The most important issue is how the circuit recovers from
a metastable state.  With modern ASIC and FPGA devices this is usually not a
problem.  ALTERA FPGAs will recover from a metastable state within 2ns.  I
have run tests for many hours of continuous operation without error which
would indicate that the above figure is correct.
Unfortunately you cannot state that there will never be a failure due to
metastability only that the probability is very small and less then other
modes of failure.


Prof. Brian C. O'Neill        | Tel: +44 0115 848 6044
Dept of Elec & Electronic Eng | Fax: +44 0115 848 6567
Nottingham Trent University   | E-mail: Brian.ONeill@xxxxxxxxx
Nottingham                    | http://eee.ntu.ac.uk/research/parallel
NG1 4BU                       |