Re: gEDA-user: one hot state machines

[John Sheahan <jrsheahan@optushome.com.au>]

Karel Kulhavý wrote:

> > What does a default: statement do?
> > What happens with no reset?
> > what happens with races or cosmic belches?
> >    
> What are races and cosmic belches? Is it something like metastable states?
>
>  
bad things happen.
perhaps an asynchronous reset goes away at the wrong time.
Perhaps a solar event makes too many high energy particles that change a 
flipflop state.
Perhaps there is a setup problem because a PLL somewhere making the 
clock is not
yet stable, running a touch quickly.
Pehaps a test mode that is false pathed is exited.
Not a metastable - just a condition that is not supposed to happen.

either way - a one-hot state machine might end up with 2 flops (or no 
flops) set.
I'm unconvinced fpga state machine synthesis is robust in this area.
(what does the default clause really do?)

Or maybe I'm just Robinson Crusoe on this.

john





> Cl<
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> > john
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