RE: [sv-bc] Mantis 1940

I filed this as Mantis 2071.

Shalom
 

> -----Original Message-----
> From: Steven Sharp [mailto:sharp@cadence.com] 
> Sent: Tuesday, October 02, 2007 9:34 PM
> To: Bresticker, Shalom; sv-bc@eda.org; doug_warmke@mentor.com
> Subject: RE: [sv-bc] Mantis 1940
> 
> >From: "Warmke, Doug" <doug_warmke@mentor.com>
> 
> >If no driver is connected
> >to a net, its value shall be high-impedance (z) unless the net is a 
> >trireg, in which case it shall hold the previously driven value.
> >
> > 
> >
> >The last sentence is a little bit troublesome.
> 
> This sentence has always been an oversimplified description 
> of a trireg.
> It describes the result that is usually observed, but not 
> always.  That may be sufficient in this section, but there 
> does need to be a full specification of how a trireg works in 
> the section on triregs.  The description that is there is 
> also inadequate.
> 
> The description of a trireg as behaving in two distinct ways 
> (depending on whether it is driven) is wrong.  It appears to 
> be true if the trireg is driven with a strength higher than 
> its capacitive strength, but is untrue if driven with the 
> same or weaker strength.  For example, a trireg that is being 
> driven, but with a drive strength that is weaker than the 
> trireg capacitive strength, will not have the driven value.  
> It will hold the old value.
> 
> The way a trireg actually works is this:  It uses the normal 
> resolution for a multi-driver wire, but with an extra input 
> that is the old output logical value with the declared trireg 
> capacitive strength.  It is that simple.  All of its 
> behaviors can be derived from that (except the decay 
> behavior, which adds some extra complexity).
> 
> Of course, that description is likely to be confusing to a 
> typical user, who may have trouble deriving the trireg 
> behavior from it.  Since the LRM came from a user guide, it 
> is not surprising that it contains a simplified description 
> of the usual visible behavior instead.  But since 
> implementators need to know what the actual behavior is, it 
> would be good if the LRM specified it.
> 
> 
> Steven Sharp
> sharp@cadence.com
> 
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