edaorg@v-ms.com wrote: > Ken Kundert wrote: >> All, >> One of the original reasons why we went with the contribution >> operator with its accumulating nature as opposed to a equality operator >> when the language was first designed was that it neatly addressed the >> question of "what happens if you apply it more than once to the same >> branch". There are two choices if you use an equality operator: >> 1. it is an error >> 2. one wins >> Neither seemed like a very good alternative, but in the case of OOMRs in >> particular it was decided that neither was desirable. Consider each case: >> 1. if it is an error to give a value to a branch more than once, then >> once a branch is given a value in a module, then it could never be given >> a value via an OOMR. Since it is likely that any branch that one might >> want to give a value to via an OOMR would already have a value, this >> seemed like too big a restriction. >> 2. if only one wins when two values are given to a branch, how is it >> decided which value wins, particularly if both values are being given >> via OOMRs. >> > That's what the solver is for, it's not some kind of race condition. The situation is ambiguous. There are two equations when there should only be one. With the accumulating nature of the contribution operator we define how they should be combined. If we remove the accumulating nature, then we must define some other way for the ambiguity to be resolved. >> Since the accumulating nature of the contribution operator was added to >> the language specifically to support OOMRs, it seems odd that we are now >> considering stripping this feature, but only for OOMRs. Has there been >> some issue with the contribution operator and OOMRs? I have heard people >> voice an opinion that they would prefer that it be changed, but nobody >> has pointed out any real problems with the way it works now or what >> significant new capability would be gained if a change were made.assignments > [Don't recall the accumulating nature of the operator having much to do > with OOMRs myself, I suspect the OOMR stuff was pretty much an > afterthought.] > > The current behavior is inconsistent in that if you wire something up > with OOMRs vs using ports then you can get different behavior. To > someone with a good understanding of electronics and digital Verilog > it's not at all obvious that an OOMR potential contribution will add in > series with other potential contributions to a branch - the obvious > behavior would be that it was in parallel (as with using ports). I am at a loss to understand why it is a problem that contributions do not behave the same as port connections, especially since doing so would make OOMR contributions inconsistent with in-module contributions. > IMO the easiest way out of the problem is to deprecate "<+" as an > operator for OOMRs, and use a new operator which is explicitly indicates > that the contribution is in series or parallel (e.g. <++,<=). Again, we seem to be proposing changes for the sake of change. Is there a documented problem we are trying to solve? -Ken > Kev. > >> -Ken >> >> Kevin Cameron wrote: >> >>> To back up a bit in the discussion, I'd agree "<+" is a bit odd, but >>> it's not that dissimilar to "+=" in C. The only objection I had was to >>> extending the summing semantic to OOMRs and between analog blocks for >>> potentials, which is why I suggested using "<++" to make that case >>> explicit. Likewise I would have no problem with introducing a >>> non-summing operator (say "<=") if that makes it clearer to users what >>> is going on, that would then probably require another operator to match >>> the "<++" (say "<=+") for symmetry. Given that modelers then use "<=" >>> instead of "<+", why would they expect any summing of potential >>> contributions for OOMRs or between blocks when using "<="? >>> >>> I think re-using operators from other languages would be a bad idea >>> unless they do exactly the same thing. >>> >>> Kev. >>> >>> Geoffrey.Coram wrote: >>> >>>> Marq - >>>> In most cases, though, the distinction doesn't matter: >>>> if you don't expect the contributions to accumulate, >>>> then you write your model with a single contrib and >>>> everything works fine; or you have multiple contribs >>>> of complicated expressions, and for efficienct, you >>>> don't want to compute the complicated expressions that >>>> you don't need. It'd be odd to have >>>> >>>> I(br) <+ (some complicated expression); >>>> if (off) >>>> I(br) <+ 0; //does not set branch current to zero! >>>> >>>> If it's off (the idea is that the user wanted to turn >>>> the current off), it seems that the user would have >>>> wanted to bypass the complicated expression: >>>> if (off) >>>> I(br) <+ 0; >>>> else >>>> I(br) <+ (some complicated expression); >>>> >>>> >>>> The curious nature of the contrib is mentioned explicitly >>>> in my tutorials for writing Verilog-A compact models ... >>>> >>>> -Geoffrey >>>> >>>> >>>> >>>> Marq Kole wrote: >>>> >>>> >>>>> All, >>>>> >>>>> I would even say that most users starting to write models for >>>>> Verilog-AMS are quite unaware of this. They write their models as >>>>> though it were a simple assignment - only in rare cases or when they >>>>> start to work on more complicated models will they start looking at >>>>> the actual LRM text and discover the contribution behavior. The same >>>>> goes for the implicit equations for that matter... >>>>> >>>>> Just my $0.02. >>>>> >>>>> >>>>> >>>> >>>> >>> >> >> > > > ------------------------------------------------------------------------ > > http://www.grfx.com <http://www.grfx.com>mailto:dkc_ f rom _grfx.com <mailto:dkc_ f rom _grfx.com> > -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.
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