Andreas wrote, on 04/29/08 16:27:
> Hello,
>
> I've got a synthesized verilog netlists containing statements, which
> connect signals to logic high or low, e. g.
>
> assign xy = 1'b0; //The net xy is tied to gnd (logic low).
>
> The problem is that I have no tiehi or tielo cells in my synthesis
> libraries.
>
> After im****ting the verilog netlist as a Virtuoso schematic into CDE,
> these verilog assignments are replaced. Sometimes they're replaced by a
> "cds_thru" cell from the "basic" library and sometimes by "short" cell
> from the designkit. I don't understand why two different cells are used.
>
> The real Problem occurs, when I want to do a LVS. The LVS tool tells me
> that it can not find the "cds_thru" or "short" cells in the layout. LVS
> results incorrect, although the layout is correct.
>
> What can I do?
>
> I already searched for an lvsIgnore property, but the "cds_thru" and
> "short" cells don't have one.
>
> Thanks for your help,
>
> Andreas
>
The "short" (I assume that's equivalent to the "patch" component in the
basic
library) effectively creates an alias in the database, which says that two
net
names are equivalent. However, aliases can only be created when one of the
nets
is internal (i.e. if both are terminals, or both globals, or one terminal
and
one global, it can't do it). In that case, a "cds_thru" is used. cds_thru
is
something I proposed a few years ago to solve precisely this problem - how
to
create a "through" connection - which is perfectly possible in Verilog,
but not
in the CDB database. What cds_thru does is get netlisted as an assign in
verilog, an "iprobe" (i.e. a zero-volt source) in spectre, a zero-volt
source in
hspice, a "small" resistor in CDL (which can be filtered out in Physical
verification tools such as Dracula, Assura and Calibre), and so on. For
Diva and
Assura using the auLvs view, you can add a removeDevice() call in your
LVS
rules (I think it's filterDevice() for Assura) which shorts the device
with a
particular name.
VirtuosoXL also knows how to treat this pseudo-device as a short.
The "patch" device shouldn't need any special handling though, as this
should be
taken care of during netlisting - it will just map to use one of the
aliased
names everywhere.
Regards,
Andrew.
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