netlist changes during simulation

Hello.
Hello,
Im trying to simulate an adder circuit which produces an output
depending on an 'input offset'. That is the offset gets added to a
temp variable and whenever the addition results in an overflow it
should produce an output.

The adder is a 4 bit adder and so whener the sum exceeds 1111 it shud
produce a carry out and the resulting sum shud substract 16 and
produce a new sum which would then be smaller than 1111. I hope u get
the point.

I made schematic for the circuit which consists of a simple 4 bit
Ripple carry adder and a register driven by the clock because I want
the addition to take place with the clock. The output of the adder
feeds the register and the reg op feeds one of the inputs to the
adder thus creating a loop structure.
After doing that I create a symbol out of it and use it in a TB to see
the behavior.

After certain simulation trials of the Test bench i get this strange
error in the spectre.out file which says that "I11 instance is an
instance of an undefined model subcircuit" . Here 'I11' refers to the
instance name of the symbol I created earlier.

This strange error disappears when i make an exact copy of the same
Test Bench schematic into another schematic. And again after a while
the same error pops up and then i recreate another schematic and then
it works..which is becoming kind of annoying. Does anyone have any
pointers as to why is it happening?

Hope to get some hints about that.

Best regards,
Aijaz Baig.

 

The same question (also posted on the Designer's Guide site) is being answered
on www.cdnusers.org - see
http://www.cdnusers.org/Forums/tabid/52/forumid/59/postid/3672/view/topic/Default.aspx

Andrew.