I was running the Verilog-A logic gates from:
http://www.designers-guide.org/VerilogAMS/functional-blocks/gates/gates.va

in a commercial simulator, and I got this warning:
   VERILOG-A PERFORMANCE WARNING:
       "gates.va", line 125: The first argument to the transition filter is a
       continuous signal, which might slow down the simulation.
       Further occurrences of this warning will be suppressed.

If I change, for example,
   V(out) <+ transition( !((V(in1) > vth) && (V(in2) > vth)) ? vh : vl, td, tt );
to
   vout = ( !((V(in1) > vth) && (V(in2) > vth))) ? vh : vl;
   V(out) <+ transition( vout, td, tt );

then I don't get the warning, and the simulation runs 40x faster!

Hmm, spectre doesn't seem to be that clever about figuring this out. Even just reformulating the same expression as:

Code:

   vout = ( !((V(in1) > vth) && (V(in2) > vth))) ? vh : vl;
   V(out) <+ transition( vout, td, tt ); 

stops the warning. I'll file a CCR to see if we can improve it.

Regards,

Andrew.

Dan,

No, but I didn't do any significant tests - mine was just to see if it compiled rather than giving it anything sensible to simulate.

Perhaps Geoffrey can comment - I'm sure he'll take this up with Cadence  ;)

Regards,

Andrew.

I think the issue is, if you were to do

V(out) <+ transition( V(in1), td, tt);

then every time V(in1) changed even the slightest, the simulator would have to schedule a timepoint at t+td to propagate this change to the output (and further timepoints up to t+td+tt to finish the transition).

I guess the simulator sees the V(in1) in the original argument and schedules these timepoints, even at timepoints where the Boolean expression isn't changing.

(sorry for resurrecting an old thread, but it seems to me the appropriate place to ask...)

I've run into this warning ("The first argument to the transition filter is a continuous signal, which might slow down the simulation") while using a simple Sample-and-Hold implemented as follows:

 parameter real Vt = 0.5 from (-inf:inf);
 
 parameter real Td = 0    from [0:inf);
 parameter real Tr = 100p from (0:inf);
 parameter real Tf = 100p from (0:inf);
 
 
 real vout_val;
 analog begin
   @(cross(V(vclk) - Vt, 1))   vout_val = V(vin);
   V(vout) <+ transition( vout_val, Td, Tr, Tf );
 end

Seems that Spectre refers to a real value as to a value that can change continuously (although it doesn't), and issues the warning.

I don't see any way to implement it without using a real value, or without using the "transition" filter (removing it will cause too abrupt changes at the output).

Any suggestions?

Thanx,
Dmitry.

P.S. I'm running MMSim 10.1

Hi,

I also have the same problem. The block is basically a sample and hold. The output signal is not continuous time, despite the warning.
I am not sure about the correct method of speed test. However, it seems to me the simulation time increases specially when it is used with other blocks.

Ali