The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Simulators >> AMS Simulators >> AMS Simulations Running Slow
https://designers-guide.org/forum/YaBB.pl?num=1256230640

Message started by QuantumSimulator on Oct 22^nd, 2009, 9:57am

Title: AMS Simulations Running Slow
Post by QuantumSimulator on Oct 22^nd, 2009, 9:57am

Hello All. I am running topLevel AMS simulations and the simulator is running extremely slow. Basically, simulations that took 11 hrs now take 130 hrs!. Sim steps get smaller and smaller or they remain the same 20.11ps, 20.11ps .............20.11ps. I tried to find anything oscillating and I could find none. I relaxed torelerances and this helped one sim but not the second sim. Any ideas I can try.....?

Thanks in advance
QuantumSimulator

Title: Re: AMS Simulations Running Slow
Post by Riad KACED on Oct 22^nd, 2009, 11:36am

Hi there,

What I tend to suspect in this kind of cases is a verilog-AMS block with an always statement without any sensitivity list. something like

Code:

always
begin
out=1.0-in;
end

Now if I add 'in' in the senstivity list, then the code gets aveluated only when 'in' changes.

Code:

always @(in)
begin
out=1.0-in;
end

This was just a guess.
AMS Designer comes with utility called 'NC Profiler'. You can invoke this tool by adding the -profile option to the ncsim command line. If you are using AMS from ADE, then you would be able to setup the profile from ADE -> Simulation -> Options -> AMS Simulator and then turn on the 'Generate runtime profile' switch. You then need to re-run and exit the simulator to get the profiler resulted dumped into the ncprof.out file.

Hope this helps !
Cheers,
Riad.

Title: Re: AMS Simulations Running Slow
Post by QuantumSimulator on Oct 24^th, 2009, 7:39pm

Thanks Raid for the suggestion. I could not find "always" statement without @.

Title: Re: AMS Simulations Running Slow
Post by Riad KACED on Oct 25^th, 2009, 4:18am

Hi,

Did you look at the profiler ?
Does your simulation spend time in the digital or Analog solver ?

When I get slow simulation, I usually look at the inserted connect modules as well. Using either the ncelab -iereport option (available from ADE as a switch) or using the TCL debug from ncsim:
> scope -aicm -rec -hier.
The ierport gives a good summary at the end.

You case is a bit odd though because your AMS simulation is even slower than your Spice level simulation, isn't it ?

This may be wither:

1. A digital statement hanging around
2. A convergence issue with Verilog-A blocs.
3. A bug somewhere.

What I usually do to narrow down this kind of issues is:

1. I run AMS Designer in Spectre direct like way. In other words, I set up the config so to reproduce the Direct Spectre simulation from within the AMS Designer. I don't use any digital, RTL ... etc. Just transistor level + Verilog-A just as run by spectre outside AMS. Ideally, you simulation would run just as fine as Spectre. If not, it means that the Spectre version used by AMS has got some issues and you need to report this to Cadence Customer Support. Otherwise, if all works fine, then you need to look at the profiler to see where the simulation spends most of the time. You may also need to report you issue to cadence Customer Support.

Cheers,
Riad.