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verilogams $rdist_normal for random resistor value in a transient simulation

AndyD93

I'm trying to generate a Gaussian random resistor value for each transient simulation with the following verilogams code:

`include "constants.vams"
`include "disciplines.vams"

module Rrandom (va, vb);

inout va, vb;
electrical va, vb;
parameter real Rmean = 200.0*63.0;
parameter real Rstdev = 100;
parameter integer seed = 23;

real Rval;

analog
begin
  Rval = $rdist_normal( seed, Rmean, Rstdev, "instance" );
  I(va, vb) <+ V(va,vb)/Rval;
end

endmodule

When I run in "Single Run, Sweeps and Corners" mode the Rval is changing on every timepoint.  Here is a plot of the Rval for one of the resistors in the netlist:

But I'd like to have a single value for the resistor throughout the simulation (not changing per timepoint).  I tried moving the $rdist_normal outside the analog block as a "parameter real" similar to the example shown in the "Cadence Verilog-A Language Reference 21.1" in the $arandom section (but using $rdist_normal) but that gives me a syntax error.

`include "constants.vams"
`include "disciplines.vams"

module Rrandom (va, vb);

inout va, vb;
electrical va, vb;
parameter real Rmean = 200.0*63.0;
parameter real Rstdev = 100;
parameter integer seed = 23;

parameter real Rval = $rdist_normal( seed, Rmean, Rstdev, "instance" );

analog
begin
  I(va, vb) <+ V(va,vb)/Rval;
end

endmodule

How can I get resistor with a Gaussian distribution that does not change every simulation timepoint in the transient run?

  • in reply to Andrew Beckett

    Thanks for the reference to the article, now I'm one step closer,  When I use $cds_get_mc_trial_number the random variable is changing vs. MC run.  That works nicely, but to get a different random number each instance...

    I understand what they are doing in the example given.  But according to the manual, the $arandom or $rdist_normal "string" argument should be able to take care of this: 

    "string is an optional argument and provides support for Monte Carlo analysis. If string is
    set to global, then one value is generated for each Monte Carlo trial. If string is set to
    instance, then one value is generated for each instance that references this value, and a
    new set of values for these instances is generated for each Monte Carlo trial."

    [Verilog-A Language Reference 21.1]

    Am I missing something here?

  • in reply to AndyD93
    AndyD93 said:
    Sorry to say it, but honestly the tb.count that is used in the example for each instance seems like a kludge to me

    I completely agree. That was precisely what I thought when I saw it. Some of these are workarounds due to gaps in the standards coverage (note that coverage of VerilogA might be slightly different from VerilogAMS; no simulator covers absolutely everything in the LRM because there are quite a lot of esoteric things in the standard which nobody is pushing for; not to say that this feature is esoteric!).

    For example, in the VerilogAMS LRM it shows using localparam to do this, but since we don't support an initialiser on a localparam, that doesn't work.

    Please contact customer support - then an Application Engineer can explore this properly, and ensure that change requests are filed to support the standard way of doing this rather than a workaround (there may be a better workaround, but unfortunately I don't have the bandwidth this week or next to explore this further myself; too many balls in the air at the moment with the day job!).

    Andrew

  • in reply to Andrew Beckett

    Andrew, thank you so much for your help here, it is highly appreciated.  I removed my "kludge" remarks from my post, just to be more polite.  :)  I will submit a ticket about this, because I am still interested in knowing how to use the "string" argument as it is presented in the documentation that I showed.  It is there for a reason, and it seems like it should work.  Like I said earlier, maybe I'm missing something or not understanding the details on how to use it properly.

    Thanks again for all your help!

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