Problem about Monte Carlo analysis

Discussion in 'Cadence' started by Yan Zhang, Sep 2, 2003.

  1. Yan Zhang

    Yan Zhang Guest

    Hi,
    I have some problem with Monte Carlo analysis.
    Each time I ran the Monte Carlo analysis, I got the same
    results. It seems that the seed of the random function is always the same.
    Is there some way to deal with it? Or I miss some options?

    Thanks.
     
    Yan Zhang, Sep 2, 2003
    #1
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  2. Yan Zhang

    Erik Wanta Guest

    I don't see a field to specify a seed in the MC GUI.

    The documentation says:
    If you do not specify a seed, then each time that you run the
    analysis, you get different results; that is, a different stream of
    pseudorandom numbers is generated. If you do not specify a seed, the
    Spectre simulator uses the Spectre process id (PID) as a seed.

    I don't see this to be the case however. If I do a Simulation->Create
    Input Files I always get seed=1 in the MC section of the netlist.
     
    Erik Wanta, Sep 3, 2003
    #2
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  3. Yes, the seed is currently fixed. There's a PCR to allow the seed to be
    specified in the UI (I don't remember the PCR number) - I think it has
    come up in the past on this group (check google).

    The idea behind having a fixed seed was to allow you to have reproducable
    results - and you can (sort of) workaround the problem by changing the starting
    run number on the form.

    I'm assuming Yan that you're not saying that the results for each point in
    the monte-carlo are the same as all the other points, but rather that the
    results for the whole set of runs stays the same?
    If the results are the same point to point, then that would suggest that
    you either have models which don't vary, or more likely that you have
    expressions for your outputs which reference the wrong set of results.

    Andrew.
     
    Andrew Beckett, Sep 3, 2003
    #3
  4. Yan Zhang

    Erik Wanta Guest

    FYI, the PCR number is 432223.
    ---
    Erik

     
    Erik Wanta, Sep 3, 2003
    #4
  5. Indeed,

    it is very useful to be able to go from a given MC run in
    results/waveform back to the state of the process and models. Around
    here, the typical designer has already pushed the envelope on disk usage
    and CPU time before he starts to do MC runs, so he will not be able to
    save all results, and he will not have the time to simply rerun the
    first 73 runs when he notices something "interesting" in the 74th MC run
    and want to know more about it.

    Even with the possibility to set the seed to the value of a given MC
    run, there is still quite some work to relate the seed value to the
    process parameter and model parameter values. And the process params are
    what you want to know so that you can understand how the process state
    makes the circuit fail, and be able to correct.
    Since MC, corner runs and such are late in the electrical design
    phase, at a time when people a also busy documenting, preparing
    presentations, meetings, and close to a deadline, they will almost never
    go through much trouble to trace back what this particular MC run was
    doing to their circuit. They will rather make assumptions (that they don
    t check by simulation), fix the circuit, start a new set of simulations
    and check only that the worst case situation doesn happen anymore.
     
    eda support guy, Sep 4, 2003
    #5
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