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 Information on dc_pivot_check option 

Last post Sat, Feb 2 2013 8:58 AM by Dmitry Babin. 4 replies.
Started by Dmitry Babin 31 Jan 2013 06:25 AM. Topic has 4 replies and 2313 views
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  • Thu, Jan 31 2013 6:25 AM

    • Dmitry Babin
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    • Joined on Mon, Jan 26 2009
    • Haifa, Israel
    • Posts 8
    • Points 130
    Information on dc_pivot_check option Reply

    Hi all,

    I'm simulating a circuit and get the following notice from Spectre:

    Bad pivoting is found during DC analysis. Option dc_pivot_check=yes is recommended for possible improvement of convergence

    However, I can't find any reference to this option in the documentation. Can someone expand on it a bit?

     

    Some background:

    I'm simulating power-up of a circuit with two supplies. Initially, one supply is on, while another one is at 0V, and most of the nets in the 2nd part of the circuit are actually floating (no MOSFET conducts). Neverthless, I would expect the DC solution for them to be 0V (due to gmin conductors installed by Spectre at each floating net). However, that's not the case: DC solution assigns voltages of up to several hundred mV to some of the nets in the powered-down part! Moreover, the voltage values change strongly and inconsistently when I switch corners or change gmin value.

    • Post Points: 35
  • Thu, Jan 31 2013 6:34 AM

    Re: Information on dc_pivot_check option Reply
    Hi Dmitry,

    Good to see you the other day.

    This is a question best asked of customer support (I'd answer it myself but I'm just about to take off on my flight back home)

    Regards,
    Andrew
    • Post Points: 20
  • Thu, Jan 31 2013 7:52 AM

    • Dmitry Babin
    • Not Ranked
    • Joined on Mon, Jan 26 2009
    • Haifa, Israel
    • Posts 8
    • Points 130
    Re: Information on dc_pivot_check option Reply

    Thanks Andrew,

    I've opened an SR on this. I just thought posting it on a forum may help other users with the same problem.

    Regards,

    Dmitry

    • Post Points: 5
  • Thu, Jan 31 2013 8:25 AM

    • Tawna
    • Top 25 Contributor
    • Joined on Fri, Jul 11 2008
    • Snohomish, WA
    • Posts 216
    • Points 5,720
    RE: Information on dc_pivot_check option Reply
    Hi Dmitry,

    RE:

    I'm simulating a circuit and get the following notice from Spectre:

    To improve dc performance, the options "dc_pivot_check=yes"

    and "pivotdc=yes" were added in MMSIM 11.1 isr11.


     
    If you see this warning in your log file, the DC analysis may be having difficulty converging.

    Setting dc_pivot_check = yes may help.

     
    This message doesn't affect anything - it just gives you a hint on how to improve the

    performance if dc analysis is slow.

     
    The difference between these two options:  pivotdc and dc_pivot_check:

    With pivotdc=yes, it will perform pivoting on every iteration of DC analysis,

    trying to overcome convergence difficulty without overhead consideration.

    With dc_pivot_check=yes, it will perform pivoting only when the pivot is worse

    than certain threshold. This way it may help improve the DC convergence or even

    performance without introducing much overhead.

     
    We will be enhancing spectre –h to include a more descriptive explanation.

    Best regards,

    Tawna
    Best regards, Tawna Wilsey Staff Support AE, Global Customer Support Cadence Design Systems, Inc.
    • Post Points: 20
  • Sat, Feb 2 2013 8:58 AM

    • Dmitry Babin
    • Not Ranked
    • Joined on Mon, Jan 26 2009
    • Haifa, Israel
    • Posts 8
    • Points 130
    Re: RE: Information on dc_pivot_check option Reply

    Thanks for the detailed explanation, Tawna. I'll give it a try.

     

    Thanks,

    Dmitry

    • Post Points: 5
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Started by Dmitry Babin at 31 Jan 2013 06:25 AM. Topic has 4 replies.