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Information on dc_pivot_check option

Dmitry Babin

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.

  • 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

  • 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

  • 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

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

     

    Thanks,

    Dmitry

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