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Simulators >> Circuit Simulators >> spectre error in transient analysis of counter example
https://designers-guide.org/forum/YaBB.pl?num=1519037370

Message started by abhilash_172 on Feb 19^th, 2018, 2:49am

Title: spectre error in transient analysis of counter example
Post by abhilash_172 on Feb 19^th, 2018, 2:49am

Hi
i am trying to simulate a counter using D flip flops. outputs are coming perfectly upto some time and then simulation log has warning
" zero diagonal found in jacobian" .i am using MMSIM 151.
i attached the screenshot.

Regards
abhilash.

Title: Re: spectre error in transient analysis of counter example
Post by abhilash_172 on Feb 19^th, 2018, 2:50am

virtuoso output log file

Title: Re: spectre error in transient analysis of counter example
Post by Geoffrey_Coram on Feb 22^nd, 2018, 8:34am

Did you write the code for the D flip flops? (Or where did you get them?)

Do they properly handle mid-rail inputs? If the input is rising, and the simulator happens to pick a timepoint right when the input hits (vdd+vss)/2, do you end up with both the P and N devices off, so that there is no conductance on the output node?

Title: Re: spectre error in transient analysis of counter example
Post by Ken Kundert on Feb 23^rd, 2018, 7:08pm

A zero diagonal is unusual but it is not an error. The message is printed in the hopes that it might give a clue as to what is happening if something does go wrong, but it in itself is not an indication that anything has gone wrong.

-Ken

Title: Re: spectre error in transient analysis of counter example
Post by abhilash_172 on Feb 25^th, 2018, 2:01am

Hi
@ Geoffrey_Coram, i am simulating a D flop using transistors not the verilog code.

1 . in the waveform picture i posted output is coming for sometime, then there is solution error.
2 . tried increasing the cmin value in spectre options, Now its working fine, but if i run it for really long time again the same issue " zero diagonal found in jacobian".