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spectre error in transient analysis of counter example (Read 377 times)
abhilash_172
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spectre error in transient analysis of counter example
Feb 19th, 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.
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Capture_016.PNG
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abhilash_172
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Re: spectre error in transient analysis of counter example
Reply #1 - Feb 19th, 2018, 2:50am
 
virtuoso output log file
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Capture1_001.PNG
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Geoffrey_Coram
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Re: spectre error in transient analysis of counter example
Reply #2 - Feb 22nd, 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?
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Ken Kundert
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Re: spectre error in transient analysis of counter example
Reply #3 - Feb 23rd, 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
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abhilash_172
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Re: spectre error in transient analysis of counter example
Reply #4 - Feb 25th, 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".
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