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IMPEDANCE SHEET ISSUES

Khurram
I have noticed that impedance calculated in electrical constraint manager doesn’t reflect real time data (correct me if I am missing something). To my perceptive, allegro constraint manager should give us the data which can be easily comparable to real time data. In impedance sheet, Allegro gives us the calculation (other than dielectric height, dielectric constant, trace width and height etc) by only considering the presence of shield layer which might not be a good parameter for calculations. My concern is that it should also take care whether the signal have copper island or voids/swell directly above the signal on plane layer, Also to my understanding it should give us cumulative impedance instead of ranges (50:90), I think it can be done by customizing the constraint manager but it will be better if they give us both options.

Also I still have not found any feature in ALLEGRO which flags any kind of error if a cline is not getting copper in adjacent shielding layer. Segment over void feature fails if an arc is near to void but not overlapping it still generate error on it.
  • Dennis Nagle

    Hello Khurram,

     If you would like more accuracy in your impedance and delay DRCs, you can set the User Preference (Setup  > User Preferences) use_accurate_delay_calculation which is found in the DRC > Delay category. Hover over the field in the dialog to get a summary of what it does.

    You are also correct that you can customize CM to give you more details on the Impedance worksheet. The last example in the Advanced Constraints tutorial (<install_dir>/doc/cmadvcnstut) has code to add a column that will display all of the segments and their impedance values that are outside of the range set for the net.

    -Dennis Nagle
    Cadence

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