Property Low Tolerance settings can cause property errors in flat mode, while clean in hier

Document created by brent_beebe on Oct 23, 2012
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Symptoms

  • LVS run clean in hier mode, errors in flat mode.


 

Solution

For example, lets say that in your Source side, you have 3 devices, each with a different hardcoded value for NF:



  •      (in cell1)           c0 Vx VSS  w=5 l=3 nf=150
         (in cell2)           c0 Vx VSS  w=5 l=3 nf=2500
         (in topcell)        c0 Vx VSS  w=5 l=3 nf=5000


  • And, in your rules file you have this:

          LVS REDUCE C PARALLEL [ TOLERANCE L 0 W 0 NF 0 ...


  • The TOLERANCE argument for LVS REDUCE says that if a property value is beyond a certain tolerance of difference, do not allow reduction of that device with others. Since these three values are greater than 0% different, they are not reduced together.


  • This is not an issue in -hier because the caps nf value in each cell matches.


  • In -flat, they are all basically in the same cell, but cannot be reduced because of the 0 tolerance.


  • Changing it as follows:

           LVS REDUCE C PARALLEL [ TOLERANCE L 0 W 0 // NF 0 ..., commented out here, or changed to some suitable value for NF

    Enables a CORRECT flat run.


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