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How to use OCEAN 'pv' command to get captab results.

RFStuff

Dear All,

I saw the Cadence-post (please see at the end of this post) titled:-  What is difference between "node" and "nodetoground" captab output, and how to access captab results.

It says that one can use the OCEAN command 'pv' to find the node-to-node and node-to-ground capacitance.

I ran a dcOP analysis and then I opened the calculator. I typed pv("/I0/ref_core/in: vssa" "Fixed" ?result "captab").  /I0/ref_core/in and vssa are two nodes in the schematic.

But, it did NOT show any value in the calculator. Perhaps I am missing something.

Could anybody please tell how to use the OCEAN command 'pv' either in calculator or in OCEAN script to find the captab results.

Kind Regards,

What is difference between "node" and "nodetoground" captab output, and how to access captab results

 

Problem

 

You are using the captab option with dc or transient analysis in spectre. This gives a table of node capacitance for the circuit. You can choose either "node", "nodetoground" or "nodetonode" detail options for this table.
"spectre -h info" gives following:
***
9 detail=node
How detailed should the capacitance table be?
Possible values are node, nodetoground or nodetonode.
***
What is the difference between the "node" and "nodetoground" ?
How to access capacitance values from captab results?

 

Solution

 

The "node" capacitance is the total node capacitance, which is the sum of node to ground (reference to ground) capacitance, and node to other nodes (reference to other nodes) capacitance.
So, if I have this topology

c1 (NODE 0) 5pf
c2 (NODE a) 1pf
c3 (NODE b) 2pf

and there are no other capacitances, then
"node" capacitance for NODE is
5pf + 1pf + 2pf = 8pf.

"nodetogroud" capacitance for NODE is
5pf

 "nodetonode" will list out c2 and c3 values of 1pf and 2pf.

The spectre netlist will contain something like:

captabInfo info what=captab detail=node where=rawfile

You can then use in ADE -> Results->Print Capacitance Table to access the captab results. You can also access the results in the Results Browser and calculator.
To access results in Results Browser, open psf/raw directory and check results created in folder like captab-info.captab.
To access results in calculator, you can use ocean ‘pv’ function something as mentioned below:

pv("NODE: NODE" "Total" ?result "captab")
or
pv("NODE: NODE" "Variable" ?result "captab")
or
pv("NODE: NODE" "Fixed" ?result "captab")

Parents

  • There are two issues here:

    1. That article is a bit confusing as the examples don't make it clear that you need a single space either side of the colon (they show no space before the colon)
    2. You cannot use schematic names (the name mapping doesn't work properly) - the reverse name mapping when  you use outputs() gets confused and shows names that will not work. You should use outputs(?map nil ?result 'captab) to show the available names

    So that means you should use pv("I0.ref_core.in : vssa" "Fixed" ?result "captab")

    I'll tweak the article to make that clearer.

    Regards,

    Andrew.

  • in reply to Andrew Beckett

    Dear Andrew,

    Thanks for clearing the confusion.

    I have three doubts as follow:-

    1:- What is the difference between Fixed and Variable capacitor. Any way in dc analysis the capacitance is the Fixed. How cadence calculates the variable capacitance.

    2:- Can the  above corrected 'pv' command be used for transient analysis (at certain points) ?

    3:- If I use the pv command (with "total") for finding the capacitance between gate and drain nodes of a MOS in the schematic, does it give the capacitance of cgd of MOS or something else. Does it include the capacitance present in the MODEL of the MOS ?

    Kind Regards,

  • in reply to RFStuff

    It's explained quite well in the spectre documentation here. (That's the SPECTRE17.1 documentation). For the second question, see How to generate a captab at multiple times throughout a transient simulation. For the third, it will include all the capacitance (from the device and any other capacitors) - so yes, it will include the capacitance in the model.

    Regards,

    Andrew.

  • in reply to Andrew Beckett

    Dear Andrew,

    Thanks a lot for the clarification.

    But the documentation still does not clarify how cadence distinguishes between the variable and the FIXED capacitance.

    Could you please explain this. This would be really helpful for the circuit designer.

    Kind Regards,

Reply
  • in reply to Andrew Beckett

    Dear Andrew,

    Thanks a lot for the clarification.

    But the documentation still does not clarify how cadence distinguishes between the variable and the FIXED capacitance.

    Could you please explain this. This would be really helpful for the circuit designer.

    Kind Regards,

Children
  • in reply to RFStuff

    The documentation does distinguish between them (it just doesn't say a great deal, mainly because there's not a great deal to say). The Fixed is the linear capacitance and the Variable is the nonlinear capacitance. Of course, you may not care that much whether the capacitance is linear or nonlinear, but it's an indication of whether the capacitance is dependent upon the operating point or not.

    Regards,

    Andrew.

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