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Simulating IBIS Model using Spectre

cADEUser

I have a question regarding simulating IBIS model using Spectre.  IBIS model generation always has the die capacitance included and in the generated IBIS file you will have this value as  “C_comp” value.  Does the Spectre accounts for this capacitance from the IBIS file while computing the time domain voltage waveform during simulation ?  If I add additional capacitance outside in the testbench, to model the die capacitance, then it will be double counting.

Does anyone know if Spectre is already accounting this C_comp during the time domain voltage wave computation from IBIS file, during simulation ?

Parents
  • Andrew Beckett

    I don't have a great deal of experience of using IBIS models, but from a quick search, I think you either need to use the ibis2subckt tool in the Spectre release to convert an IBIS file to a spectre subckt, or use the ibis_include statement in spectre to include the file (which I think does the conversion behind the scenes). Simply using ibis_buffer doesn't include package parasitics.

    See this application note Using Spectre to Simulate IBIS Models and this article: How to include and simulate IBIS buffer models in ADE which include package parasitics

    Andrew

  • cADEUser
    in reply to Andrew Beckett

    Actually I was not asking about the package parasitic portion. I know that those are not included. I was asking about the C_comp inside the IBIS file which represents die capacitance component. Does the spectre simulator account of this C_comp value  while simulating IBIS model ? If it does that I should not be adding any additional explicit capacitance outside at the output to represent the die capacitance. Otherwise it will be double counting.  The question is not regarding the package parasitic. 

  • Andrew Beckett
    in reply to cADEUser

    If you can provide an IBIS file containing this die capacitance, I can investigate. As I mentioned, I don't have a great deal of experience of using IBIS and so don't have any examples to hand. If you can't provide that here, I suggest you contact customer support where you hopefully can provide the file. BTW, I did do some research and didn't find any internal reference describing whether the die capacitances are or aren't included.

    Andrew.

  • cADEUser
    in reply to Andrew Beckett

    Hi Andrew,

    This is a propriety model that I am not able to share here. Let me contact customer support on this.

  • cADEUser
    in reply to cADEUser

    Hi Andrew,

    Actually I found a document in the support site that you referenced above that shows that that C_comp is included by the  spectre. This clearly outlines how spectre simulates IBIS. Very good and in details. I have my answer now. 

    https://support.cadence.com/apex/techpubDocViewerPage?xmlName=spectremod.xml&title=Spectre%20Circuit%20Simulator%20Components%20and%20Device%20Models%20Reference%20--%20Circuit%20Components%20-%20IBIS%20I/O%20buffer%20(ibis_buffer)&hash=pgfId-1039584&c_version=19.1&path=spectremod/spectremod19.1/Chap1.html#pgfId-1039584

    For C_comp : 

    Switching Coefficients

    Switching coefficients, if not given, are calculated from the test waveforms and loads. If two waveforms are available for rising transition (V1r, V2r) and two for falling (V1f, V2f), then linear system of equations is solved for pairs of switching coefficients (Wdr, Wur) and (Wdf, Wuf) at each time t:

        Ipd(Vr1-Vgnd)*Wdr + Ipu(Vr1-Vpwr)*Wur = I0(Vr1) 
        Ipd(Vr2-Vgnd)*Wdr + Ipu(Vr2-Vpwr)*Wur = I0(Vr2) 
        Ipd(Vf1-Vgnd)*Wdf + Ipu(Vf1-Vpwr)*Wuf = I0(Vf1) 
        Ipd(Vf2-Vgnd)*Wdf + Ipu(Vf2-Vpwr)*Wuf = I0(Vf2)

    Where Ipd(V), Ipu(V) are pull-down and pull-up current source fonctions. Load current I0(V) for each load resistance Rl, voltage Vl, and capacitance Cl is calculated as:

        I0(V) = (Ccomp+Cl)*dV/dt - (V-Vl)/Rl - Igc(V-Vgndc) - Ipc(V-Vpwrc)

    So it is included. Thanks Andrew for all the help. 

  • Andrew Beckett
    in reply to cADEUser
    cADEUser said:
    Actually I found a document in the support site that you referenced above

    Rookie mistake on my part. If all else fails, I should read the manual before answering!

    Glad you got the answer you wanted.

    Andrew.

Reply
  • Andrew Beckett
    in reply to cADEUser
    cADEUser said:
    Actually I found a document in the support site that you referenced above

    Rookie mistake on my part. If all else fails, I should read the manual before answering!

    Glad you got the answer you wanted.

    Andrew.

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