Re: [IBIS-Users] Ramp vs. Rising/Falling waveforms, Cref missing ...

hi Tom
In the s2i file we have following switches: [Vinl], [Vinh], [Vmeas], 
[Vref], [Rref] and [Cref] .
Are these values really used while generating ibis model or these are 
just the information in ibis
file  for board level simulations.

thanks
sujit


Tom Dagostino wrote:

> Yes, you still need to have C_comp in the model.  Most simulators, 
> from what I have heard, don't use C_comp when they are using the 
> buffer as a driver.  But when the buffer is receiving a signal, such 
> as a reflection, the simulator puts C_comp into the simulation.. Ask 
> your specific simulator company for details on how they handle this 
> situation.
>  
>  
>
> Tom Dagostino
> Teraspeed(R) Labs
> 13610 SW Harness Lane
> Beaverton, OR 97008
> 503-430-1065
> tom@teraspeed.com
> www.teraspeed.com
>
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> 401-284-1827
>
>     -----Original Message-----
>     From: NG,MEI-YEE [mailto:mei-yee.ng@avagotech.com]
>     Sent: Wednesday, December 20, 2006 3:26 PM
>     To: tom@teraspeed.com; April.Hachenburg@smsc.com;
>     ibis-users@server.eda.org
>     Cc: akvarma@ncsu.edu
>     Subject: RE: [IBIS-Users] Ramp vs. Rising/Falling waveforms, Cref
>     missing ...
>
>     Hi Tom,
>
>      
>
>     I find your reply very useful. Thank you for the information.
>     There is one question regarding C_comp that I would like to ask.
>     When I run simulation to extract V-T curves, I used an extracted
>     RC netlist, which means it has taken into account parasitic R and
>     C of the buffer. In that case, do I still need to specify another
>     C_comp value? Would that mean double-calculating this C_comp?
>
>      
>
>     Kindly advice.
>
>
>     Thank you.
>
>      
>
>     Best regards,
>
>     Mei Yee
>
>     IC Design Engineer
>
>     Avago Technologies
>
>      
>
>     ------------------------------------------------------------------------
>
>     From: owner-ibis-users@server.eda.org
>     [mailto:owner-ibis-users@server.eda.org] On Behalf Of Tom Dagostino
>     Sent: Thursday, December 21, 2006 4:14 AM
>     To: April.Hachenburg@smsc.com; ibis-users@server.eda.org
>     Cc: akvarma@ncsu.edu
>     Subject: RE: [IBIS-Users] Ramp vs. Rising/Falling waveforms, Cref
>     missing ...
>
>      
>
>     April
>
>      
>
>     R_load is the load the buffer is placed under when the ramp data
>     is acquired.  50 Ohms is the typical value and it is usually
>     terminated to ground for the dV/dT rise and to power for the dV/dT
>     fall. 
>
>      
>
>     R_fixture is the load the buffer is placed under when the VT
>     waveforms as acquired.  This value is typically 50 Ohms and is
>     terminated to ground for both a rising and falling waveform and
>     then terminated to power for the other set of rising and falling
>     waveforms.  C_fixture would be any capacitance associated with the
>     R_fixture.  But its not a good idea to have any C_fixture when
>     extracting VT waveforms.  I've seen simulator give very wrong
>     answers when C_fixture is present.
>
>      
>
>     Rref is the manufacturer's timing test load's resistive
>     component.  This can be any value (within reason) and can be
>     terminated to any reasonable voltage Vref the manufacturer deems
>     appropriate for the buffer technology.  It is usually terminated
>     to the typical value the buffer would see in an actual
>     application.  If the buffer was an ECL output then 50 Ohms to
>     Vcc-2Volts would be the load.  Most CMOS devices have just a Cref,
>     a capacitor terminated to ground.
>
>      
>
>     C_comp is the capacitance associated with the I/O buffer. It
>     includes all the capacitance from metal, active or passive devices
>     on the silicon.  It forms the majority of the buffer's impedance
>     reactive component.
>
>      
>
>     R_fixture should match the load the buffer is seeing in an actual
>     application.  Most circuit boards trace impedance runs between 40
>     and 70 Ohms or there about.  Most buffers are characterized into a
>     50 Ohm load which is close to the board impedance.  Since the
>     model developer cannot predict every application 50 Ohms is a good
>     compromise for most models.  There are some technologies that are
>     better served with a non 50 Ohm load such as RAMBUS, which is
>     specified into 27 Ohms or very weak CMOS drivers that cannot drive
>     a 50 Ohm load to Vinl or Vinh.  I'd caution against using a 500
>     Ohm load in your model making unless there is a very good reason
>     to do so. 
>
>      
>
>     The Ramp key word is a legacy key word that was used before the VT
>     waveforms were added to IBIS.  It is still required to be present
>     in the model. If R_fixture is not specified it defaults to 50 Ohm
>     in the model.
>
>      
>
>      
>
>     Tom Dagostino
>     Teraspeed(R) Labs
>     13610 SW Harness Lane
>     Beaverton, OR 97008
>     503-430-1065
>     tom@teraspeed.com
>     www.teraspeed.com
>
>     Teraspeed Consulting Group LLC
>     121 North River Drive
>     Narragansett, RI 02882
>     401-284-1827
>
>         -----Original Message-----
>         From: owner-ibis-users@server.eda.org
>         [mailto:owner-ibis-users@server.eda.org] On Behalf Of
>         April.Hachenburg@smsc.com
>         Sent: Wednesday, December 20, 2006 9:39 AM
>         To: ibis-users@server.eda.org
>         Cc: akvarma@ncsu.edu
>         Subject: [IBIS-Users] Ramp vs. Rising/Falling waveforms, Cref
>         missing ...
>
>
>         IBIS Users,
>         I wondered if anyone could clarify for me the usage of Rload
>         vs. Rref vs. Rfixture and similarly Cref vs. Cfixture.
>
>         I am using s2ibis3 and discovered that the Cref parameter
>         specified in my .s2i file does not appear in my .ibs model
>         file.  After looking at the netlists and reading the IBIS
>         documentation, it appears Cref is not used for any
>         simulations.  What is the purpose of Cref?  And why isn't it
>         showing up in the .ibs file?
>
>         Also, I read that C_comp is taken into account for the
>         rising/falling V-T waveforms though I cannot find that
>         paramater in any of the corresponding netlists.  How is C_comp
>         used?
>
>         As for Rref vs. Rfixture, I thought I read somewhere that for
>         best results Rref should match Rfixture.   Is that true?  In
>         my model, I am using the keywords Rising/Falling Waveform.  I
>         found that if I do not specify Rload even though I am not
>         specifying the [Ramp] keyword, then 50ohms is used in one of
>         the transient simulations despite Rref and Rfixture being set
>         to 500 ohms.  Should I be specifying the [Ramp] keyword along
>         with the [Rising/Falling Waveform] keywords? If so, how do
>         they work together?
>
>         - April
>
>         ---------------------------------------------------
>         April Hachenburg
>         Core Technology Design Engineer
>         SMSC
>

-- 
thanks
sujit kumar




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