Re: [IBIS-Users] Re: [IBIS] Different PMOS and NMOS driver impedance

From: Sudarshan H N <hn.sudarshan_at_.....>
Date: Mon Aug 18 2008 - 23:26:48 PDT
HI,

I had a similar discussion about the value of the Rfixture some time back
with the IBIS group, and that time the conclusion was, as you said we should
select Rfixture close to the impedance seen by the driver on the board i.e,
around 50ohm. But the exact value might be different and hence these  V-t
curves in IBIS will just give one set of values for given Rfixture,  and
board level simulation tools will use these V-t curves as a reference to get
the actual V-t waveforms for a given load.

Still my doubt is, the pad will see some value of RLC on the board, and IBIS
discourages to use reactive loads when generating Vt curves. So how much
accuracy these V-t curves will give , assuming its only generated with
Rfixture and not with other reactive loads.

Regards
Sudarshan

On Mon, Aug 18, 2008 at 10:08 PM, Tom Dagostino <tom@teraspeed.com> wrote:

>  No, no and no.
>
>
>
> The load resistor used for extracting the VT waveforms should represent the
> load seen by the driver on the circuit board.  Most reasonable circuit
> boards will have trace impedances between 40 and 75 Ohms and usually the
> design targets 50 Ohms.  So the IBIS model should characterize the buffer
> into an impedance close to that. The generally accepted value is 50 Ohms.
> It has nothing at all to do with the output impedance of the driver.  You
> are not trying to match load/output impedance or maximize power transfer,
> you are trying to get a macro model to be characterized near its operating
> conditions.
>
>
>
> There are no such things as IV VT curves. There are no loads associated
> with extracting IV curves.
>
>
>
> Extending the simulation time may or may not "fix" IV/VT curve mismatches.
> Extending simulations times will only fix mismatches if the original
> simulations did not allow the VT waveforms to reach their final value.  If
> you have a problem understand the cause before jumping to a solution.  Just
> jumping to a solution in most cases will not work.
>
>
>
> Tom Dagostino
> Teraspeed(R) Labs
> 13610 SW Harness Lane
> Beaverton, OR 97008
> 503-430-1065
>
> 503-430-1285 FAX
> tom@teraspeed.com
> www.teraspeed.com
>
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> 401-284-1827
>
>
>
> *From:* owner-ibis-users@server.eda.org [mailto:
> owner-ibis-users@server.eda.org] *On Behalf Of *Chetana Raghuwanshi
> *Sent:* Monday, August 18, 2008 6:51 AM
> *To:* Sudarshan H N
> *Cc:* ibis@server.eda.org; ibis-users@server.eda.org;
> owner-ibis-users@server.eda.org
> *Subject:* Re: [IBIS-Users] Re: [IBIS] Different PMOS and NMOS driver
> impedence
>
>
>
> Hi Sudarshan,
>
> You are right. My concern is IV VT mismatch error.
> How would it disappear by increasing the simulation time ?
>
> Best Regards
> Chetana
>
> --------------------------------------------------------
> Chetana Raghuwanshi
> CTO / Process & Library Technology
> NXP Semiconductors India
> NXP Block C, 4th Floor
> MFAR Manyata Tech Park
> Nagavara, Bangalore-560045
> Tel : +91 80 4024 7072
> --------------------------------------------------------
> [image: Inactive hide details for "Sudarshan H N" <hn.sudarshan@gmail.com>]"Sudarshan
> H N" <hn.sudarshan@gmail.com>
>
>   *"Sudarshan H N" <hn.sudarshan@gmail.com>*
> Sent by:
>
> owner-ibis-users@server.eda.org
>
> 2008-08-18 06:43 PM
>
> To
>
>
> "Chetana Raghuwanshi" <chetana.raghuwanshi@nxp.com>
>
> cc
>
>
> ibis-users@server.eda.org, ibis@server.eda.org
>
> Subject
>
>
> [IBIS-Users] Re: [IBIS] Different PMOS and NMOS driver impedence
>
>
>
>
> Hello Chetana,
>
> You can use any of the 2 values for Rfixture to generate Vt curves. There
> is no rule that , your Rfixture should match to the PMOS or NMOS driver
> resistance. But you should always use one value to generate the one set of
> rising and falling waveforms(experts, correct me if i am wrong). If you are
> seeing any errors with respect to IV and Vt curve mismatch , just simulate
> for more time so that it will reach the saturation.
>
> Regards
> Sudarshan
>
> On Mon, Aug 18, 2008 at 6:37 PM, Chetana Raghuwanshi <
> chetana.raghuwanshi@nxp.com> wrote:
>
> Hello Experts,
>
> One of my IO cell has a buffer with different PMOS and NMOS impedences.
> PMOS impedence is approx 200 Ohms and that for NMOS is 96 Ohms.
> In this case what should be the value of Rfixture for calculating IV VT
> curves ?
> Would it be Rfixture1 for calculating pull down and falling waveforms and
> Rfixture2 for calculating pull up and rising waveforms ?
> If I do rise/pullup and fall/pulldown simulations with different Rfixture
> values, is it acceptable ?
>
> Best Regards
> Chetana
>
>
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