Date: Tue Aug 19 2008 - 08:15:26 PDT
Are you concerned about the mismatch between your driver's NMOS/PMOS impedances and the 50 ohm load used to generate the V-T curves?
I wouldn't be. The 50 ohm load is meant to be a reasonable match to the PCB impedance, as Tom said - not the driver's output impedance, which can vary substantially. Simulators don't use the V-T data directly - they use the V-T curves to extract turn-on/turn-off timing data for the pullup and pulldown devices. The simulator computes how much the pullup/pulldown is "on" or "off" at each point in time, and uses that information in conjunction with the I-V curves (which aren't predicated on a reference resistance) to predict the output's response.
The topics of V-T curves and Kt coefficients have been presented and discussed at length. I don't have a pointer handy, but I'm guessing Arpad does ...
Todd.
Todd Westerhoff VP, Software Products SiSoft 6 Clock Tower Place, Suite 250 Maynard, MA 01754 (978) 461-0449 x24 twesterh@sisoft.com www.sisoft.com
Sudarshan H N wrote:
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
503-430-1065503-430-1285 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
--------------------------------------------------------
"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
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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