RE: [IBIS-Users] Load Line Question.


Subject: RE: [IBIS-Users] Load Line Question.
From: Tom Dagostino (tom@teraspeed.com)
Date: Tue Jan 28 2003 - 10:49:07 PST


IBIS sums all the appropriate IV curves before it calculates the voltage
from the load line. In your example, the open drain model, IBIS would sum
the pulldown, the ground clamp (if present in the model) and powerclamp
(again, if present in the model). It would then compute the voltage from
the load line based on the summed IV curves.

Are you including everything?

Tom Dagostino
Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
2926 SE Yamhill St. Device Modeling Division
Portland, OR 97214 13610 SW Harness Lane
                                   Beaverton, OR 97008
http://www.teraspeed.com 503-430-1065
tom@teraspeed.com

-----Original Message-----
From: owner-ibis-users@server.eda.org
[mailto:owner-ibis-users@server.eda.org]On Behalf Of whiz kid
Sent: Tuesday, January 28, 2003 10:22 AM
To: ibis-users@server.eda.org
Subject: [IBIS-Users] Load Line Question.

Hi Gurus,
I have a question regarding plotting load lines based
on the V-I tables.

1) I have a open drain model with V_fixture=1.26v and
R_fixture = 50ohms.There are two V-T tables ( one
rising and one falling with V_fixture= 1.26 and
R_fixture = 50ohms ). I calculated the load line based
on the values as I=0 at V=Vcc and I = 1.26/50 =25.2ma.
I then plotted this loadline on the Pulldown curve by
drawing a line with co-ordinates (1.26v,0ma),
(0v,25.2ma) and noted for the intersection of the VI
curve and the loadline. The intersection point that I
get is way off from the one the tool calculates, which
in turn is off from the VT table falling endpoint. My
question is what is it that I am missing so that I get
a different value from the tool and also fixing the
model so that I fall within the 1% tolerance of the VT
end point.

Thanks,
Rahul.

--- Peter LaFlamme <plaflamm@amcc.com> wrote:
> Hi John,
> These types of warnings are very common. It appears
> to me that you are either using an old IBISCHK or
> you are creating a Version 2.1 model because the
> latest IBISCHK will give errors for any mismatch
> greater than 1% or so. If your I/O is single-ended
> then
> you can determine where the problem is pretty
> easily. You need to determine what your output load
> is. In this case 50 ohms. Then plot your I-V curves
> and V-T curves. On the I-V curves you need to draw a
> 50-ohm load line. The load line intersection with
> the
> I-V curve should match the end point of the V-T
> curve (for rising waveform use the pull-up curve,
> Falling waveform use pull-down).
>
> Is this a differential output? If so what technology
> is it? Creating an IBIS model of a differential
> output can be very tricky. IBIS actually models the
> differential output as a single-ended entity which
> it is definitely not. My experience with LVDS, PECL
> and CML models has been that P and N output voltage
> levels are dependent on each other. For example on
> one of our LVDS I/O cells the sum of the P and N
> voltages will be 2.4V. So if the P side is 1.0V the
> N side will try to be 1.4V. To create a model as
> such you need to set up your simulation such that
> the N side load is a voltage source that is
> dependent on the P output.
>
> I have read the papers on the IBIS website and they
> are somewhat helpful, however the cases presented
> are applicable to certain cases only. There needs to
> be an IBIS cookbook that explains both the theory
> behind creating the models and the how to.
>
> Well enough about Differential.
>
> I hope that this helps.
>
> Peter
>
>
> Duane Mattheisen wrote:
> >
> > John
> > I was writing for a bench measurement not a
> simulation. The 50 ohms
> > is a common standard for the AC measurements. And
> if your on a AC bench
> > measuring of actual devices, a capacitor helps
> when put it on the power
> > supply side of the R_fixture not on the device
> side, what that does is
> > take any voltage drop out of the wires going from
> the power supply to
> > the R_fixture. For the DC measurement again on
> the bench the resistance
> > to make as low as possible is the fixture
> resistance which is between
> > the Voltage/Current measurement tool and the
> device.
> >
> > On another note I down loaded s2ibis2_fix_tar.Z
> and s2ibis2_tar.Z but
> > did not know how to work with them. Is there any
> advice you can give me
> > to start?
> >
> > Thanks Duane Matheisen
> > John Phillippe wrote:
> > >
> > > Duane Mattheisen wrote:
> > > > John Take out as much fixture resistance as
> possible in the DC
> > > > measurement.
> > > > Also for the AC measurement put a big
> capacitor (0.1uf and a >2.2uF
> > > > tantalum cap) to ground on the Rfixture having
> the other side of the
> > > > Rfixture that connects to the output as close
> as possible to the
> > > > output.
> > > > Sincerely
> Duane Mattheisen
> > > > "Angulo, John" wrote:
> > > >
> > > >>The following is posted on behalf of John
> Phillippe at Motorola. Please send any private
> replies to john.phillippe@motorola.com only.
> > > >>
> > >
> >>--------------------------------------------------
> > > >>
> > > >>Ibis Guru's,
> > > >> I get quite a few waveform mismatches
> between the rising/falling
> > > >>edges and the I/V curves like the one shown
> below.
> > > >>
> > > >>WARNING - Model pad_26v_fast: The [Falling
> Waveform]
> > > >> with [R_fixture]=50 Ohms and
> [V_fixture_max]=0V
> > > >> has MAX column DC endpoints of 0.05V
> and 1.75v, but
> > > >> an equivalent load applied to the
> model's I-V tables yields
> > > >> different voltages ( 0.16V and 1.91V),
> > > >> a difference of 6.66% and 8.94%,
> respectively.
> > > >>
> > > >>I'm curious if this is a common problem, or if
> there are some simple
> > > >>solutions to get everything to match up. I
> check the waveforms and set
> > > >>the sim time long enough to make sure the
> waveforms are stable. Any
> > > >>help/suggestions would be greatly appreciated.
> > >
> > > Duane,
> > > Since I'm using s2ibis2, I should make rload
> as small as possible?
> > > On the order of 1ohm? I read in this group
> before to use an rload of
> > > 50ohms since that is what most people are using,
> and that it is a bit of
> > > a "standard." I've also been told to have NO
> capacitance in the AC
> > > measurements, and again to only use a 50ohm
> resistor.
> > > I did try a run with a 0.1uF cap on the pad
> for the AC measurements,
> > > but that just slows the edge down.
> > >
> > > --
> > > John Phillippe
> > > SPS, 32 Bit Embedded Controller Division, IC
> Creation
> > > Motorola - 512-895-1835
> > > Austin, TX - john.phillippe@motorola.com
> >
>
|------------------------------------------------------------------
> > |For help or to subscribe/unsubscribe, email
> majordomo@eda.org
> > |with just the appropriate command message(s) in
> the body:
> > |
> > | help
> > | subscribe ibis <optional e-mail
> address, if different>
> > | subscribe ibis-users <optional e-mail
> address, if different>
> > | unsubscribe ibis <optional e-mail
> address, if different>
> > | unsubscribe ibis-users <optional e-mail
> address, if different>
> > |
> > |or email a written request to
> ibis-request@eda.org.
> > |
> > |IBIS reflector archives exist under:
> > |
> > | http://www.eda.org/pub/ibis/email_archive/
> Recent
> > | http://www.eda.org/pub/ibis/users_archive/
> Recent
> > | http://www.eda.org/pub/ibis/email/
> E-mail since 1993
>
> --
> Peter LaFlamme
>
> Applied Micro Circuits Corp.
> System Applications Engineer
> 200 Minuteman Rd, 3rd Floor
> Andover, MA 01810
>
> 978-247-8470 phone
> 978-623-0055 Fax
>
|------------------------------------------------------------------
> |For help or to subscribe/unsubscribe, email
> majordomo@eda.org
> |with just the appropriate command message(s) in the
> body:
> |
> | help
> | subscribe ibis <optional e-mail address,
> if different>
> | subscribe ibis-users <optional e-mail address,
> if different>
> | unsubscribe ibis <optional e-mail address,
> if different>
> | unsubscribe ibis-users <optional e-mail address,
> if different>
> |
> |or email a written request to ibis-request@eda.org.
> |
> |IBIS reflector archives exist under:
> |
>
=== message truncated ===

__________________________________________________
Do you Yahoo!?
Yahoo! Mail Plus - Powerful. Affordable. Sign up now.
http://mailplus.yahoo.com
|------------------------------------------------------------------
|For help or to subscribe/unsubscribe, email majordomo@eda.org
|with just the appropriate command message(s) in the body:
|
| help
| subscribe ibis <optional e-mail address, if different>
| subscribe ibis-users <optional e-mail address, if different>
| unsubscribe ibis <optional e-mail address, if different>
| unsubscribe ibis-users <optional e-mail address, if different>
|
|or email a written request to ibis-request@eda.org.
|
|IBIS reflector archives exist under:
|
| http://www.eda.org/pub/ibis/email_archive/ Recent
| http://www.eda.org/pub/ibis/users_archive/ Recent
| http://www.eda.org/pub/ibis/email/ E-mail since 1993

|------------------------------------------------------------------
|For help or to subscribe/unsubscribe, email majordomo@eda.org
|with just the appropriate command message(s) in the body:
|
| help
| subscribe ibis <optional e-mail address, if different>
| subscribe ibis-users <optional e-mail address, if different>
| unsubscribe ibis <optional e-mail address, if different>
| unsubscribe ibis-users <optional e-mail address, if different>
|
|or email a written request to ibis-request@eda.org.
|
|IBIS reflector archives exist under:
|
| http://www.eda.org/pub/ibis/email_archive/ Recent
| http://www.eda.org/pub/ibis/users_archive/ Recent
| http://www.eda.org/pub/ibis/email/ E-mail since 1993



This archive was generated by hypermail 2b28 : Tue Jan 28 2003 - 11:08:58 PST