[SI-LIST] Re: Question on IBIS: rising/falling waveform - H ow complete waveform created?
- From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 24 Jan 2003 08:18:08 -0800
Jack,
First of all, my first name is "Arpad", not "Muranyi", so when you write
to me, please address me as "Arpad".
Tom Dagostino gave a pretty good answer to your question, I just want to
add a little to it. Regarding ramps, there are only two of them. If
you have a model that has only a pullup or only a pulldown, i.e. model
type open-something, then one ramp will give you how it turns on, and
the other ramp tells you how it turns off. However, for a buffer that
has both pullup and pulldown in it, the ramp only tells you how the
pullup and pulldown turns on and the turning off information is missing.
This is because the way the load resistor is connected to the buffer
during the measurement.
This is why there is a very strong recommendation for having ***at =
least***
four waveforms in the model for the complementary buffers. That way you
can provide the information for on/off for both pullup and pulldown.
An additional benefit for using waveforms is that being waveforms, they
give you more detail about how the transients (turning on/off) happen
with respect to time. The ramp only tells you an overall slope. This
can be important when you have a multi stage buffer, because the stages
can be turning on/off so that the edge is not straight, but a little =
wavy.
A waveform will show that detail correctly, a ramp will not.
Regarding the math, I can't derive the whole thing here, but if you have
two falling waveforms, with Rfixture connected to Vcc and GND, =
Vpd_on(t),
Vpu_off(t), respectively, and you have the IV curves of the pu and pd, =
you
can set up two equations for finding the two unknowns which are the
scaling coefficients of the pullup and pulldown IV curves with respect
to time, kpu_fall(t) and kpd_fall(t). You can do the same for the two
rising edges also to get kpu_rise(t) and kpd_rise(t). Once you have =
that
you can scale the IV curves with respect to time, where time is the =
elapsed
time from when the buffer was triggered to do a rising or falling edge,
and your output will start generating the waveform depending on what the
load is (remember these IV curves are also a function of the pad =
voltage,
so the load has also a say in how much current you get, not just the
scaling coefficient).
I hope this helps.
Arpad
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-----Original Message-----
From: Jack W.C. Lin [mailto:JackWCLin@xxxxxxxxxxxx]
Sent: Thursday, January 23, 2003 5:34 PM
To: Muranyi, Arpad; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Question on IBIS: rising/falling waveform - H
ow complete waveform created?
Hi Muranyi:
In theoretical, if we have pull up and pull down I-V curve (HIGH and LOW
curve) plus rise/fall time (ramp), the simulator should can create a
waveform. But this waveform will not the same as the waveform which is
created by simulator when include pull up, pull down, 4 sets V-T tables. =
I
can't understand that if we have I-V curve and ramp info, why we should =
have
4 sets V-T tables? Can you explain more about the roles of V-T tables? =
How
to help simulator to create more accurate waveform? I can't understand =
what
you said=20
" Mathematically, if you have a complementary buffer i.e. it has a
pullup and a pulldown, with four waveforms in the IBIS model you can set =
up
two equations with two unknowns for each edge (rise/fall)
and solve for the unknowns which are the coefficients vs. time for the
pullup and pulldown."=20
Thanks
Jack
-----Original Message-----
From: Muranyi, Arpad [mailto:arpad.muranyi@xxxxxxxxx]
Sent: Friday, January 24, 2003 12:18 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Question on IBIS: rising/falling waveform - How
complete waveform created?
This is to answer Jack's and Adeel's questions about how waveforms
are used in IBIS.
To give an intuitive answer, you can think of waveforms as percentage
vs. time tables. They are telling the simulator is how to scale
the IV curves as a function of time as the buffer turns on/off.
Mathematically, if you have a complementary buffer i.e. it has a
pullup and a pulldown, with four waveforms in the IBIS model you
can set up two equations with two unknowns for each edge (rise/fall)
and solve for the unknowns which are the coefficients vs. time for
the pullup and pulldown.
Since the IV curve is a function of the pad voltage, the load of the
actual simulation doesn't have to be the same as the R_fixture that
was used for the waveform tables in the IBIS model. The actual
simulation waveform will adjust to the loading conditions.
I hope this helps to understand how the IBIS model works in simulators.
Arpad Muranyi
Intel Corporation
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