RE: [IBIS-Users] IV Curves

Just a couple of refinements to the answers I have seen
so far.

1) Actually none of the IV curves are GND relative any more,
this is just a terminology that is left over from the first
version of the IBIS specification.

Note that all four IV curves now have an associated [*** Reference]
keyword, which defines the voltage for the origin of the IV curve.
Therefore a pulldown IV curve for an RS232 driver could be referenced
to -12 V, for example, which is not GND relative by any means...
It is true, in most cases we reference the pulldown and GND clamp IV
curves to 0 V, so it is easy to forget about this capability.

2) Lynne said it right, but I would like to add, that even if you
have a perfect open at the end of the T-line, the 2Vcc or -Vcc
doubling will only happen if your driver is superconductive,
i.e. very low impedance. This is because the T-line voltage
doubling doubles the incident step, which is determined by the
voltage division based on the T-line impedance, and the driver
impedance. So the 2Vcc and -Vcc voltages are kind of an absolute
maximum condition which will most likely never happen.

Arpad Muranyi
Intel Coproation
====================================================================
 

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf Of lgreen
Sent: Monday, January 10, 2005 8:43 AM
To: 'lee yang'; ibis-users@eda.org
Subject: RE: [IBIS-Users] IV Curves

Hello, Lee,
Please see answers inserted.
- Lynne

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of lee yang
Sent: Sunday, January 09, 2005 5:58 PM
To: ibis-users@eda.org
Subject: [IBIS-Users] IV Curves

Hi IBIS experts:

I have a few questions regarding IBIS data:
1) What is the reason of ground clamp and pulldown data is ground relative
while power clamp and pullup data is vcc relative?
-- This was an arbitrary decision made in the IBIS 1.0 specification.
Note that GND-and Pulldown tables currents shift relative to ground
(and currents usually go through 0A at GND), while Power- and Pullup tables
shift when Vcc changes (and currents usually go through 0A at Vcc).

2) The IV curves data is covered from -vcc to 2vcc, in what condition the
voltage would become -vcc? (eg1: output is driving vcc and the receiver end
is being shorted, the reflection coefficient is -1, then the maximum
negative voltage is 0V (vcc + (-1*vcc)=0); eg2: output is driving 0v and the

reciever end is being shorted, the reflection coefficient is -1 as well,
would it be any reflection if the output is driving 0V with reciever end
being shorted to ground?)
-- A load with a small capacitance has a reflection coefficient close to +1.
For an open circuit termination, the reflection coefficient is +1.
A rising waveform into an "open circuit" load can cause 2*Vcc at the driver,
and a falling reflection (starting at Vcc) can cause -Vcc at the driver.

Hope you all could shed some light here.
Thanks.

Yours Faithfully,
LY

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Received on Mon Jan 10 08:58:32 2005