Hello Everyone,
I find it a happy coincidence that we are talking about the referencing
of S-parameter models and the GND verbiage cleanup at the same time,
because there is a very strong relationship between these two topics.
This through struck me in the editorial meeting today when Walter
showed the equations that Bob also repeated below.
[Pullup]: Vtable = [Pullup Reference] - Voutput
[Power Clamp]: Vtable = [Power Clamp Reference] - Voutput
[Pulldown]: Vtable = Voutput - [Pulldown Reference]
[Gnd Clamp]: Vtable = Voutput - [Gnd Clamp Reference]
ECL:
[Pulldown]: Vtable = [Pulldown Reference] - Voutput
where [Pulldown Reference] = [Pullup Reference]
Since the right side of these equations involve voltages, they
can all be treated as if they were ports, i.e. two terminals.
For example, the [Pullup Reference] voltage needs a reference
with respect to which it is measured. Similarly, the Voutput
voltage also needs a reference with respect to which it is
measured. IBIS does not spell out what these references are,
but I think we can safely say that it assumes that they are the
same for all of the voltages shown in the equations above. I
think it is also safe to say that this reference is the terminal
of the power supply box which serves as the reference for the other
terminals of the same power supply box, usually the +5V, +12V and
-12V in the power supplies of age, and more recently the lower
voltages, such as +3.3V, +1.5V, etc... In simulations this
"universal reference terminal" might be node 0, but doesn't
have to be.
Walter is correct in pointing out that we should define the same
equations for the [Pulldown Reference] and [GND Clamp Reference]
that we have for the "Vcc relative" [Pullup Reference] and [Power
Clamp Reference] keywords. I used to say in my IBIS classes that
the pulldown and GND clamp I-V curves are "GND relative" the same
way as the other two are "Vcc relative".
I agree with Bob, that these definitions should NOT refer to pins,
and especially not GND pins. As the recent discussion pointed out,
there are devices out there which have a negative and a positive
supply and no "GND pin". Although this subject is highly debatable,
because there is nothing wrong with connecting a +/-15 volt Op-Amp
to +30 volt and GND. So is the -15V "negative supply" pin of this
device ground, or a negative supply? It all depends on how the
designer connects it to his/her power supply "terminology"... :)
The same thought process also applies to ECL vs. PECL... The question
of which terminal on the power supply is given the name GND is an
arbitrary thing. Even with normal 5V TTL/CMOS technology, I could
call the positive terminal of the power supply the GND terminal and
the negative terminal of the power supply the -5V supply. The GND
question becomes only an issue when we start making connections to
the chassis and/or Earth Ground, "where carrots and potatoes are grown"
for safety and/or EMI reasons, and when we have coax, and/or other
(non-isolated) wiring/signaling going between different boxes/buildings,
etc... Fun subject...
Radek is also correct that the only other thing that is missing in
IBIS the definition of the reference for the signal voltages. This
also includes the question of what Vinh and Vinl is referenced to.
This is a more complicated question, though, than just picking a
suitable arbitrary reference, such as the "GND terminal" of the
power supply box. The reason is because Vinh and Vinl are used
to "evaluate" the signal that the input sees on the die. For that
reason the reference of Vinh or Vinl would most likely have to be
something on the die. But what this is on the die may be circuit
design dependent, so I am leaning towards inventing keywords or
subparameters which allow the model maker to define the reference
of these voltages, since they are the ones who know what it is.
In addition, we may also need to consider to define these reference
as an expression, such as:
([Pullup Reference] + [Pulldown Reference]) / 2
not just as a fixed pin/pad, or other "location" or "voltage". The
"Ext_ref" feature in IBIS is a (lame) example for this need.
These were some of the thoughts that came to me in the editorial
meeting today, we were just running short on time and I didn't get
a chance to bring them up.
Thanks,
Arpad
=====================================================================
From: ibis-editorial-bounce@xxxxxxxxxxxxx
[mailto:ibis-editorial-bounce@xxxxxxxxxxxxx] On Behalf Of Bob Ross
Sent: Friday, March 04, 2016 1:32 PM
To: ibis-editorial@xxxxxxxxxxxxx
Subject: [ibis-editorial] FW: [ibis-interconn] Re: What does a Voltage mean in
IBIS
Also to the Editorial Committee.
Bob
From:
ibis-interconn-bounce@xxxxxxxxxxxxx<mailto:ibis-interconn-bounce@xxxxxxxxxxxxx>
[mailto:ibis-interconn-bounce@xxxxxxxxxxxxx] On Behalf Of Bob Ross
Sent: Friday, March 04, 2016 11:28 AM
To: ibis-interconn@xxxxxxxxxxxxx<mailto:ibis-interconn@xxxxxxxxxxxxx>
Subject: [ibis-interconn] Re: What does a Voltage mean in IBIS
Walter,
My confusion was that the "Other Notes" in Slides 2 appear under the
[Pullup]/.. keywords.
However, in refining the terminology, we should clarify that Vcc is just a
generic name for a reference. It does not need to be a "pin", and I can show
some data sheets that use internal charge pump voltage rails where the Vcc
"pin" is not exposed externally. (We cannot use [Pin Mapping] for those
devices.) We are sloppy here and need to understand that "Vcc" for [Pullup
Reference] can have a different value than a "Vcc" for the [Power Clamp
Reference]. Separate terms may be needed.
For slide 3, we may be overloading the word "GND" when we really might mean the
[Pulldown Reference] Vee voltage. The [Gnd Clamp Reference] can be different
from the [Pulldown Reference].
We might also state that Vcc/Vee is generic in this document - they could mean
Vdd/Vss or other reference names used by a component.
Regarding Slide 4, we may not spell this out. But the real rules for entering
values in table based on DUT extraction is:
[Pullup]: Vtable = [Pullup Reference] - Voutput
[Power Clamp]: Vtable = [Power Clamp Reference] - Voutput
[Pulldown]: Vtable = Voutput - [Pulldown Reference]
[Gnd Clamp]: Vtable = Voutput - [Gnd Clamp Reference]
ECL:
[Pulldown]: Vtable = [Pulldown Reference] - Voutput
where [Pulldown Reference] = [Pullup Reference]
All Reference voltages entries are values with respect to an assumed internal 0
V local reference, whether or not such a local reference is exposed externally
for the [Model].
Slide 5:
The notion is mostly correct for Vinl and VInh. They are adjusted for the
voltages of the clamp references for most cases
However, while not common, it would be legal to change the references
differently - Vdd+9 V, Vee + 10 V, or they could be compressed.
Then the Vinh and Vinh rules should follow the technology and data sheet
intent. Here are 3 common rules for Vinl and Vinh:
TTL: Always an offset from Vee (0.8, 2.0 V)
CMOS: 30% and 70% of Vdd (with Vss = gnd) meaning that threshold difference
could get rescaled
PECL: Always an offset from Vcc.
----
So, part of fixing all references in IBIS is to be precise about our
terminology and application.
Bob
