Regarding clamp voltages that differ from other supply voltages, I have
always regarded that as a concession to the situation where the forward
bias voltages built into diode-like structures used for clamping do not
give the right amount of both leeway and protection. It only makes sense
that while PC and GC need to be somewhat stable relative to PU and PD
respectively, the buffer only cares about the PU and PD rails for
operation. Therefore the lower of those two (PD) becomes the obvious
choice as the reference for Vinl/Vinh.
Should IBIS say that, or are there legitimate cases where someone has
assumed otherwise?
Mike
From: ibis-editorial-bounce@xxxxxxxxxxxxx
[mailto:ibis-editorial-bounce@xxxxxxxxxxxxx] On Behalf Of Walter Katz
Sent: Friday, April 15, 2016 1:26 PM
To: ibis-editorial@xxxxxxxxxxxxx
Subject: [ibis-editorial] Comparing simulated Vpad with VinL
All,
When determining Vpad to an IBIS threshold value (e.g. VinL) we do the
following:
1. When only [Voltage Range] is specified or [Pulldown
Reference]=[GND Clamp Reference]=0. (assumes Pdref=Gcref)
a. Compare (Vpad-gcref) to the value of VinL
b. One might argue that this is really a function of (Vpad-gcref) and
(Pcref-Gcref) and (Puref- Gcref), but IBIS does not tell you that either.
2. When [Pulldown Reference]=[GND Clamp Reference]=DUT_Reference !=
0. (assumes Pdref=Gcref)
a. Compare (Vpad-(gcref- DUT_Reference)) to the value of VinL
3. When [Pulldown Reference] != [GND Clamp Reference] (Pdref !=
Gcref)
a. Can do one of two things
i.
Compare (Vpad-(gcref- [GND Clamp Reference] )) to the value of VinL
ii.
Compare (Vpad-(pdref- [Pulldown Reference] )) to the value of VinL
All SiSoft cares about is 1. Bob's answer to 2 and 3 is to generate
multiple models with different values of [Pulldown Reference] and [GND
Clamp Reference] for each condition that he plans to simulate with these
models.
IBIS is silent on the answer to 3. I point out that since there does not
seem to exist a case in nature (other than IBIS allows it), I think it
will be impossible to find out which is correct 3.a.i or 3.a.ii, since
nobody has ever made (or plans to make) such an I/O buffer.
I feel I am wasting my time in these meetings by trying to resolve what to
do for item 3, and I suggest that we simply state that although IBIS
allows this impossible construct, we leave it up to the User/EDA tool on
how do handle measurement thresholds. We do this already for VinL and VinH
(are they fixed voltage (relative to the local Pdref and Gcref) or are
they really meant to represent percentage of the voltage swing.
I hope that MM's introductory paragraph on this will let us move on.
Walter
Walter Katz
<mailto:wkatz@xxxxxxxxxx> wkatz@xxxxxxxxxx
Phone 303.449-2308
Mobile 303.335-6156
