Subject: RE: [IBIS] Time scale for VT plots on IBIS model.
From: Baumann, Hans-Gerhard (HGBaumann@ti.com)
Date: Fri Nov 15 2002 - 01:09:51 PST
All,
thanks for your helpful hints and recommendations to this subject. My
conclusion is not to use a negative delay.
Actually I came to this subject as I generated the IBIS model for a new part
which features to have a normal buffer (delay 2-4ns) or zero delay buffer
(PLL) in its signal path. The zero delay in the PLL-mode of course only
looks like 0ns delay, but it behaves like this once the PLL is locked.
As I completed the VT-tables for the buffer (2-4ns delay on WEAK,NOM,STRONG
columns) with its relation on time scale (strong model transition is before
weak model transition), I thought about how to setup the VT-tables if the
device works in the PLL-mode. Actually the process and the VDD dependency
for a zero delay buffer (PLL-mode) is significant lower and I concluded to
remove the time shift as it is shown for the real buffer mode.
Next step was how to set the starting point for the transition, very simple
for the real buffer. 50% of swing at the output represents the propagation
delay (50% at input is set to 0ns). To set a 50% point at the output to 0ns
for the PLL mode buffer would require a start <0ns to print the whole
transition. That was my trigger to get your recommendation.
I do not care too much about the "large" lead-in time (normal buffer), since
I think that most of all IBIS simulators provide the option to strip (or
keep) non-switching time from the VT-tables if the delay of a buffer is not
of any interest.
Finally I set the delay for the PLL to the shortest value possible to keep
>0ns. As I run the model I noticed of course that WEAK/STRONG variance in
delay still is larger than the PLL actually would have, but in fact that is
something what can not be modeled in IBIS at all. It is a result of the
function of the internal feedback, which compensates for process, VDD and
load. So the setting for the initial delay finally gets less important too.
Thanks and Best Regards,
Gerd Baumann
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