Subject: Re: [IBIS-Users] Buffer ramp rates & loading question.
From: John Phillippe (john.phillippe@motorola.com)
Date: Fri Feb 14 2003 - 12:14:49 PST
Lynne Green wrote:
>
> 2. I have some slow I/O drivers that take a long time to settle for the
> rising and falling curves. To help the curves settle faster, I want to
> use 300 Ohms for the load. Normally I would use 50 Ohms load for the I/O
> drivers. I am wondering if I can use 300 Ohms load for the slow drivers?
> Is it going to be a simulator problem if some of the buffers use 50 Ohm
> loads, and others 300 Ohms loads?
>
>
>>>V-t tables should be built with the nominal load you expect to see in
>>
> an actual PCB trace. If the driver is intended for use with a wide
> range of trace impedances, you could build one set of V-t tables for the
> maximum load (say 300 Ohms), and another set of V-t tables for the
> minimum load (say 50 Ohms). IBIS supports up to 100 waveform tables.
>
>>>Simulators can use more than one set of V-t tables. A model is
>>
> generally more accurate if one set of V-t tables is "near" the actual
> load. (For example, if a driver could be used with any load from 50-300
> Ohms, one could build a set of V-t tables at 50, another at 300, and
> perhaps others in between.)
I remember a while back that some people on this group had recommended
to just use the 50 ohm load and nothing else for V/T curves. Now what I
am sort of infering from the above post is to try and use a loading that
would accurately represent how the chip will be used in a board
environment. So this brings up a few new questions. FYI - I am using
S2ibis2 for ibis creation.
1. I have been under the assumption that in an IBIS model, the I/V
curves are what the simulator uses when doing board simulations. The
V/T curves are simply a nice little reference, but are not used for any
simulations. I.E. If I set my ibis model up in a simulator and then
put a bunch of 50 ohm loads to ground, and then simulate some edges, the
V/T curves are just there to "double check" the simulations. Is this a
correct way of thinking?
2. If the above is correct, than it would make sense to add more than
just the 50 ohm load models. I would want to add capacitative loads and
what not to simulate the original design specs. I.E. If a buffer was
designed to drive a 25pF load, than I would want to add that 25pF V/T
curve for reference.
3. In the V/T curve section (of s2ibis2 scripts) you can add bond wire
LRC values for the pin. It seems to me that I should leave this blank
when modeling, because when I put the chip together the LRC bond wire
values will be placed up in the pin list, and I would be "double
counting" the LRC values if I modeled them with the individual buffers
and also put them in the PIN list of the ibis model.
Thanks for all the good information so far.
-- John Phillippe SPS, 32 Bit Embedded Controller Division, IC Creation Motorola - 512-895-1835 Austin, TX - john.phillippe@motorola.com|------------------------------------------------------------------ |For help or to subscribe/unsubscribe, email majordomo@eda.org |with just the appropriate command message(s) in the body: | | help | subscribe ibis <optional e-mail address, if different> | subscribe ibis-users <optional e-mail address, if different> | unsubscribe ibis <optional e-mail address, if different> | unsubscribe ibis-users <optional e-mail address, if different> | |or email a written request to ibis-request@eda.org. | |IBIS reflector archives exist under: | | http://www.eda.org/pub/ibis/email_archive/ Recent | http://www.eda.org/pub/ibis/users_archive/ Recent | http://www.eda.org/pub/ibis/email/ E-mail since 1993
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