Subject: RE: [IBIS-Users] Rising/Falling Waveform generation
From: Lynne Green (lgreen@cadence.com)
Date: Mon Jul 01 2002 - 15:13:04 PDT
Hello, John,
When driving a 50-Ohm load, the swing is not 0-Vcc. (One only gets
full swing when driving a capacitive load.) If you draw a load line for
your driver FET and 50-ohm load, the two intersection points should
match the endpoints in your SPICE simulation. (One load line for
rising and one for falling, since the two FETs probably do not have
perfectly matched I-V curves.)
The value for Rfixture should be near the typical transmission line
load. The designed transmission line impedance (load) varies with
logic family, but values are typically between 50 and 150 Ohms.
Best regards,
Lynne
Dr. Lynne Green
Senior Member of Consulting Staff
Cadence Design Systems, Inc.
"All the world's an analog stage, whereon digital plays bit parts."
-----Original Message-----
From: John Phillippe [mailto:ra6314@email.sps.mot.com]
Sent: Thursday, June 27, 2002 9:28 AM
To: ibis-users@eda.org
Subject: [IBIS-Users] Rising/Falling Waveform generation
Dear Everyone,
I can't seem to generate decent rising and falling waveforms in IBIS.
After looking at the generated spice files, I found that the problem was
with the Rfixture and Vfixture. I found that if I didn't use a test
fixture of any kind, then the waveforms came out correct. However I had
to manually generate them. So I went back and tried monkeying with the
Rfixture values, and found that a 100K resistor seems to allow IBIS to
generate a good waveform.
I've never used an IBIS model in a simulation environment before, and
so I guess my question is regarding the Rfixture, and what it is used
for. Could I manually generate my edges and then specify an Rfixture of
0 ohms in the ibis file? Is having an Rfixture of 100k going to cause
any problems for the end users?
-- John Phillippe SPS, 32 Bit Embedded Controller Division, IC Creation Motorola - 512-895-1835 Austin, TX - ra6314@email.sps.mot.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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