[SI-LIST] Re: Skew and Jitter

Good point, Douglas.  I was assuming, but not stating, that either:

a) the HSpice model somehow modeled the whole path through the device, or
b) the rise/fall delays from the device input to the input of the output
buffer were balanced

If either of those assumptions are true, the HSpice results should be
correct.

Todd.

Todd Westerhoff
High Speed Design Specialist
Cisco Systems
1414 Massachusetts Ave - Boxboro, MA - 01719
email:twesterh@xxxxxxxxx
ph: 978-936-2149
============================================

"When did the choices get so hard, with so much more at stake?
 Life gets mighty precious when there's less of it to waste"

- Bonnie Raitt, "Nick of Time"


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of SI Eng
Sent: Monday, March 31, 2003 4:35 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Skew and Jitter


Todd,  Thanks for your response. Correct me if I'm wrong here, but if the
issue is that time 0 wouldn't be identical for Rise and Fall edges, this
would apply to a Spice simulation of the I/O cell as well. If the prop.
delays for Rise and Fall are different through a device, then to set time 0
at the input to the I/O buffer - where the stimulus is applied - would give
an erroneous result when the Rise and Fall waveforms are merged for an eye
diagram. The relative positions between Rise and Fall would be wrong. One
would have to know the exact delay of Rise vs. Fall of the internal path
from the output flop to the input of the I/O Buffer in order to set the
relative position of Rise and Fall corrrectly. No ? Douglas  ----- Original
Message ----- From: "Todd Westerhoff" <twesterh@xxxxxxxxx>To:
<si-list@xxxxxxxxxxxxx>Sent: Monday, March 31, 2003 10:28 AMSubject:
[SI-LIST] Re: Skew and Jitter

>
> IBIS simulates output behavior only, it doesn't try to predict the
behavior
> through the part in question.  If you can simulate the pieces of your
chain
> individually and then sum the jitter contributions from the pieces, IBIS
may
> work for you.
>
> Eye diagrams and IBIS are a special discussion in their own right.  You
> should be aware that rising and falling waveforms are usually extracted
> independently for IBIS mnodels, and that "time 0" in the waveform tables
is
> not necessarily meaningful (as in, it might be the time the output starts
to
> switch, and it might not).  If you're going to run pattern simulations and
> look at eye-digrams using IBIS models, you need to be sure that "time 0"
is
> correlated across the rising and fallinng waveform table data.  If it
isn't,
> you could be making assumptions that aren't necessarily valid.
>
> If you have the time and resources to do so, it's worthwhile to study the
> problem using both Spice and IBIS models initially, then move over to IBIS
> if you find good correlation between the two approaches.
>
> Todd.
>
> Todd Westerhoff
> High Speed Design Specialist
> Cisco Systems
> 1414 Massachusetts Ave - Boxboro, MA - 01719
> email:twesterh@xxxxxxxxx
> ph: 978-936-2149
> ============================================
>
> "When did the choices get so hard, with so much more at stake?
>  Life gets mighty precious when there's less of it to waste"
>
> - Bonnie Raitt, "Nick of Time"
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of SI Eng
> Sent: Sunday, March 30, 2003 9:39 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Skew and Jitter
>
>
>
> Gurus :
>
> How does one determine the eye diagram at a receiver from simulation using
> IBIS models, when there are multiple devices in the path from the driver ?
> Each device in the path will contribute to the jitter and skew which will
> affect the quality of the eye at the receiver. Does this need to be done
> using Spice ?
>
> Thanks,
>
>     Douglas
>
>
>
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