[SI-LIST] Re: Relation between slew rate and ISI
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>
- Date: Thu, 8 May 2008 09:24:54 -0700
I'm surprised at the quick dismissal of midbus probing. A search of
"PCI Express Midbus probe" at Agilent, Tek, or CATC's websites (perhaps
others) will show solutions that were developed for that bus. That's
not to say that the solution is trivial, but it's possible, at least up
to those frequencies. And yes, you will have to find a solution that
ensures adequate "eyes" at both the logic analyzer and bus receivers,
each with independent parameters.
Of course, it won't be feasible for every bus (there's dependencies on
the termination scheme, etc.), but I don't think it can be automatically
excluded as a possible solution. The alternative (being able to probe
directly at the receiver) often isn't practical.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Preethi Ramaswamy
Sent: Thursday, May 08, 2008 8:49 AM
To: Curt McNamara
Cc: wolfgang.maichen@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Relation between slew rate and ISI
I don't deny what you guys are saying. Wolfang, my simulations are
totally
in sync with what you are saying.
Like I said, these simulation are to prove that we have to let go off
this
technology of probing at any place other than at the receiver.
This is the first time I posted on this site and I must say I'm totally
impressed by the number of responses.
Thanks guys !
Preethi
On 5/8/08, Curt McNamara <CurtM@xxxxxxxxxxx> wrote:
>
> In your case this may not be a good plan. If the signal edge were
perfect
> and the logic analyzer thresholds set exactly to your receiver it
could be
> OK. However we have been talking about reflections which cause edge
> discontinuities. These could overlap either switching threshold or
occur in
> the transition region between the defined Vih and Vil. Any of these
> situations could cause sampling errors.
>
>
>
> And as Wolgang has noted, the signal may be good at the receiver yet
look
> bad in the line, or be bad at the receiver yet look good in the line.
>
>
>
>
> Curt
>
>
>
> Curt McNamara, P.E. // principal electrical engineer
> Logic Product Development
> 411 Washington Ave. N. Suite 400
> Minneapolis, MN 55401
> T // 612.436.5178
> F // 612.672.9489
> www.logicpd.com
> / / / / / / / / / / / / / / / / / / / / / / / / /
> This message (including any attachments) contains confidential
information
> intended for a specific individual and purpose, and is protected by
law.
> If you are not the intended recipient, you should delete this message
and
> are hereby notified that any disclosure, copying, or distribution of
this
> message, or the taking of any action based on it, is strictly
prohibited.
>
>
>
>
>
>
>
> *From:* Preethi Ramaswamy [mailto:preethi.gowtham@xxxxxxxxx]
> *Sent:* Wednesday, May 07, 2008 7:34 PM
> *To:* wolfgang.maichen@xxxxxxxxxxxx
> *Cc:* Curt McNamara; si-list@xxxxxxxxxxxxx;
si-list-bounce@xxxxxxxxxxxxx
> *Subject:* Re: [SI-LIST] Re: Relation between slew rate and ISI
>
>
>
> Totally agree. From these simulations trying to prove that at higher
speeds
> this kind of probing is not viable. Just need to clarify that, this is
not
> scope probing, this is logic analyzer probing. So I really don't care
much
> about how the signal looks. All I care is if the bit is sampled
correctly as
> a 1 or a 0.
>
> On 5/7/08, *wolfgang.maichen@xxxxxxxxxxxx*
<wolfgang.maichen@xxxxxxxxxxxx>
> wrote:
>
>
> Probing in the middle of the line can have its pitfalls when trying to
make
> sense of the observed waveform (and relating those to the "true"
waveforms
> seeing at the receive)r. One such issue arises when the receiver does
not
> provide perfectly matched termination (i.e. receiver impedance
different
> from line impedance, but also - which is always the case - when the
receiver
> has some input capacitance). In this case - if your probe is further
away
> from the receiver than a small fraction of a rise time - you will
observe
> two distict partial partial transitions, while at the receiver you
would see
> both lumped together at the same time (i.e. only a single transition).
>
> Extreme case as an example (easy to simulate e.g. in PSpice): 50 Ohm
driver
> driving a 50 Ohm transmission line with a 1V step, receiver is
completely
> unterminated (high impedance). The driver is thus launching a 500mV
step
> into the line (voltage divider - 50 Ohm driver + 50 Ohm transmission
line).
> The receiver will see a full 1V step (500mV incident + 500mV
reflected),
> but a probe looking at the middle of the line will see a 500mV step,
and
> then (one round-trip delay to the receiver and back later) as second
500mV
> step on top of it.
>
> Trouble is, if you are "somewhat" close to the receiver those two
> transitions may partially overlap and you may think the waveform
fidelity is
> poor, when in reality all is fine at the receiver end.
>
> Wolfgang
>
>
>
>
> *"Preethi Ramaswamy" <preethi.gowtham@xxxxxxxxx>*
> Sent by: si-list-bounce@xxxxxxxxxxxxx
>
> 05/07/2008 04:12 PM
>
> To
>
> "Curt McNamara" <CurtM@xxxxxxxxxxx>
>
> cc
>
> si-list@xxxxxxxxxxxxx
>
> Subject
>
> [SI-LIST] Re: Relation between slew rate and ISI
>
>
>
>
>
>
> Hi Curt
> For my application, I'm trying to tap the signal to feed to a
probe.
> The best point to place a probe is at the receiver but it isn't always
> possible.
> Hence, I'm trying out a few options, one of them being tapping the
signal
> before the receiver.
> Preethi
>
> On 5/7/08, Curt McNamara <CurtM@xxxxxxxxxxx> wrote:
> >
> > This may have been covered already by Wolfgang's excellent
responses:
> > The wave arrives at the load and any reflected energy is almost no
prop
> > delay away.
> > In contrast, at other points on the line the reflected energy from
the
> > load (or other discontinuity) arrives 2 x tpd later (where tpd is
the
> delay
> > from the measurement point to the reflection).
> >
> > This is one reason we may not care about intermediate points on the
line:
> > at the load we need to see signal quality and establish that there
are no
> > voltage levels beyond our limits. At the source signal quality may
not be
> as
> > critical, but checking voltage levels is important.
> >
> > Why would we care about intermediate points? They could affect
crosstalk
> > or emissions. Any other perspectives on this?
> >
> > Curt
> >
> >
> > Curt McNamara, P.E. // principal electrical engineer
> > Logic Product Development
> > 411 Washington Ave. N. Suite 400
> > Minneapolis, MN 55401
> > T // 612.436.5178
> > F // 612.672.9489
> > www.logicpd.com
> > / / / / / / / / / / / / / / / / / / / / / / / / /
> > This message (including any attachments) contains confidential
> information
> > intended for a specific individual and purpose, and is protected by
law.
> If
> > you are not the intended recipient, you should delete this message
and
> are
> > hereby notified that any disclosure, copying, or distribution of
this
> > message, or the taking of any action based on it, is strictly
prohibited.
> >
> >
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of Preethi Ramaswamy
> > Sent: Tuesday, May 06, 2008 5:52 PM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Relation between slew rate and ISI
> >
> > I'm looking for some information on the relation between slew rate
and
> ISI
> > effect on high speed memory data signals.
> > From my SI simulation I'm observing that a signal looks much worse
with a
> > higher slew rate than a lower slew rate. But the point I'm tapping
is not
> > at
> > the receiver but a point before the receiver. I don't expect the
signal
> to
> > look as good as at the receiver but I was hoping that the trends
match.
> At
> > the receiver itself, the fast corner signal looks better than the
slow
> > corner signal. The bus is properly terminated.
> > Looking at the waveform, I see that in the fast corner case,
whenever
> > there
> > is a 1010 pattern, the signal is not reaching its intended Vhigh and
Vlow
> > level. A similar thing happens at the slow corner but the signal
swing is
> > much better. Hence, the eye diagram in the fast corner is almost
closed
> > whereas the slow corner looks open.
> >
> > While searching some artciles on this, I found the opposite stated.
It
> > said
> > that ISI effects are more pronounced in the slow corner case and
> Crosstalk
> > is more pronounced in the fast corner case.
> >
> > Any experience or insight into this topic would be very helpful.
> >
> >
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