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From: rickman on 2 Mar 2010 08:07
On Mar 2, 1:08 am, -jg <jim.granvi...(a)gmail.com> wrote:
> On Mar 2, 6:15 pm, rickman <gnu...(a)gmail.com> wrote:
>
>
>
> > I'm talking about the boards.  I get virtually no ringing with the
> > slower edge rates.  The fastest, 20 mA/FAST, setting gives pronounced
> > ringing and 16 mA has some as well.  They are both 2 ns or less rise
> > times.  At 12 ns the ringing is gone and I only see the tiny notch in
> > the rising edge that I am convinced is not ringing or reflection.
>
> It depends on your terminology.
> The notch I mention below, I would call ringing, as it
> comes from a LCR+Slope model. It is effectively ringing summed onto
> the slowish rise time, and is not huge, but large enough to see a
> slight reverse in voltage.
>
> > BTW, a transmission line does not ring.  You can get reflections, but
> > unless there is some sort of feedback in your IO driver that is
> > oscillating, you won't see true ringing with a transmission line...
> > unless there is something with transmission lines that I didn't
> > learn.  It is often that reflections look like ringing because they
> > echo several times at lower amplitude each time around.  No?
>
> > Rick
>
> Just for fun, I added some guestimate numbers from what you said, and
> added a probe to the driving end, and voila, guess what?
>
> A small notch appears, on the driving end ;)
>
> Which end were you probing, your end, or connector, or the customers
> FPGA pin ?
>
> -jg

The driver. Does it show up at the same time for both 8 and 12 mA
drive?

Rick
From: -jg on 2 Mar 2010 14:25
On Mar 3, 2:07 am, rickman <gnu...(a)gmail.com> wrote:
> On Mar 2, 1:08 am, -jg <jim.granvi...(a)gmail.com> > > A small notch appears, on the driving end ;)
>
> > Which end were you probing, your end, or connector, or the customers
> > FPGA pin ?
>
> > -jg
>
> The driver.  Does it show up at the same time for both 8 and 12 mA
> drive?

That would be by varying the slew rate in the PWL source, and yes, it
comes and goes as the slew rate hits a critical threshold.

Spice says the waveform at the receive end does not have that notch.

-jg
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