[SI-LIST] Re: Ring back
- From: "Douglas Burns" <dburns@xxxxxxxxxx>
- To: <leeritchey@xxxxxxxxxxxxx>, "'Madhusudhan Kulkarni'" <madhusudhan@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 22 May 2006 15:26:24 -0400
Lee,
I would argue that overshoot and undershoot are specifying the same =
thing,
while ringback is it's own issue. Overshoot is how much a signal exceeds =
a
desired voltage level and is generally associated with VDD and VSS. Thus =
a
signal is in an overshoot condition when it exceeds VDD (or VSS). I see
people call the event overshoot when the signal exceeds VDD and =
undershoot
when it exceeds VSS...but in reality.. The signal is overshooting the =
target
voltage, independent of VDD or VSS)
Many vendors specifications specify AC and DC overshoot. The max DC =
voltage
a device can accept (example VDD+0.3V or VSS -0.3v) would be a DC =
overshoot
limit. In addition, some vendors provide a time and with another max =
voltage
spec (ie VDD + 1v for <=3D 2ns). This second spec would be an AC =
overshoot
spec.=20
Ringback on the otherhand does not require a signal to overshoot. =
Ringback
occurs when a signal makes multiple crossings of a voltage threshold and
affects timing margins. Use some standard LVTTL logic. Thresholds are =
0.8V
and 2.0V. If a rising signal exceeds 2.0V, the signal is said to have
switched. If however, the signal crosses 2.0V, rises to 3.0V (below VDD =
of
3.3)and then falls below 2.0 and then some time later crosses 2.0V and
settles out, when is the signal valid from a timing perspective. Based =
upon
worst case, it would be the second crossing. The ring between the 1st =
and
2nd crossing is called ringback. Ringback occurs on both rising and =
falling
edges.=20
To help reduce the uncertainty in timing points caused by ringback, =
vendors
have added AC and DC switching levels. A signal must than cross the AC
level, but is allowed to ringback to the DC level without effecting the
device timing.
The IBIS specification does a good job documenting these events.
Douglas J Burns
Vice-President, Consulting Services
Signal Integrity Software, Inc. (SiSoft)
6 Clock Tower Place
Suite 250
Maynard, MA 01754
978-461-0449 x14
dburns@xxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Lee Ritchey
Sent: Monday, May 22, 2006 11:02 AM
To: Madhusudhan Kulkarni; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ring back
Ringback is just another name for undershoot. I don't know why someone =
had
to invent another name for something that already had a name.
When you have undershoot and you use series terminations, the terminator
value it too high or the output impedance of the driver is too high. =
When
you have undershoot and you use parallel terminations, the value of the
terminator is too low. Fix which ever problem you have by changing the
value appropriately.
> [Original Message]
> From: Madhusudhan Kulkarni <madhusudhan@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 5/22/2006 9:28:26 AM
> Subject: [SI-LIST] Ring back
>
> Hi,
> How to solve ringback problem and what is the disadvantages of=20
> ringback
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