From: DAmbrosia, John F (firstname.lastname@example.org)
Date: Fri Apr 14 2000 - 13:18:39 PDT
In the past few days there have been some questions regarding BLVDS and HOT
SWAP applications. I am with AMP, and we have been very busy investigating
glitches that have been seen in applications requireing HOT SWAP. It was
questioned whether this was an issue in a BLVDS application, or would the
differential nature make it a non-issue. A comment was made that the
phenomenon would not be seen since it was seen on both signals of the
differential pair simulatenously.
The testing we have done in our labs (for 2mm HM product, as well as any
other separable conductive interface) has shown that we ALMOST NEVER connect
the multiple beams of the contact at the same point in time. For typical
card insertion speeds (this also includes all speeds within certain
specifications), we have measured the time between these connections (of
different contact beams ON THE SAME PIN) on the order of micro-seconds.
For different pins within the same connector, the time between these
connections can be much longer. This separation time depends highly on the
tolerances and design of the mechanical cage supporting the inserted cards,
the tolerancing in the alignment and guidance of the connector housings, and
the human factor (operators or maintenance personnel will insert cards at
different speeds, sometimes not insert them fully, insert them partially
before finishing the insertion, etc., etc.).
Sooo...we have definitely seen (during live insertion scenarios) occurrences
of inserted cards impacting the individual legs of a differential pair
separately. This impact could be separated by micro-seconds or longer.
In addition to these relatively long (with respect to a high-speed waveform)
micro-second separations, we have also recently discovered a much faster
discontinuity in our labs. This new phenomenon also occurs during hot-swap
events and occurs right at the initial time when the conductive interfaces
come in contact with one another. The connections/disconnections between
backplane contact and card contact happen as quickly as nanoseconds and as
slow as micro-seconds. We have been referring to the phenomenons as
nanosecond discontinuities. We have also seen this occur at any highly
conductive separable interface.
The nanosecond discontinuities are extremely erratic and never seem occur
at the same positions or in the same timeframes consistently (even for the
same contact, let alone discrete contacts within the same connector).
The combination of mechanical tolerancing and nanosecond discontinuities
provides us with an extremely wide range of time with which we can separate
the impacts of a hot-swapped card on the individual legs of a differential
waveform. Depending on the logic family you are using, opposing potential
levels (i.e. 0.0-5.0V, 0.0-3.3V, 0.5-1.5V, etc) during an insertion can
resulting in significant current flow to/from the backplane from the
inserted card. The capacitive loading, termination, topology, driver
capabilities, and sampling of the system will all play key roles in the
worst-case analysis of such 'glitches'. We have measured (on 5V systems)
voltage disruptions as high as ~2 V (on a logic level 'low' waveform). On
lower voltage systems (like LVDS or ECL or GTL), we can measure similar
percentages of voltage alteration to an operating waveform.
For those who are interested in a presentation we gave on this phenomenon,
go to www.amp.com/simulation <http://www.amp.com/simulation> . Go to the
"documents" section, and choose "Conference Presentations." Select
"Nanosecond Discontinuity Impact on Hot Swap."
Have a nice day!
AMP Circuits & Design
Program Manager, Strategic Accounts
Tel (717) 986-5692
Fax (717) 986-5095
Email: email@example.com <mailto:firstname.lastname@example.org>
URL: http://www.amp.com/simulation <http://www.amp.com/simulation>
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