DiffSense Timing

Bill Galloway BillG at breatech.com
Thu Nov 19 19:34:37 PST 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* "Bill Galloway" <BillG at breatech.com>
*
I have been reviewing SPI-3 looking at the timing for driver mode changes.
The only timing that I can find is in section 7.2.5.2 LVD DIFFSENS receiver.
This section states that a device shall not change its mode until a new mode
has been present for at least 100ms.

I seem to remember that the large value of 100ms was chosen to rule out the
possibility of 60Hz noise disrupting the system by causing mode changes.

Now for the problems.  There is no maximum time specified.  With no maximum
time,
the initiator does not know how long it must delay after a mode change. The
bus is
dead until all devices switch to the new mode.  I think the maximum should
be about
300ms.

There are no timing specifications on the terminator whatsoever.  This seem
to
defeat the desire that 60Hz noise not disrupt the system. I think the
terminators
should obey the same timing rules as devices.

I have talked to a few people who said it was never intended that mode
transitions
happen in a real system, it is only a stocking issue. This is NOT what
companies are
shipping.  Companies are shipping systems that expect hot plug to work, even
if it
involves a mode change.  For mode changes to work in live systems these
times have
to be specified.


I will write up a proposal along these lines unless someone points out the
error in
my ways :-)


Bill Galloway
BREA Technologies, Inc.
(281) 988-0358
BillG at breatech.com

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