Fast-80 (98-153r6)

Bill Galloway BillG at
Tue Oct 13 19:18:51 PDT 1998

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* "Bill Galloway" <BillG at>

I noticed that your document specifies a "System Deskew Delay" of 45ns.
This number does not change with negotiated speed.

In previous versions of SPI this number decreased with increasing bus
speeds.  Was this change intentional?

The reason I noticed is it changes the timing of ATN to create an attention
condition.  The current spec is that ATN shall be asserted 2 system deskew
delays (90ns) before the negation of ACK.  The more I look at this the more
this seems broken.  The current spec makes sense for async transfers but
starts breaking down for faster sync transfers.  The entire nagation period
for Fast-40 is only 8ns.  It does not make sense to setup ATN 90ns before an
8ns negation.  It gets even worse when you add double transition.

I think the attention condition spec is ok for async transfers but should be
changed for sync data transfers.  I think that ATN should be asserted a
"Transmit Setup Time" before the negation of ACK.

Bill Galloway
BREA Technologies Inc.
(281) 530-3075
BillG at

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