Fast-80 (98-153r6)

Gene_Milligan at Gene_Milligan at
Wed Oct 14 06:34:03 PDT 1998

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* Gene_Milligan at
Bill wrote:

<<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?>>

     Yes this issue was specifically addressed in the working group
meetings when I raised the question as to what it was used for and why it
changed with bus speed. The conclusion was that system deskew delay is not
used during synchronous transfers. It is only used during asynchronous
transfers and other non synchronous bus usage. Then why did it change with
the negotiated bus rate? The conclusion was that it was not intended to. It
was just that the bus rates were defined at different times and as silicon
became faster the engineers sharpened the pencil and for a while forgot
that this time factor was not in the synchronous timing calculations. To
avoid this error in the future the timing table was divided into two tables
to clarify which times were related to the synchronous negotiation.

Bill went on to point out discrepancies in attention condition timing and
wrote: <<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.>>

     I suggest that John Lohmeyer add this issue to the T10 working group
agenda and ask John to decide if this is a protocol WG or physical WG issue
(I assume protocol but silicon implemented). The additional CRC timing from
98-177r6 factors into the discussion.


* For T10 Reflector information, send a message with
* 'info t10' (no quotes) in the message body to majordomo at

More information about the T10 mailing list