Bill Galloway BillG at
Tue Feb 2 18:20:52 PST 1999

* From the T10 Reflector (t10 at, posted by:
* "Bill Galloway" <BillG at>
I am still trying to clean up the ATN documentation for SPI-3.

As a general rule the initiator must set up ATN two system deskew delays
before the negation of ACK. I found two exceptions to this rule. I know of
no reason why these exceptions are here. Is there a reason for these
exceptions or are they just vestiges of a bygone era???

ATN Assertion:

Only for a MSG OUT retry does the initiator have to set up the assertion of
ATN to the asserting edge of ACK. I assume that the target is going to
sample it on the negating edge of ACK (like all other times), so why is this
requirement here???  Here is the paragraph for your reference. MESSAGE OUT phase exception condition handling
If the target detects one or more parity error(s) on the message
byte(s)received, it may indicate its desire to retry the message(s) by
asserting the REQ signal after detecting the ATN signal has gone false and
prior to changing to any other phase. The initiator, upon detecting this
condition, shall resend all of the previous message byte(s) in the same
order as previously sent during this phase. When resending more than one
message byte, the initiator shall assert the ATN signal at least two system
deskew delays prior to asserting the ACK signal on the first byte and shall
maintain the ATN signal asserted until the last byte is sent as described in

ATN Negation:

Why is ATN assertion setup to the negating edge of ACK but ATN negation set
up to the asserting edge of ACK? It would make more sense to me to make them
both to the negating edge of ACK. Here is a paragraph out of 11.2.1 for your

The initiator shall keep the ATN signal asserted if more than one byte is to
be transferred. The initiator may negate the ATN signal at any time except
it shall not negate the ATN signal while the ACK signal is asserted during a
MESSAGE OUT phase. Normally, the initiator negates the ATN signal while the
REQ signal is true and the ACK signal is false during the last REQ/ACK
handshake of the MESSAGE OUT phase.

Comments appreciated...

Bill Galloway
BREA Technologies, Inc.
P: (281) 530-3063
F: (281) 988-0358
BillG at

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