LSI LSI53C896, Technical Manual

Page: 138 / 370

4-26 Registers
IARB Immediate Arbitration 1
Setting this bit causes the SCSI core to immediately
begin arbitration once a Bus Free phase is detected
following an expected SCSI disconnect. This bit is useful
for multithreaded applications. The ARB[1:0] bits in
SCSI
Control Zero (SCNTL0) are set for full arbitration and
selection before setting this bit.
Arbitration is retried until won. At that point, the
LSI53C896 SCSI function holds SBSY and SSEL
asserted, and waits for a select or reselect sequence.
The Immediate Arbitration bit is cleared automatically
when the selection or reselection sequence is completed,
or times out.
An unexpected disconnect condition clears IARB with it
attempting arbitration. See the SCSI Disconnect
Unexpected bit (
SCSI Control Two (SCNTL2) , bit 7) for
more information on expected versus unexpected
disconnects.
It is possible to abort an immediate arbitration sequence.
First, set the Abort bit in the
Interrupt Status Zero
(ISTAT0) register. Then one of two things eventually
happens:
•
The Won Arbitration bit ( SCSI Status Zero (SSTAT0),
bit 2) will be set. In this case, the Immediate
Arbitration bit needs to be cleared. This completes the
abort sequence and disconnects the chip from the
SCSI bus. If it is not acceptable to go to Bus Free
phase immediately following the arbitration phase, it is
possible to perform a low level selection instead.
•
The abort completes because the LSI53C896 SCSI
function loses arbitration. This is detected by the
clearing of the Immediate Arbitration bit. Do not use
the Lost Arbitration bit (
SCSI Status Zero (SSTAT0),
bit 3) to detect this condition. In this case take no
further action.
SST Start SCSI Transfer 0
This bit is automatically set during SCRIPTS execution
and should not be used. It causes the SCSI core to begin
a SCSI transfer, including SREQ/ and SACK/
handshaking. The determination of whether the transfer
is a send or receive is made according to the value
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