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4-20
SYM53C876/876E Data Manual
Registers
SCSI Registers
Register 00h
SCSI Control Zero (SCNTL0)
Read/Write
Bit 7
ARB1 (Arbitration Mode bit 1)
Bit 6
ARB0 (Arbitration Mode bit 0)
Simple Arbitration
1. The SYM53C876 SCSI function waits for a
bus free condition to occur.
2. It asserts SBSY/ and its SCSI ID (contained
in the SCID register) onto the SCSI bus. If
the SSEL/ signal is asserted by another SCSI
device, the SYM53C876 SCSI function
deasserts SBSY/, deasserts its ID and sets the
Lost Arbitration bit (bit 3) in the SSTAT0
register.
3. After an arbitration delay, the CPU should
read the SBDL register to check if a higher
priority SCSI ID is present. If no higher
priority ID bit is set, and the Lost Arbitration
bit is not set, the SYM53C876 SCSI function
wins arbitration.
4. Once the SYM53C876 SCSI function wins
arbitration, assert SSEL/ via the SOCL for a
bus clear plus a bus settle delay (1.2
μ
s) before
a low level selection is performed.
Full Arbitration, Selection/Reselection
1. The SYM53C876 SCSI function waits for a
bus free condition.
2. It asserts SBSY/ and its SCSI ID (the highest
priority ID stored in the SCID register) onto
the SCSI bus.
3. If the SSEL/ signal is asserted by another
SCSI device or if the SYM53C876 SCSI
function detects a higher priority ID, the
SYM53C876 SCSI function deasserts BSY,
deasserts its ID, and waits until the next bus
free state to try arbitration again.
4. The SYM53C876 SCSI function repeats
arbitration until it wins control of the SCSI
bus. When it wins, the Won Arbitration bit is
set in the SSTAT0 register, bit 2.
5. The SYM53C876 SCSI function performs
selection by asserting the following onto the
SCSI bus: SSEL/, the target’s ID (stored in
the SDID register), and the SYM53C876’s
ID (stored in the SCID register).
6. After a selection is complete, the Function
Complete bit is set in the SIST0 register, bit
6.
7. If a selection time-out occurs, the Selection
Time-Out bit is set in the SIST1 register, bit
2.
Bit 5
START (Start Sequence)
When this bit is set, the SYM53C876 starts
the arbitration sequence indicated by the
Arbitration Mode bits. The Start Sequence bit
is accessed directly in low-level mode; during
SCSI SCRIPTS operations, this bit is con-
trolled by the SCRIPTS processor. Do not
start an arbitration sequence if the connected
(CON) bit in the SCNTL1 register, bit 4,
indicates that the SYM53C876 is already con-
nected to the SCSI bus. This bit is automati-
cally cleared when the arbitration sequence is
complete. If a sequence is aborted, check bit 4
ARB1
ARB0
STAR
T
5
WATN
EPC
RES
AAP
TRG
7
6
4
3
2
1
0
Default >>>
1
1
0
0
0
X
0
0
ARB1
ARB0
Arbitration Mode
0
0
1
1
0
1
0
1
Simple arbitration
Reserved
Reserved
Full arbitration, selection/reselection