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4-64
SYM53C876/876E Data Manual
Registers
SCSI Registers
Bit 1
SOZ (SCSI Synchronous Offset
Zero)
This bit indicates that the current synchro-
nous SREQ/SACK offset is zero. This bit is
not latched and may change at any time. It is
used in low level synchronous SCSI opera-
tions. When this bit is set, the SYM53C876
SCSI function, as an initiator, is waiting for
the target to request data transfers. If the
SYM53C876 SCSI function is a target, then
the initiator has sent the offset number of
acknowledges.
Bit 0
SOM (SCSI Synchronous Offset
Maximum)
This bit indicates that the current synchro-
nous SREQ/SACK offset is the maximum
specified by bits 3-0 in the SCSI Transfer reg-
ister. This bit is not latched and may change at
any time. It is used in low level synchronous
SCSI operations. When this bit is set, the
SYM53C876 SCSI function, as a target, is
waiting for the initiator to acknowledge the
data transfers. If the SYM53C876 SCSI func-
tion is an initiator, then the target has sent the
offset number of requests.
Register 4Dh
SCSI Test One (STEST1)
Read/Write
Bit 7
SCLK
When set, this bit disables the external SCLK
(SCSI Clock) pin, and the chip uses the PCI
clock as the internal SCSI clock. If a transfer
rate of 10 Mb/s (or 20 MB/s on a wide SCSI
bus) is desired on the SCSI bus, this bit must
be reset and at least a 40 MHz external SCLK
must be provided.
Bit 6
ISO_MODE (SCSI Isolation Mode)
This bit allows the SYM53C876 SCSI func-
tion to put the SCSI bi-directional and input
pins into a low power mode when the SCSI
bus is not in use. When this bit is set, the
SCSI bus inputs are logically isolated from the
SCSI bus.
Bit 5
Reserved
Bit 4
Reserved
Bit 3
DBLEN (SCLK Doubler Enable)
This bit, when reset, powers down the internal
clock doubler circuit, which doubles the
SCLK 40 MHz clock to an internal 80 MHz
SCSI clock required for Wide Ultra SCSI
operation. Both the SCLK Doubler Enable
DBLEN and SCLK Double Select DBLSEL
bits must be set in either SCSI function to get
the internal 80 MHz SCSI clock.
Bit 2
DBLSEL (SCLK Doubler Select)
This bit, when set, selects the output of the
internal clock doubler for use as the internal
SCSI clock. When reset, this bit selects the
clock presented on SCLK for use as the inter-
nal SCSI clock.
SCLK
7
Default >>>
0
ISO
6
RES
5
RES
4
DBLEN
3
DBLSEL
2
RES
1
RES
0
0
X
X
0
0
X
X