![](http://datasheet.mmic.net.cn/390000/SYM53C876_datasheet_16836340/SYM53C876_158.png)
4-66
SYM53C876/876E Data Manual
Registers
SCSI Registers
Setting this bit places all the open-drain
48 mA SCSI drivers into a high-impedance
state. This is to allow internal loopback mode
operation without affecting the SCSI bus.
Bit 2
AWS (Always Wide SCSI)
When this bit is set, all SCSI information
transfers are done in 16-bit wide mode. This
includes data, message, command, status and
reserved phases. Normally, deassert this bit
since 16-bit wide message, command, and
status phases are not supported by the SCSI
specifications.
Bit 1
EXT (Extend SREQ/SACK
Filtering)
Symbios Logic TolerANT SCSI receiver tech-
nology includes a special digital filter on the
SREQ/ and SACK/ pins which causes the dis-
regarding of glitches on deasserting edges.
Setting this bit increases the filtering period
from 30ns to 60ns on the deasserting edge of
the SREQ/ and SACK/ signals.
Note: Never set this bit during fast SCSI
(greater than 5M transfers per second)
operations, because a valid assertion could
be treated as a glitch.
Bit 0
LOW (SCSI Low level Mode)
Setting this bit places the SYM53C876 SCSI
function in low level mode. In this mode, no
DMA operations occur, and no SCRIPTS
execute. Arbitration and selection may be per-
formed by setting the start sequence bit as
described in the SCNTL0 register. SCSI bus
transfers are performed by manually asserting
and polling SCSI signals. Clearing this bit
allows instructions to be executed in SCSI
SCRIPTS mode.
Note: It is not necessary to set this bit for access
to the SCSI bit-level registers (SODL,
SBCL, and input registers).
Register 4Fh
SCSI Test Three (STEST3)
Read/Write
Bit 7
TE (TolerANT Enable)
Setting this bit enables the active negation
portion of Symbios Logic TolerANT technol-
ogy. Active negation causes the SCSI Request,
Acknowledge, Data, and Parity signals to be
actively deasserted, instead of relying on exter-
nal pull-ups, when the SYM53C876 SCSI
function is driving these signals. Active deas-
sertion of these signals occurs only when the
SYM53C876 SCSI function is in an informa-
tion transfer phase. When operating in a dif-
ferential environment or at fast SCSI timings,
TolerANT Active negation should be enabled
to improve setup and deassertion times.
Active negation is disabled after reset or when
this bit is cleared. For more information on
Symbios Logic TolerANT technology, see
Chapter 1.
Note: Set this bit if the Enable Ultra SCSI bit in
SCNTL3 is set.
Bit 6
STR (SCSI FIFO Test Read)
Setting this bit places the SCSI core into a test
mode in which the SCSI FIFO is easily read.
Reading the least significant byte of the
SODL register causes the FIFO to unload.
The functions are summarized in the table
below.
TE
7
STR
6
HSC
5
DSI
4
DIFF
3
TTM
2
CSF
1
STW
0
Default >>>
0
0
0
0
X
0
0
0
Register
Name
Register
Operation
FIFO Bits
FIFO
Function
SODL
SODL0
SODL1
Read
Read
Read
15-0
7-0
15-8
Unload
Unload
None