
4-26
SYM53C876/876E Data Manual
Registers
SCSI Registers
Bit 3
EWS (Enable Wide SCSI)
When this bit is clear, all information transfer
phases are assumed to be eight bits, transmit-
ted on SD7-0/, SDP0/. When this bit is
asserted, data transfers are done 16 bits at a
time, with the least significant byte on SD7-0/
, SDP/ and the most significant byte on
SD15-8/, SDP1/. Command, Status, and
Message phases are not affected by this bit.
Bits 2-0 CCF2-0 (Clock Conversion Factor)
These bits select a factor by which the fre-
quency of SCLK is divided before being pre-
sented to the SCSI core. The synchronous
portion of the SCSI core can be run at a dif-
ferent clock rate for fast SCSI, using the Syn-
chronous Clock Conversion Factor bits. The
bit encoding is displayed in the table below.
All other combinations are reserved.
Note: It is important that these bits are set to the
proper values to guarantee that the
SYM53C876 meets the SCSI timings as
defined by the ANSI specification.
Register 04h
SCSI Chip ID (SCID)
Read/Write
Bit 7
Reserved
Bit 6
RRE (Enable Response to
Reselection)
When this bit is set, the SYM53C876 SCSI
function is enabled to respond to bus-initiated
reselection at the chip ID in the RESPID0
and RESPID1 registers. Note that the chip
does not automatically reconfigure itself to
initiator mode as a result of being reselected.
Bit 5
SRE (Enable Response to Selection)
When this bit is set, the SYM53C876 SCSI
function is able to respond to bus-initiated
selection at the chip ID in the RESPID0 and
RESPID1 registers. Note that the chip does
not automatically reconfigure itself to target
mode as a result of being selected.
Bit 4
Reserved
Bits 3-0 Encoded Chip SCSI ID, bits 3-0
These bits store the SYM53C876 SCSI func-
tion encoded SCSI ID. This is the ID which
the chip asserts when arbitrating for the SCSI
bus. The IDs that the SYM53C876 SCSI
function responds to when selected or rese-
lected are configured in the RESPID0 and
RESPID1 registers. The priority of the 16
possible IDs, in descending order is:
SCF2
CCF2
SCF1
CCF1
SCF0
CCF0
Factor
Frequency
SCSI Clock
(MHz)
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
SCLK/3
SCLK/1
SCLK/1.5
SCLK/2
SCLK/3
SCLK/4
SCLK/6
SCLK/8
50.01-75.0
16.67-25.0
25.01-37.5
37.51-50.0
50.01-75.0
75.01-80.00
120
160
RES
7
Default >>>
X
RRE
6
SRE
5
RES
4
ENC3
3
ENC2
2
ENC1
1
ENC0
0
0
0
X
0
0
0
0
Highest
4
Lowest
7
6
5
3
2
1
0
15 14
13 12
11
10
9
8