
2-22
SYM53C876/876E Data Manual
Functional Description
SCSI Functional Description
Designing a Wide Ultra SCSI System
Migrating an existing SCSI design from Fast
SCSI to Wide Ultra SCSI requires minor software
modifications as well as consideration for some
hardware design guidelines. Since Wide Ultra
SCSI is based on existing SCSI standards, it can
use existing software programs as long as the soft-
ware is able to negotiate for Wide Ultra SCSI syn-
chronous transfer rates.
In the area of hardware, the primary area of con-
cern in single-ended systems is to maintain signal
integrity at high data transfer rates. To assure reli-
able operation at Wide Ultra SCSI transfer
speeds, follow the system design parameters rec-
ommended in the Wide Ultra SCSI Parallel Inter-
face draft standard, which is available from the
SCSI Web Site referenced at the beginning of this
manual. Chapter 6 contains Wide Ultra SCSI
timing information. In addition to the guidelines
in the draft standard, make the following software
and hardware adjustments to accommodate Wide
Ultra SCSI transfers:
n
Set the Wide Ultra SCSI Enable bit to enable
Wide Ultra SCSI transfers.
n
Set the TolerANT Enable bit, bit 7 in the
STEST3 register whenever the Wide Ultra
SCSI Enable bit is set.
n
Do not extend the SREQ/SACK filtering
period with STEST2 bit 1.
n
Use a 40 MHz SCSI clock with an internal
clock doubler.
Figure 2-7: Determining the Synchronous Transfer Rate
SCF2
0
0
0
1
0
SCF1
0
1
1
0
0
SCF0
1
0
1
0
0
SCF
Divisor
1
1.5
2
3
3
CCF2
0
0
0
1
0
CCF1
0
1
1
0
0
CCF0
1
0
1
0
0
CCF
Divisor
1
1.5
2
3
3
TP2
0
0
0
0
1
1
1
1
TP1
0
0
1
1
0
0
1
1
TP0
0
1
0
1
0
1
0
1
XFERP
Divisor
4
5
6
7
8
9
10
11
SCLK
40 MHz
SCF
Divider
CCF
Divider
This point must not
exceed 80 MHz
This point must not
exceed 25 MHz
Synchronous
Divider
Asynchronous
SCSI Logic
To
SCSI Bus
Example:
SCLK = 40 MHz, SCF = 1, XFERP = 4,
SCSI transfer rate = 10 MHz, CCF = 2
(40 MHz 1 = synchronous core rate)
(40 MHz 4 = 10 MHz synchronous rate = 10 MB/s
on an 8-bit SCSI bus)
Send
Clock
Receive
Clock
Divide by 4
CLK
Doubler