2 designing an ultra scsi system, 1 using the scsi clock doubler, Designing an ultra scsi system – Avago Technologies LSI8751D User Manual
Page 28: Using the scsi clock doubler, Section 2.2, “designing an ultra scsi system
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Functional Description
SDMS software includes a SCSI BIOS to manage all SCSI functions
related to the device. It also provides a series of SCSI device drivers that
support most major operating systems. SDMS software supports a
multithreaded I/O application programming interface (API) for user
developed SCSI applications. SDMS software supports both the ASPI
and CAM SCSI software specifications.
2.2 Designing an Ultra SCSI System
Migrating an existing SE SCSI design from SCSI-2 to Ultra SCSI requires
minor software modifications as well as consideration for some hardware
design guidelines. Since Ultra SCSI is based on existing SCSI standards,
it can use existing software programs as long as the software is able to
negotiate for Ultra SCSI synchronous transfer rates.
In the area of hardware, the primary area of concern in SE systems is to
maintain signal integrity at high data transfer rates. To assure reliable
operation at Ultra SCSI transfer speeds, follow the system design
parameters recommended in the SCSI-3 Ultra Parallel Interface
standard.
Chapter 7, “Instruction Set of the I/O Processor,”
contains Ultra
SCSI timing information. In addition to the guidelines in the draft
standard, make the following software and hardware adjustments to
accommodate Ultra SCSI transfers:
•
Set the Ultra Enable bit to enable Ultra SCSI transfers.
•
Set the TolerANT Enable bit, bit 7 in the
register whenever the Ultra Enable bit is set.
•
Do not extend the SREQ/SACK filtering period with
bit 1.
2.2.1 Using the SCSI Clock Doubler
The LSI53C875 can double the frequency of a 40–50 MHz SCSI clock,
allowing the system to perform Ultra SCSI transfers in systems that do
not have 80 MHz clock input. This option is user selectable with bit
settings in the
,
and
registers. At power-on or reset, the
doubler is disabled and powered down. Follow these steps to use the
clock doubler: