2.2.5 Designing an Ultra SCSI System

Since Ultra SCSI is based on existing SCSI standards, it can use existing driver programs as long as the software is able to negotiate for Ultra SCSI synchronous transfer rates. Additional software modifications are needed to take advantage of the new features in the LSI53C875A.

For additional information on Ultra SCSI, refer to the SPI-2 working document which is available from the SCSI BBS referenced at the beginning of this manual. Chapter 6, “Electrical Specifications,”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 SCSI Test Three (STEST3) register, whenever the Ultra Enable bit is set.

Do not extend the SREQ/SACK filtering period with SCSI Test Two (STEST2) bit 1. When the Ultra Enable bit is set, the filtering period is fixed at 15 ns for Ultra SCSI, regardless of the value of the SREQ/SACK Filtering bit.

Use the SCSI clock quadrupler.

A 20 or 40 MHz input must be supplied if using the SCSI clock quadrupler for an Ultra design.

2.2.5.1Using the SCSI Clock Quadrupler

The LSI53C875A can quadruple the frequency of a 20 MHz SCSI clock, allowing the system to perform Ultra SCSI transfers. This option is user selectable with bit settings in the SCSI Test One (STEST1), SCSI Test Three (STEST3), and SCSI Control Three (SCNTL3) registers. At power-on or reset, the quadrupler is disabled and powered down. Follow these steps to use the clock quadrupler:

Step 1. Set the SCLK Quadrupler Enable bit (SCSI Test One (STEST1), bit 3).

Step 2. Poll bit 5 of the SCSI Test Four (STEST4) register.

The LSI53C875A sets this bit as soon as it locks in the quadrupled frequency. The frequency lockup takes approximately 100 s.

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Functional Description

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LSI 53C875A technical manual Designing an Ultra Scsi System, Using the Scsi Clock Quadrupler