
SCSI Functional Description
2-21
When bit 5 (LED_CNTL) in the
General Purpose Pin Control (GPCNTL)
register is set and bit 6 (Fetch Enable) in the GPCNTL register is cleared
and the SYM53C896 is not performing an EEPROM autodownload, then
bit 3 (CON) in the
Interrupt Status Zero (ISTAT0)
register will be presented
at the GPIO_0 pin.
The CON (Connected) bit in
Interrupt Status Zero (ISTAT0)
will be set
anytime the SYM53C896 is connected to the SCSI bus either as an
initiator or a target. This will happen after the SYM53C896 has
successfully completed a selection or when it has successfully
responded to a selection or reselection. It will also be set when the
SYM53C896 wins arbitration in low level mode.
2.2.5 Designing an Ultra2 SCSI System
Since Ultra2 SCSI is based on existing SCSI standards, it can use
existing driver programs as long as the software is able to negotiate for
Ultra2 SCSI synchronous transfer rates. Additional software
modifications may be needed to take advantage of the new features in
the SYM53C896.
In the area of hardware, LVD SCSI is required to achieve Ultra2 SCSI
transfer rates and to support the longer cable and additional devices on
the bus. All devices on the bus must have LVD SCSI capabilities to
guarantee Ultra2 SCSI transfer rates. For additional information on Ultra2
SCSI, refer to the SPI-2 working document which is available from the
SCSI BBS referenced at the beginning of this manual.
Chapter 6,
“Specifications”
contains Ultra2 SCSI timing information. In addition to the
guidelines in the draft standard, make the following software and
hardware adjustments to accommodate Ultra2 SCSI transfers:
Set the Ultra Enable bit to enable Ultra2 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 the
SCSI Test
Two (STEST2)
register bit 1. When the Ultra Enable bit is set, the
filtering period will be fixed at 8 ns for Ultra2 SCSI or 15 ns for Ultra
SCSI, regardless of the value of the SREQ/SACK filtering bit.
Use the SCSI clock quadrupler.