SOMSCSI Synchronous Offset Maximum0

This bit indicates that the current synchronous SREQ/, SACK/ offset is the maximum specified by bits [3:0] in the SCSI Transfer (SXFER) register. This bit is not latched and may change at any time. It is used in low level synchronous SCSI operations. When this bit is set, the LSI53C875A, as a target, is waiting for the initiator to acknowledge the data transfers. If the LSI53C875A is an initiator, then the target has sent the offset number of requests.

Register: 0x4D

SCSI Test One (STEST1)

Read/Write

7

6

5

 

4

 

3

2

1

 

0

SCLK

ISO

 

R

 

 

QEN

QSEL

R

 

0

0

 

x

 

x

 

0

0

x

 

x

SCLK

 

SCSI Clock

 

 

 

 

 

 

7

 

 

When set, this bit disables the external SCLK (SCSI

 

 

Clock) pin, and the chip uses the PCI clock as the

 

 

 

internal SCSI clock. When set, it will also select the PCI

 

 

clock as the internal SCSI clock if the internal clock

 

 

 

quadrupler is enabled and selected.

 

 

 

ISO

 

SCSI Isolation Mode

 

 

 

 

6

 

 

This bit allows the LSI53C875A to put the SCSI

 

 

 

bidirectional and input pins into a low power mode when

 

 

the SCSI bus is not in use. When this bit is set, the SCSI

 

 

bus inputs are logically isolated from the SCSI bus.

 

R

 

Reserved

 

 

 

 

 

 

[5:4]

QEN

 

SCLK Quadrupler Enable

 

 

 

3

This bit, when set, powers up the internal clock quadrupler circuit, which quadruples the SCLK. A 40 MHz clock is quadrupled to an internal 160 MHz SCSI clock, as required for Ultra SCSI operation. The output from a 20 MHz SCLK is 80 MHz. When cleared, this bit powers down the internal quadrupler circuit.

4-88Registers

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Image 180
LSI 53C875A technical manual Register 0x4D, Sclk, Iso, Qen