SCF[2:0]

Synchronous Clock Conversion Factor

[6:4]

 

These bits select a factor by which the frequency of

 

SCLK is divided before being presented to the

 

 

synchronous SCSI control logic. Write these to the same

 

value as the Clock Conversion Factor bits below unless

 

fast SCSI operation is desired. See the SCSI Transfer

 

(SXFER) register description for examples of how the

 

SCF bits are used to calculate synchronous transfer

 

periods. See the table under the description of bits [7:5]

 

of the SXFER register for the valid combinations.

 

EWS

Enable Wide SCSI

3

 

When this bit is clear, all information transfer phases are

 

assumed to be eight bits, transmitted on SD[7:0]/ and

 

 

SDP0/. When this bit is asserted, data transfers are done

 

16 bits at a time, with the least significant byte on

 

 

SD[7:0]/ and SDP0/ and the most significant byte on

 

 

SD[15:8]/, SDP1/. Command, Status, and Message

 

 

phases are not affected by this bit.

 

CCF[2:0]

Clock Conversion Factor

[2:0]

 

These bits select a factor by which the frequency of

 

SCLK is divided before being presented to the SCSI core.

 

The synchronous portion of the SCSI core can be run at

 

a different clock rate for fast SCSI, using the

 

Synchronous Clock Conversion Factor bits. The bit

 

encoding is displayed in the table below. All other

 

combinations are reserved.

 

 

 

 

 

 

 

 

SCF2

SCF1

SCF0

Factor

SCSI Clock

 

CCF2

CCF1

CCF0

Frequency

(MHz)

 

 

 

 

 

 

 

0

0

0

SCLK/3

50.01–75.0

 

 

 

 

 

 

 

0

0

1

SCLK/1

16.67–25.0

 

 

 

 

 

 

 

0

1

0

SCLK/1.5

25.01–37.5

 

 

 

 

 

 

 

0

1

1

SCLK/2

37.51–50.0

 

 

 

 

 

 

 

1

0

0

SCLK/3

50.01–75.0

 

 

 

 

 

 

 

1

0

1

SCLK/4

75.01–80.00

 

 

 

 

 

 

 

1

1

0

SCLK/6

120

 

 

 

 

 

 

 

1

1

1

SCLK/8

160

 

 

 

 

 

 

Note: It is important that these bits are set to the proper values to guarantee that the LSI53C875A meets the SCSI timings as defined by the ANSI specification.

SCSI Registers

4-29

Page 121
Image 121
LSI 53C875A SCF20 Synchronous Clock Conversion Factor, Ews, Enable Wide Scsi, CCF20 Clock Conversion Factor