group codes. If this bit is set, the device does not reload the Block Move byte count, regardless of the group code.

WSRWide SCSI Receive0

When read, this bit returns the value of the Wide SCSI Receive (WSR) flag. Setting this bit clears the WSR flag. This clearing function is self-clearing.

The WSR flag indicates that the SCSI core received data from the SCSI bus, detected a possible partial transfer at the end of a chained or nonchained block move command, and temporarily stored the high-order byte in the SCSI Wide Residue (SWIDE) register rather than passing the byte out the DMA channel. The hardware uses the WSR status flag to determine what behavior must occur at the start of the next data receive transfer. When the flag is set, the stored data in SWIDE may be “residue” data, valid data for a subsequent data transfer, or overrun data. The byte is read as normal data by starting a data receive transfer.

Performing a SCSI send operation clears this bit. Also, performing any nonwide transfer clears this bit.

Register: 0x03

SCSI Control Three (SCNTL3)

Read/Write

7

6

 

 

4

3

2

 

0

USE

 

 

SCF[2:0]

 

EWS

 

CCF[2:0]

 

 

 

 

 

 

 

 

 

0

0

 

0

0

0

0

0

0

 

 

 

 

 

 

 

 

 

USE

 

Ultra SCSI Enable

 

 

 

7

 

 

Setting this bit enables Ultra SCSI synchronous transfers.

 

 

The default value of this bit is 0. This bit should remain

 

 

cleared if the LSI53C875A is not operating in Ultra SCSI

 

 

mode.

 

 

 

 

 

When this bit is set, the signal filtering period for SREQ/ and SACK/ automatically changes to 15 ns for Ultra SCSI, regardless of the value of the Extend REQ/ACK Filtering bit in the SCSI Test Two (STEST2) register.

Note: Set this bit to achieve Ultra SCSI transfer rates in legacy systems that use an 80 MHz clock.

4-28Registers

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LSI 53C875A technical manual WSRWide Scsi Receive0, Scsi Control Three SCNTL3 Read/Write, Use, Ultra Scsi Enable