Register: 0x21

Chip Test Four (CTEST4)

Read/Write

7

6

5

4

3

 

2

 

 

0

BDIS

FBL3

ZSD

SRTM

MPEE

 

 

FBL[2:0]

 

 

 

 

 

 

 

 

 

 

 

0

0

0

0

0

 

0

0

 

0

 

 

 

 

 

 

 

 

 

 

BDIS

Burst Disable

 

 

 

 

 

7

 

When set, this bit causes the LSI53C875A to perform

 

back-to-back cycles for all transfers. When this bit is

 

cleared, back-to-back transfers for opcode fetches and

 

burst transfers for data moves are performed.

 

FBL3

FIFO Byte Control

 

 

 

 

 

6

 

This bit is used with FBL[2:0]. See Bits [2:0] description

 

in this register.

 

 

 

 

 

 

ZSD

SCSI Data High Impedance

 

 

 

5

 

Setting this bit causes the LSI53C875A to place the SCSI

 

data bus SD[15:0] and the parity lines SDP[1:0] in a high

 

impedance state. In order to transfer data on the SCSI

 

bus, clear this bit.

 

 

 

 

 

 

SRTM

Shadow Register Test Mode

 

 

 

4

 

Setting this bit allows access to the shadow registers

 

used by Memory-to-Memory Move operations. When this

 

bit is set, register accesses to the Temporary (TEMP) and

 

Data Structure Address (DSA) registers are directed to

 

the shadow copies STEMP (Shadow TEMP) and SDSA

 

(Shadow DSA). The registers are shadowed to prevent

 

them from being overwritten during a Memory-to-Memory

 

Move operation. The DSA and TEMP registers contain

 

the base address used for table indirect calculations, and

 

the address pointer for a call or return instruction,

 

 

respectively. This bit is intended for manufacturing

 

 

diagnostics only and should not be set during normal

 

operations.

 

 

 

 

 

 

 

MPEE

Master Parity Error Enable

 

 

 

3

 

Setting this bit enables parity checking during master

 

data phases. A parity error during a bus master read is

 

detected by the LSI53C875A. A parity error during a bus

 

master write is detected by the target, and the

 

SCSI Registers

4-59

Page 151
Image 151
LSI 53C875A technical manual Bdis, FBL3, Zsd, Srtm, Mpee