
4-22
SYM53C876/876E Data Manual
Registers
SCSI Registers
Register 01h
SCSI Control One (SCNTL1)
Read/Write
Bit 7
EXC (Extra Clock Cycle of Data
Setup)
When this bit is set, an extra clock period of
data setup is added to each SCSI send data
transfer. The extra data setup time can pro-
vide additional system design margin, though
it affects the SCSI transfer rates. Clearing this
bit disables the extra clock cycle of data setup
time. Setting this bit only affects SCSI send
operations.
Bit 6
ADB (Assert SCSI Data Bus)
When this bit is set, the SYM53C876 SCSI
function drives the contents of the SCSI Out-
put Data Latch Register (SODL) onto the
SCSI data bus. When the SYM53C876 SCSI
function is an initiator, the SCSI I/O signal
must be inactive to assert the SODL contents
onto the SCSI bus. When the SYM53C876
SCSI function is a target, the SCSI I/O signal
must be active to assert the SODL contents
onto the SCSI bus. The contents of the
SODL register can be asserted at any time,
even before the SYM53C876 SCSI function
is connected to the SCSI bus. Clear this bit
when executing SCSI SCRIPTS. It is nor-
mally used only for diagnostics testing or
operation in low level mode.
Bit 5
DHP (Disable Halt on Parity Error
or ATN) (Target Only)
The DHP bit is only defined for target mode.
When this bit is cleared, the SYM53C876
SCSI function halts the SCSI data transfer
when a parity error is detected or when the
SATN/ signal is asserted. If SATN/ or a parity
error is received in the middle of a data trans-
fer, the SYM53C876 SCSI function may
transfer up to three additional bytes before
halting to synchronize between internal core
cells. During synchronous operation, the
SYM53C876 SCSI function transfers data
until there are no outstanding synchronous
offsets. If the SYM53C876 SCSI function is
receiving data, any data residing in the DMA
FIFO is sent to memory before halting.
When this bit is set, the SYM53C876 SCSI
function does not halt the SCSI transfer when
SATN/ or a parity error is received.
Bit 4
CON (Connected)
This bit is automatically set any time the
SYM53C876 SCSI function is connected to
the SCSI bus as an initiator or as a target. It is
set after the SYM53C876 SCSI function suc-
cessfully completes arbitration or when it has
responded to a bus initiated selection or rese-
lection. This bit is also set after the chip wins
simple arbitration when operating in low level
mode. When this bit is clear, the SYM53C876
SCSI function is not connected to the SCSI
bus.
The CPU can force a connected or discon-
nected condition by setting or clearing this bit.
This feature is used primarily during loopback
mode.
Bit 3
RST (Assert SCSI RST/ Signal)
Setting this bit asserts the SRST/ signal. The
SRST/ output remains asserted until this bit is
cleared. The 25
μ
s minimum assertion time
defined in the SCSI specification must be
timed out by the controlling microprocessor
or a SCRIPTS loop.
Bit 2
AESP (Assert Even SCSI Parity
(force bad parity))
When this bit is set, the SYM53C876 SCSI
function asserts even parity. It forces a SCSI
parity error on each byte sent to the SCSI bus
from the chip. If parity checking is enabled,
then the SYM53C876 SCSI function checks
data received for odd parity. This bit is used
EXC
7
Default >>>
0
ADB
6
DHP
5
CON
4
RST
3
AESP
2
IARB
1
SST
0
0
0
0
0
0
0
0