
SYM53C876/876E Data Manual
4-67
Registers
SCSI Registers
Bit 5
HSC (Halt SCSI Clock)
Asserting this bit causes the internal divided
SCSI clock to come to a stop in a glitchless
manner. This bit is used for test purposes or
to lower I
DD
during a power down mode.
Bit 4
DSI (Disable Single Initiator
Response)
If this bit is set, the SYM53C876 SCSI func-
tion ignores all bus-initiated selection
attempts that employ the single-initiator
option from SCSI-1. In order to select the
SYM53C876 SCSI function while this bit is
set, the SYM53C876 SCSI function’s SCSI
ID and the initiator’s SCSI ID must both be
asserted. Assert this bit in SCSI-2 systems so
that a single bit error on the SCSI bus is not
interpreted as a single initiator response.
Bit 3
CHECKHI (Check High Parity)
If this bit is set, all devices in the SCSI system
implementation are assumed to be 16 bits.
This causes the SYM53C876 to always check
the parity bit for SCSI IDs 15-8 during bus-
initiated selection or reselection, assuming
parity checking has been enabled. If an 8-bit
SCSI device attempts to select the
SYM53C876 while this bit is set, the chip
ignores the selection attempt, because the par-
ity bit for IDs 15-8 is undriven. See the
description of the Enable Parity Checking bit
in the SCNTL0 register for more information.
Bit 2
TTM (Timer Test Mode)
Asserting this bit facilitates testing of the
selection time-out, general purpose, and
handshake-to-handshake timers by greatly
reducing all three time-out periods. Setting
this bit starts all three timers and if the respec-
tive bits in the SIEN1 register are asserted, the
SYM53C876 SCSI function generates inter-
rupts at time-out. This bit is intended for
internal manufacturing diagnosis and should
not be used.
Bit 1
CSF (Clear SCSI FIFO)
Setting this bit causes the “full flags” for the
SCSI FIFO to be cleared. This empties the
FIFO. This bit is self-resetting. In addition to
the SCSI FIFO pointers, the SIDL, SODL,
and SODR full bits in the SSTAT0 and
SSTAT2 are cleared.