
Functional Description
Synchronous Operation
SYM53C875/875E Data Manual
2-19
SCNT L3 Register, bits 2–0 (CCF2–0)
T he CCF2-0 bits select the factor by which the
frequency of SCLK is divided before being pre-
sented to the asynchronous SCSI core logic. T his
divider must be set according to the input clock
frequency in the table.
SX FE R Register, bits 7–5 (T P2–0)
T he T P2-0 divider bits determine the SCSI syn-
chronous transfer period when sending synchro-
nous SCSI data in either initiator or target mode.
T his value further divides the output from the
SCF divider.
Achieving Optimal SCSI Send Rates
To achieve optimal synchronous SCSI send tim-
ings, the SCF divisor value should be set high, to
divide the clock as much as possible before pre-
senting the clock to the T P divider bits in the
SX FER register. T he T P2-0 divider value should
be as low as possible. For example, with a 80 MHz
clock to achieve a 20 MB/s Ultra SCSI send rate,
the SCF bits can be set to divide by 1 (001) and
the T P bits to divide by 4 (000). To set for a 10
MB/s send rate for Fast SCSI-2, the SCF bits can
be set to divide by 2 (011) and the T P bits set to
divide by 4 (000).
Ultra SCSI
Synchronous Data Transfers
Ultra SCSI is simply an extension of current Fast
SCSI-2 synchronous transfer specifications. It
allows synchronous transfer periods to be negoti-
ated down as low as 50 ns, which is half the 100 ns
period allowed under Fast SCSI-2. T his will allow
a maximum transfer rate of 40 MB/s on a 16-bit
SCSI bus. T he SYM53C875 requires an 80MHz
SCSI clock input to perform Ultra SCSI transfers.
In addition, the following bit values affect the
chip’s ability to support Ultra SCSI synchronous
transfer rates:
1. Clock Conversion Factor bits, SCNT L3
register bits 2-0 and Synchronous Clock
Conversion Factor bits, SCNT L3 register bits
6-4. T hese fields now support a value of 101
(binary), allowing the SCLK frequency to be
divided down by 4. T his allows systems using
an 80 MHz clock or the internal clock doubler
to operate at Fast SCSI-2 transfer rates as well
as Ultra SCSI rates, if needed.
2. Ultra Mode Enable bit, SCNT L3 register bit
7. Setting this bit enables Ultra SCSI
synchronous transfers in systems that have an
80 MHz clock or use the internal SCSI clock
doubler.