
SYM53C876/876E Data Manual
4-27
Registers
SCSI Registers
Register 05h
SCSI Transfer (SXFER)
Read/Write
Note: When using Table Indirect I/O
commands, bits 7-0 of this register are
loaded from the I/O data structure.
Note: For additional information on how the
synchronous transfer rate is determined,
refer to Chapter 2,
Functional Description
.
Bits 7-5 TP2-0 (SCSI Synchronous Transfer
Period)
These bits determine the SCSI synchronous
transfer period used by the SYM53C876
SCSI function when sending synchronous
SCSI data in either initiator or target mode.
These bits control the programmable dividers
in the chip.
Note: For Wide Ultra SCSI transfers, the ideal
transfer period is 4, and 5 is acceptable.
Setting the transfer period to a value
greater than 5 is not recommended.
The synchronous transfer period the
SYM53C876 should use when transferring
SCSI data is determined as in this exam-
ple.The SYM53C876 is connected to a hard
disk that can transfer data at 10 MB/s syn-
chronously. The SYM53C876 SCSI func-
tion’s SCLK is running at 40 MHz. The
synchronous transfer period (SXFERP) is
found as follows:
SXFERP = Period/SSCP + ExtCC
Period = 1
÷
Frequency = 1
÷
10 MB/s = 100 ns
SSCP = 1
÷
SSCF = 1
÷
40 MHz = 25 ns
(This SCSI synchronous core clock is determined in SCNTL3 bits 6-4
ExtCC = 1 if SCNTL1 bit 7 is asserted and the 53C876 is sending data.
ExtCC = 0 if the 53C876 is receiving data)
SXFERP = 100
÷
25 = 4
Key:
SXFERP = Synchronous transfer period
SSCP = SCSI Synchronous core period
SSCF = SCSI Synchronous core frequency
ExtCC = Extra clock cycle of data setup
TP2
7
Default >>>
0
TP1
6
TP0
5
MO4
4
MO3
3
MO2
2
MO1
1
MO0
0
0
0
X
0
0
0
0
TP2
TP1
TP0
XFERP
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
4
5
6
7
8
9
10
11
Table 4-4: Examples of Synchronous Transfer
Periods for SCSI-1 Transfer Rates
CLK
(MHz)
SCSI
CLK
÷
SCNTL3
bits 6-4
XFERP
Synch.
Transfer
Period
(ns)
Synch.
Transfer
Rate
(MB/s)
40
80
÷
4
÷
2
4
4
200
200
5
5
Table 4-5: Example Transfer periods for Fast
SCSI and Wide Ultra SCSI transfer rates
CLK
(MHz)
SCSI
CLK
÷
SCNTL3
bits 6-4
XFERP
Synch.
Transfer
Period
(ns)
Synch.
Transfer
Rate
(MB/s)
80
80
40
÷
1
÷
2
÷
1
4
4
4
50
100
100
20
10
10