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SCSI Operating Registers
SYM53C825A/825AE Data Manual
5-47
Register 49 (C9)
SCSI T imer One (ST IME1)
Read/Write
Bit 7
Reserved
Bit 6
HT HBA (Handshake-to-Handshake
T imer Bus Activity E nable)
Setting this bit causes this timer to begin test-
ing for SCSI REQ/ACK activity as soon as
SBSY/ is asserted, regardless of the agents par-
ticipating in the transfer.
Bit 5
GE NSF (General Purpose T imer
Scale Factor)
Setting this bit causes this timer to shift by a
factor of 16.
Bit 4
HT HSF (Handshake to Handshake
T imer Scale Factor)
Setting this bit causes this timer to shift by a
factor of 16.
Bits 3-0 GE N3-0 (General Purpose T imer
Period)
T hese bits select the period of the general pur-
pose timer. T he time measured is the time
between enabling and disabling of the timer.
When this timing is exceeded, the GEN bit in
the SIST 1 register is set. Refer to the table
under ST IME0, bits 3-0, for the available
time-out periods.
Note: To reset a timer before it has expired and
obtain repeatable delays, the time value
must be written to zero first, and then
written back to the desired value. T his is
also required when changing from one time
value to another. See Chapter 2,
“Functional Description,” for an
explanation of how interrupts will be
generated when the timers expire.
RES
7
HTHBA
6
GENSF
5
HTHSF
4
GEN3
3
GEN2
2
GEN1
1
GEN0
0
Default>>>
X
0
0
0
0
0
0
0
HTH 7-4, SEL 3-0,
GEN 3-0
Minimum Time-out
(50 MHz Clock)
GENSF= 0
GENSF=1
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
Disabled
100
μ
s
200
μ
s
400
μ
s
800
μ
s
1.6 ms
3.2 ms
6.4 ms
12.8 ms
25.6 ms
51.2 ms
102.4 ms
204.8 ms
Disabled
1.6 ms
3.2 ms
6.4 ms
12.8 ms
25.6 ms
51.2 ms
102.4 ms
204.8 ms
409.6 ms
819.2 ms
1.6 sec
3.2 sec
These values will be correct if the CCF bits in the SCNTL3
register are set according to the valid combinations in the bit
description.
1101
1110
1111
409.6 ms
819.2 ms
1.6 sec
6.4 sec
12.8 sec
25.6 sec
HTH 7-4, SEL 3-0,
GEN 3-0
Minimum Time-out
(50 MHz Clock)
GENSF= 0
GENSF=1
These values will be correct if the CCF bits in the SCNTL3
register are set according to the valid combinations in the bit
description.