inter
I/O
INTERFACING
tWD:
Data
hold after write
(IOWC#
rise)
(2 x
CLK2
period) -
PAL
RegOut +
PAL
RegOut
+ xcvr. Disable
(2
x 31.25) -
12
+ 0
+ 2
= 52.5 nanoseconds
tR
V:
command recovery time
(11 x
CLK2
period)
(11 x 31.25)
= 331.75 nanoseconds
-
PAL
RegOut
Max
12
+
PAL
RegOut Min
+ 0
Many
peripherals require a minimum recovery time between back-to-back accesses. This
recovery time
is
usually provided in software by a series
of
NOP
instructions. A
JMP
to the
next instruction also provides a delay because it flushes the
80386 Prefetch Queue; this
method has a more predictable execution time
than
the
NOP
method.
In
80386 systems, the instructions
that
provide recovery time are executed more quickly
than
in earlier systems.
For
software compatibility with earlier microprocessor generations,
hardware must
guarantee
the recovery time. However, the circuitry to delay bus commands
selectively for the specific instance of back-to-back accesses to a particular device
is
typically
more complex
than
the frequency
of
such accesses justifies. Therefore, the preferred solution
is
to delay all
I/0
cycles by the minimum recovery time. Because most
I/0
accesses are
relatively infrequent, performance
is
not degraded.
The
I/0
access timings
of
the basic interface are compatible with all of the currently avail-
able Intel peripherals.
(In
some cases, the high-speed versions
of
these peripherals are
required). Table 8-2 compares several peripheral timings with the timings provided by bus
controller.
Table
8-2.
Timings
for
Peripherals
using
Basic
I/O
Interface
tAR
tAW
tRR
tWW
tRA
tWA
tAO
tRO
tDF
tOW
tOWF
tWO
tRY
bus cntrlr
39 39
269
300 180
211
335
253
241
289
-
52
331
8259-2 0 0
160 190
0 0
200
120
85
160
-
0 190
8254-2
30
0
95
95
0 0
185
85
65
85
- 0
165
82C54-2 0
0
95
95
0 0
185
85
65
85
-
0
165
82C55-2
0 0
150 100
0
20
-
120
75
100
-
30
300
8272
0 0
250 250
0 0
-
200
100
150
- 5 -
82064
0 0
200
200
0 0 -
70
200
160
-
0 300
8041
0 0
250 250
0 0
225 225
100 150
-
0 2500
8042
0 0 160 160 0 0 130 130
85
130
- 0
1120
8251
0 0
250
250
0 0
-
200
100
150
-
20
-
8273-4
0 0
250
250
0 0 300
200
100 150
-
0
1920
8274 0
0
250 250
0
0
200 200
120 150
-
0 300
8291
0 0 140
170
0 0 250
100
60
130
- 0
-
8292
0 0
250
250
0 0
225
225
100 150
- 0 -
8-13
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