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I/O
INTERFACING
8.2.3 16-Bit I/O
To avoid extra bus cycles and to simplify device selection, 16-bit I/O devices should be
assigned to even addresses.
If
I/O addresses are located on adjacent word bciundaries,
address decoding must generate the Bus Size
16
(BS16#) signal
so
that the Intel386
DX
microprocessor performs a 16-bit bus
cycle.
If
the addresses are located on every other
word boundary (every doubleword address), BS16#
is
not needed.
8.2.4 32-Bit I/O
To avoid extra bus cycles and to simplify device selection, 32-bit devices should be
assigned to addresses that are even multiples of four. Chip select for a 32-bit device
should be conditioned
by
all byte enables (BE3#-BEO#) being active.
8.2.5 Linear Chip Selects
Systems with
14
or fewer I/O ports that reside only in the I/O space or that require more
than one active select (at least one high active and one
low
active) can use linear chip
selects to access
I/O devices. Latched address lines A2-A15 connect directly to I/O
device selects as shown in Figure
8-2.
8.3 BASIC I/O INTERFACE
In a typical Intel386
DX
microprocessor system design, a number of slave I/O devices
can be controlled through the
same local bus interface. Other I/O devices, particularly
those capable of controlling the local bus, require more complex interfaces. This section
presents a basic interface for slave peripherals.
The high performance and flexibility of the Intel386
DX
microprocessor local bus inter-
face plus the increased availability of programmable and semi-custom logic make it fea-
sible to design custom bus control logic that meets the requirements of particular system.
The basic
I/O interface shown
in
Figure
8-3
can be used to connect the Inte1386
DX
microprocessor to virtually all slave peripherals. The following list includes some com-.
mon peripherals compatible with this interface:
8259A Programmable Interrupt Controller
8237
DMA
Controller (remote mode) .
82258
Advanced
DMA
Controller (remote mode)
8253,
8254 Programmable Interval Timer
8272
Floppy Disk Controller
82064 Fixed Disk Controller
8274
Multi-Protocol Serial Controller
8255
Programmable Peripheral Interface
8041,
8042 Universal Peripheral Interface
8-4