Intel MCS48 User Manual page 272

8212
III.
Bi-Directional Bus
Driver
A
pair of 8212's wired (back-to-back) can be used
as a
symmetrical
drive, bi-directional
bus
driver.
The
devices
are controlled_by the data bus
input control
which
is
connected
to
DS1 on
the
first
8212 and
to
DS2
on
the
second.
One
device
is
active,
and
acting as a
straight
through
buffer the other
is
in
3-state
mode.
This
is
a very
useful circuit
in
small system
design.
BI-DIRECTIONAL BUS DRIVER
n^
IV.
Interrupting Input Port
This use
of
an 8212
is
that of a
system
input port that
accepts a strobe from
the
system
input source,
which
in
turn clears the service request
flip-flop
and
interrupts
the
processor.
The
processor then goes through
a service
routine, identifies
the
port,
and causes
the device
selection
logic to
go
true
—
enabling the
system
input
data
onto
the
data
bus.
INTERRUPTING INPUT PORT
TO
PRIORITY
CKT
(ACTIVE LOW)
L
^y—
TO CPU
INTERRUPT
INPUT
V. Interrupt Instruction
Port
The 8212 can be used
to gate the interrupt instruction,
normally
RESTART
instructions,
onto
the data bus.
The
device
is
enabled from
the interrupt
acknowledge
signal
from
the
microprocessor and from
a
po
rt
se
lection signal.
This signal
is
normally
tied to
ground. (DS1 could be used
to
multiplex a variety of interrupt instruction ports
onto
a
common
bus).
INTERRUPT INSTRUCTION
PORT
RESTART
I
N
INSTRUCTION
^
\
(RSTO
—
RST
7)
^
—
V
(DSD
PORT SELECTION
INTERRUPT
ACKNOWLEDGE
—
5h
7-34