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SCHOTTKY BIPOLAR 8214
FUNCTIONAL DESCRIPTION
General
'he
8214
is
a device specifically designed for use
in
real
time, interrupt driven, microcomputer systems. Basically it
is
an eight (8) level priority control
unit
that
can accept
eight different interrupt requests, determine which has
the
highest priority, compare
that
level
to
a software maintained
current status register and issue an interrupt
to
the
system
based on this comparison along with vector information
to
indicate
the
location
of
the
service routine.
Priority Encoder
The eight requests inputs, which are active low, come into
the Priority Encoder. This circuit determines which request
input
is
the
most important (highest priority) as preassigned
by
the
designer. (R7)
is
the
highest priority
input
to
the
8214 and
(RO)
is
the
lowest.
The
logic
of
the
Priority
En-
coder
is
such
that
if
two
or
more input levels arrive
at
the
same time then
the
input having
the
highest priority will take
presidence and a three
bit
output,
corresponding
to
the
act-
ive
level
(modulo 8) will be sent out.
The
Priority Encoder
also contains a latch
to
store
the
request input. This latch
is
controlled by
the
Interrupt Disable Flip-flop so
that
once
an
interrupt has been issued by
the
8214
the
request latch
is
no longer open. (Note
that
the
latch does
not
store inact-
ive
requests.
In
order for a request
to
be monitored by
the
8214 it must remain present until it has been serviced.)
Current
Status
Register
In
an interruptdriven microcomputer system it
is
important
to
not
only prioritize incoming requests
but
to
ascertain
whethersuch a request
is
a higher priority than
the
interrupt
currently being serviced.
The Current Status Register
is
a simple 4-bit latch
that
is
treated as an addressable
outport
port
by
the
microcom-
puter system. It
is
loaded when
the
ECS
input
goes low.
Maintenance
of
the
Current
Status
Register
is
performed
as
a portion
of
the
service routine. Basically, when an interrupt
is
issued
to
the
system
the
programmer
outputs
a binary
code (modulo
8)
that
is
the
compliment
of
the
interrupt
level. This value
is
stored
in
the
Current
Status
Register and
is
compared
to
all further prioritized incoming requests by
the Priority Comparator. In essence, a copy
of
the
current
interrupt
level
is
written into
the
8214
to
be used as a ref-
erence for comparison. There
is
no restriction
to
this main-
tenance,
other
level values can be written into this register
as
references so
that
groups
of
interrupt
requests may be
disallowed under complete control
of
the
programmer.
5-155
Note
that
the
fourth bit
in
the
register
is
SGS. This input
is
part
of
the
value written
out
by
the
programmer and per-
forms a special function.
The
Priority Comparator will only
issue an
output
that
indicates
the
request level
is
greater than
the
Current
Status
Register. If
both
comparator
inputs are
equal
to
zero no
output
will be present.
The
SGS input
allows
the
programmer
to,
in
effect, disable
this
comparison
and allow
the
8214
to
issue an
interrupt
to
the
system
that
is
based only
on
the
logic
of
the
priority encoder.