Intel MCS48 User Manual page 342

8259A
POLL
In this
mode
the
microprocessor
internal Interrupt
Enable
flip-flop
is reset,
disabling
its
interrupt input.
Service
to
devices
is
achieved by programmer
initiative
using a
Poll
command.
The
Poll
command
is
issued by
setting
P
=
"1"
inOCW3.
The 8259A
treats
the next
RD
pulse
to
the
8259A
(i.e.,
RD =
0,
C§"
=
0)
as an
interrupt
acknowledge,
sets the
appropriate
IS
bit
if
there
is
a request,
and
reads the
priority
level.
Interrupt
is
frozen
from
WR
to
RD.
The word
enabled onto
the data
bus
during
R~D" is:
D7 06 05
04 03 02 01 00
W2
W1
WO
W0-W2:
Binary
code
of
the highest
priority
level
requesting
service.
I:
Equal
to
a "1"
if
there
is
an
interrupt.
This
mode
is
useful
if
there
is
a
r
outine
command
com-
mon
to several levels so
that the
INTA sequence
is
not
needed
(saves
ROM
space).
Another
application
is
to
use
the
poll
mode
to
expand
the
number
of priority
levels to
more
than
64.
END OF
INTERRUPT
(EOI)
The
In
Service
(IS) bit
can be
reset either
automat
ically
following the
trailing
edge
of
the
last in
sequence INTA
pulse (when
AEOI
bit in
ICW1
is
set) or
by a
command
word
that
must
be issued
to
the
8259A
before
returning
from a service routine (EOI command).
An
EOI
command
must
be issued
twice,
once
for
the
master and once
for
the
corresponding
slave
if
slaves are
in
use.
There are
two
forms
of
EOI
command:
Specific
and Non-
specific.
When
the
8259A
is
operated
in
modes
which
preserve the
fully
nested
structure,
it
can determine
which
IS
bit
to reset
on
EOI.
When
a Non-Specific EOI
command
is
issued the
8259A
will
automatically reset
the highest
IS bit
of
those
that
are
set,
since
in
the
nested
mode
the highest
IS level
was
necessarily the
last level
acknowledged and
serviced.
However, when
a
mode
is
used which
may
disturb the
fully
nested
structure, the
8259A may
no longer be able
to
determine the
last level
acknowledged.
In this
case a
Specific
End
of Interrupt (SEOI)
must be issued which
includes as
part of
the
command
the
IS level to
be
reset.
EOI
is
issued
whenever EOI =
1
,
in
OCW2,
where
L0-L2
is
the binary
level of
the IS
bit to
be
reset.
Note
that
although
the
Rotate
command
can be
issued together with
an EOI
where EOI =
1,
it
is
not necessarily
tied to
it.
It
should be noted
that
an
IS bit
that
is
masked by an
IMR
bit will
not
be
cleared
by a
non-specific
EOI
if
the
8259A
is in
the Special
Mask
Mode.
AUTOMATIC END OF INTERRUPT
(AEOI)
MODE
If
AEOI =
1
in
ICW4, then the
8259A
will
operate
in
AEOI
mode
continuously
until
reprogrammed by ICW4.
In
this
mode
the
8259A
will
automatically
perform
a non-
specific
EOI operation
at
the
trailing
edge
of
the
last
interrupt
acknowledge
pulse (third
pulse
in
MCS-80/85,
second
in
MCS-86). Note
that
from
a
system
standpoint,
this
mode
should be used
only
when
a nested
multilevel
interrupt
structure
is
not required within a single 8259A.
To
achieve automatic
rotation (Rotate
Mode
A)
within
AEOI,
there
is
a special
rotate
flip-flop.
It
is
set
by
OCW2
with
R=1, SEOI =
0,
EOI
=
0,
and
cleared with
R =
0,
SEOI =
0,
EOI
=
0.
ROTATING
PRIORITY
MODE
A (AUTOMATIC
ROTATION)
FOR EQUAL
PRIORITY DEVICES
In
some
applications there are a
number
of interrupting
devices
of
equal
priority. In this
mode
a device, after
being serviced, receives the lowest
priority,
so a device
requesting an
interrupt
will
have
to wait,
in
the worst
case
until
each
of 7
other devices are serviced
at
most
once.
For example,
if
the
priority
and
"in
service" status
is:
Before Rotate
(IR4
the highest
priority requiring service)
IS7 IS6 ISS IS4 IS3 IS2
IS1
ISO
"IS" Status
|o|l|o|l|o|o|o|o|
Lowttt
Priority
Highest
Priority
Priority
Status
7^6|5|4|3|2|
1
p0~
|
|
1
[
| | | | | |
Highest
Priority
Lowest
Priority
h^
o
[
7+frT|4|3i
After Rotate
(IR4
was
serviced,
all
other
priorities
rotated correspondingly)
IS7 IS« ISS IS4 IS3 IS2 IS1 ISO
"IS" Status
Priority
Status
The
Rotate
command mode
A
is
issued
in
OCW2
where:
R =
1
,
EOI =
1
,
SEOI
=
0.
Internal status
is
updated by an
End
of Interrupt
(EOI
or
AEOI)
command.
If
R =
1,
EOI =
0,
SEOI
=
0,
a
"Rotate-A"
flip-flop
is
set.
This
is
useful in
AEOI, and described
under Automatic
End
of Interrupt.
ROTATING PRIORITY
MODE
B
(ROTATION BY
SOFTWARE)
The programmer can
change
priorities
by
programming
the
bottom
priority
and
thus fixing
all
other
priorities;
i.e., if
IR5
is
programmed
as the
bottom
priority
device,
then IR6
will
have
the highest one.
The
Rotate
command
is
issued
in
OCW2
where:
R =
1,
SEOI =
1;
L0-L2
is
the binary priority level
code
of
the
bottom
priority
device.
Observe
that
in
this
mode
internal status
is
updated by
software control
during
OCW2.
However,
it
is
independ-
ent of
the
End
of Interrupt (EOI)
command
(also exe-
cuted by
OCW2).
Priority
changes can be executed
dur-
ing
an EOI
command
or independently.
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