Intel MCS48 User Manual page 63

SINGLE
COMPONENT
SYSTEM
this
would include high noise margin
inputs
(V<x/
2),
unusual
logic level inputs
as from a diode
isolated
keyboard, analog channel expansion, and
direct capacitive
touchpanel
interface.
The com-
parator action
is
automatic
and
the port
is
read
just
as any
other
port.
2.16.2 High Current
Outputs
Very
high current drive
is
desirable for minimizing
external parts required
to
do
high
power
control.
P10
and P1
1
have been
designated high drive out-
puts
capable
of sinking
7mA
at
Vss
+
2.5
volts.
(For
clarity,
this
is
7mA
to
Vss
with
a
2.5
volt
drop
across the
buffer.)
These
pins
may,
of
course,
be
paralleled for
14mA
drive
if
the output
logic
states
are
always
the
same.
2.16.3
Expanded
I/O
In
addition to the
26
digital
I/O
lines
contained on-
board the 8022, a user can
obtain additional
I/O
lines
by
utilizing
the
Intel®
8243
I/O expander
chip
or
standard TTL.
The 8243
interfaces to
4
port
lines of
the
8022
(lower
half
of port 2)
and
is
strobed by the
PROG
line
of
the
8022.
The
interface
procedure
is
exactly the
same
as
with the
8021
—
see
Section 2.9.3.
2.17 Test
and
Interrupt
Inputs
In
addition to
the
24
general
purpose I/O
lines
which comprise
ports
0,1, and
2,
the
8022 has
two
inputs
which are
testable
via
conditional
jump
instructions,
TO and T1
.
These
pins allow inputs to
cause program branches
without the necessity
to
load an
input port into
the accumulator.
TO
and T1
have
other functions
as
well.
The
Test
pin
serves as an
external
interrupt
in-
put
as
well
as a
testable
input.
An
interrupt
se-
quence
is
initiated
by
applying
a low
"0"
level input
to
the
TO
pin
when
external interrupt
is
enabled.
Interrupt
is
level
triggered
and
active
low
to allow
"WIRE ORING"
of
several
interrupt
sources
at
the
input
pin.
When
an
interrupt
is
detected,
it
causes
a "jump
to
subroutine"
at location
3
in
program
memory
as soon as
all
other cycles
of
the current
instruction
are complete. At
this time,
the
program
counter contents are
saved
in
the
program
counter stack, but the remaining status
of
the
pro-
cessor
is
not.
Unlike the
8048,
the
8022
does
not
contain
a program
status
word. Thus,
when
ap-
propriate,
the carry
and
auxiliary
carry
flags are
saved
in
software,
as
is
the accumulator.
The
rou-
tine
shown
below saves
the
accumulator and
the
carry flags
in
only four bytes.
Instructions Bytes
Comments
MOV
R6,A
1
;save accumulator
CLR A
1
;clear accumulator
DA A
1
;convert carry flags into
sixes
MOV
R7,A
1
;save status
of
carry flags
The end
of
an
interrupt
service subroutine
is
marked
by the execution
of
a
Return from
Interrupt
instruction
(RETI). Prior to returning
from the
inter-
rupt subroutine,
however, the status
of
the accu-
mulator
and
the carry
flags
are restored
in
software.
The
following routine
restores the
sta-
tus of
the
accumulator
and
the carry
flags,
which
was
previously saved,
in
five bytes.
Instructions Bytes
Comments
MOV
A,R7
1
;restore carry flags status to
Add
A,#0AAH
2
;accumulator and set/clear carry
flags
MOV
A,R6
1
;restore
accumulator
RETI
1
;return
The
interrupt
system
is
single
level
in
that
once an
interrupt
is
detected,
all
further interrupt
requests
are ignored
until
execution
of
a RETI
re-enables
2^
V
LC
TTL
GATE
-o
FREQUENCY REFERENCE
OPTIONS
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