SINGLE
COMPONENT
SYSTEM
this
would include high noise margin inputs
(Vee
1 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.
P 1 a and P
11
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
1/0
In
addition to the
26
digital I I 0 lines contained
on-
board the 8022, a user can obtain additional
1/0
lines by utilizing the
Intel(!)
8243 I
10
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 I 0 lines
which comprise ports 0,
1,
and
2,
the 8022 has
two inputs which are
testable via conditional jump
instructions,
TO
and T
1.
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 a
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
r--_--~
XTAL 1
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
MeV
R6.A
1
;aave
accumulator
CLR
A
DAA
MeV
R7.A
;clear accumulator
;convert carry flags into sixes
;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
MeV
A,R7
1 ;restore carry flaga status to
Add
A,#OAAH
2 ;accumulator
and
set/clear carry flags
MeV
A,RS
;reatore accumulator
RETI
;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
+5V
r---
.......
---i
XTAL 1
1K
OPTIONAL
C=20-5OpF
...
_1_
by'LC
L..-_--
...
XTAL2
INDUCTOR
FREQUENCY
REFERENCE
OPTIONS
J<>--
__
--~XTAL
1
D
1 MEGQ
'----
.......
----.
XTAL 2
.6-3.6
MHz
NC
XTAL2
CRYSTAL
EXTERNAL
2-27
