Intel MCS48 User Manual page 59

SINGLE
COMPONENT
SYSTEM
2.9.1
T1
Input
The
8021 T1
input
line
can be used
as
an
input
for
the following functions:
• Event
Counter
(external input)
•
Test
input
for
branch
instructions
•
Zero
voltage crossing detection
The
operation
of
T1 as an
input
to
the Event
Counter
is
described
in
the
Timer/Event
Counter
section.
When
used
as a test input,
theJTI and JNT1
instructions
test
fori
and
levels,
respectively.
The
T1
pin
can
also
be used
to
detect the
zero crossing
of
slowly
moving
AC
signals
(60 Hz).
The
self-biasing circuit
shown
in
Figure 4 permits the Test
1
input to detect
when
the input voltage crosses zero
within
±5%;
the voltage
is
then
coupled through
a
1.0/xf
capacitor.
Maximum
input voltage
is
3V
peak-to-peak.
The
zero cross detection
is
especially useful
in
SCR
control
of
60
Hz
power and
in
developing time-of-day and
other timing
routines.
As
a
ROM
mask
option there
is
a pullup
resistor that
is
useful
for
switch contact input or standard
TTL.
2.9.2
High
Current Outputs
Very high current
drive
is
desirable
for
(a)
ZERO CROSS DETECT
EXTERNAL
CAPACITOR
>->-
minimizing external parts required
to
do
high
power
control.
P10and
P11 have
been
designated high
drive
outputs 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.
2.9.3
Expanded
I/O
The
8021 can be used with the 8243
I/O
expander
chip,
which
provides
additional
I/O capability with a
limited
number
of
overhead
pins.
This chip has 4
directly
addressable
4-bit ports.
It
connects
to
the
PROG
pin,
which
provides a clock,
and
pins
P20-P23, which provide address
and
data.
These
ports
can be
written with a
MOVD
P,A;
ANLD
P,A;
and
ORLD
P,A
for
Ports 4-7.
A
high
to
low
transition
on
PROG
signifies
that
address
and
control are available
on
P20-P23.
The
previous data
on P20-P23
before
an output expander
instruction
is
lost.
Therefore,
when
using
an output
expander
P20-P23
are
not
useful
for
general input/output.
Reading
is
via
the
MOVD
A,
P.
This
circuit
configuration
is
shown
in
Figure
5.
The
timing
diagram
is
shown
in
Figure
6.
The
8021
can
also
use standard low
cost
TTL
to
expand
the
number
of
I/O
lines.
An
example
is
shown
in
the Applications
section.
->—
I
(b)
OPTIONAL PULLUP
RESISTOR
EXTERNAL
SWITCH
>H>-
1
->J
FIGURE
4.
TEST
1
PIN FIGURE
5.
I/O
EXPANDER
INTERFACE
2-23