Intel MCS48 User Manual page 204

ID8243
125
<
J
~
100
_o
H
75
LLI
DC
DC
D
O
z
C/J
<
I-
o
50
25
GUARANTEED WORST
CASE
CURRENT
SINKING
CAPABILITIES
OF
ANY
I/O
PORT
PIN
vs.
TOTAL
SINK
CURRENT OF ALL
PINS
1
8 9 10
11
12 13
MAXIMUM
SINK
CURRENT
ON ANY
PIN@.45V
MAXIMUM
Iql
WORST
CASE
PIN (mA)
Sink Capability
The
8243 can
sink 5
mA@.45V
on each
of
its
16
I/O
lines
simultaneously.
If,
however,
all
lines
are not sinking
simultaneously
or
all
lines
are not
fully
loaded, the drive
capability of
any
individual
line
increases as
is
shown
by
the
accompanying
curve.
For example,
if
only 5 of the 16
lines
are
to
sink current
at
one
time,
the curve
shows
that
each
of
those
5 lines
is
capable
of
sinking 9
mA@.45V
(if
any
lines
are to sink
9
mA
the
total
l
OL
must
not
exceed 45
mA
or
five
9
mA
loads).
Example:
How
many
pins
can
drive 5
TTL
loads
(1
.6
mA)
assuming
remaining pins are unloaded?
I
0L
=
5
x 1.6
mA =
8
mA
£l 0L
=
60
mA
from curve
# pins
=
60
mA
+
8 mA/pin
=
7.5
=
7
In this
case, 7
lines
can
sink 8
mA
for
a
total
of
56 mA.
This leaves 4
mA
sink current capa-
bility
which can be
divided
in
any
way
among
the
remaining
8
I/O lines of
the 8243.
Example: This example
shows how
the
use
of the
20
mA
sink capability of Port 7 affects the sink-
ing capability of
the other
I/O lines.
An
8243
will
drive
the following loads simul-
taneously.
2
loads
—
20
mA@1V
(port
7
only)
8 loads
—
4
mA@.45V
6 loads
—
3.2
mA@.45V
Is
this
within the specified limits?
£l 0L
=(2
x
20)
+
(8
x
4)
+
(6
x
3.2)
=
91
.2
mA.
From
the curve:
for
Iol=4 mA,
£l
0L
~
93
mA
since
91.2
mA
<
93
mA
the loads are within
specified
limits.
Although
the
20
mA@1V
loads are
used
in
calculating
tl
L.
it
"
s the largest current
re-
quired®. 45V which determines
the
maximum
allowable
cl 0L
.
Note:
A
10 to
50Kn
pullup resistor to
+5V
should be
added
to
8243
outputs
when
driving to
5V
CMOS
directly.
6-52