Ports - Intel MCS48 User Manual

EXPANDED
MCS-48
SYSTEM
another
using standard
MCS-80/85
I/O
devices;
and
a third
using the combination
memory/I/O expander
devices the 8155,
8355,
and
8755.
It
is
also
possibleto
expand
using standard
TTL
devices as
shown
in
Chapter
5.
3.3.1
I/O
Expander
Device
The most
efficient
means
of
I/O
expansion
for
small
systems
is
the
8243
I/O
Expander De-
vice
which
requires only
4
port
lines
(lower
half of
Port
2) for
communication
with the
8048.
The 8243
contains
four
4-bit
I/O ports
which serve as
extension
of
the
on
chip
I/O
and
are addressed as
ports
#4-7.
The
follow-
ing operations
may
be performed on
these
ports:
1.
Transfer
Accumulator
to Port.
2.
Transfer Port
to
Accumulator.
3.
AND
Accumulator
to Port.
4.
OR
Accumulator
to Port.
A
4-bit
transfer
from a
port
to
the
lower
half of
the Accumulator
sets the
most
significant four
bits
to zero.
All
communication between
the
8048 and
the
8243
occurs over
Port
2
lower
(P20-P23)
with timing provided
by an
output
pulse
on
the
PROG
pin of
the processor.
Each
transfer consists of
two
4-bit
nibbles:
The
first
containing the
"op code" and
port
address and
the
second
containing the actual
4
bits
of data.
Nibble
1
Nibble 2
3
2
1
3 2
1
I I
A A
d d d d
Instruction
Code
Port
Address
data
AA
00
Read
00- -Port
#4
01 Write
01-
-Port
#5
10
OR
10-
-Port
#6
11
AND
11-
-Port
#7
EXPANDER INTERFACE
PROG
8048
P20-P23
<^0. t>"°
/I
:>
1
INPUTS
<^r—
4
CHIP SELECT
CONNECTION
IF
MORE
THAN ONE EXPANDER
IS
USED
CS
PROG
8243
DATA
IN
P2
P4
P5
|v
4
V
I/O
I/O
I/O
I/O
OUTPUT EXPANDER
TIMING
PROG
P20-P23
\
-<ZJC
/
>
ADDRESS
(4-BITS)
DATA
(4-BITS)
AND OPCODE
JITS
0,1
BITS
2,3
PORT
ADDRESS
00
~
01
10
11_
READ
WRITE
OR
AND
3-6