Intel MCS48 User Manual page 75

EXPANDED
MCS-48
SYSTEM
A
high
to
low
transition of
the
PROG
line indi-
cates that
address
is
present while
a low
to
high transition
indicates the
presence
of data.
Additional 8243's
may
be added
to
the four
bit
bus and
chip selected using additional
output
lines
from
the
8048/8748.
I/O
Port Characteristics
Each
of
the
four 4-bit
ports
of
the
8243
can
serve as
either input
or output
and can
pro-
vide high drive capability
in
both the high
and
low
state.
3.3.2 I/O
Expansion with Standard
Peripherals
Standard MCS-80/85
type
I/O
devices
may
be
added
to
the
MCS-48
using the
same
bus
and
timing
used
for
Data
Memory
expan-
sion. I/O
devices
reside
on
the
Data
Memory
bus and
in
the data
memory
address space
and
are
accessed
with the
same
MOVX
instructions.
See
the previous
section
on
data
memory
expansion
for
a
description
of timing.
The
following are a
few
of
the
Standard
MCS-80
devices
which
are very useful
in
MCS-48
systems.
8214
Priority Interrupt
Encoder
8251
Serial
Communications
Interface
8255
General
Purpose Programmable
I/O
8279 Keyboard/Display
Interface
8253
Interval
Timer
See
Chapter
7 for detailed
data sheets
on
these
and
other
components.
3.3.3
Combination
Memory
and
I/O
Expanders
As
mentioned
in
the sections
on program and
data
memory
expansion
the
8355/8755 and
8155 expanders
also contain I/O
capability.
8355/8755:
These
two
parts are
ROM
and
EPROM
equivalents
and
therefore contain
the
same
I/O structure. I/O
consists
of
two
8-bit
ports
which
normally reside
in
the
exter-
nal
data
memory
address space and
are
ac-
cessed
with
MOVX
instructions.
Associated
with
each
port
is
an
8-bit
Data
Direction
Reg-
ister
which
defines
each
bit in
the port
as
either
an
input or
an
output.
The
data
direction
registers
are
directly
addressable thereby
al-
lowing the user
to
define
under software con-
trol
each
individual
bit
of
the ports
as
either
input or output.
All
outputs are
statically
latched
and
double
buffered. Inputs are not
latched.
8155/8156:
I/O
on
the 8155/8156
is
config-
ured as
two
8-bit
programmable
I/O ports
and one
6-bit
programmable
port.
These
three registers
and
a Control/Status
regis-
ter
are accessible as external data
memory
with the
MOVX
instructions.
The
contents
of
the control
register
determines the
mode
of
the three
ports.
The
ports
can be
programmed
as input or output with or
without associated
handshake
communi-
cation
lines.
In
the
handshake mode,
lines
of
the
six-bit
port
become
input
and
out-
put strobes
for
the
two
8-bit ports.
See
the
8048
INT
P20
RD
WR
BUS
-<
c
INT
C/D
8279
_
KEYBOARD
RD
DISPLAY
WR
"N
DATA
-j/
BUS
CS
"X
K£
KEYBOARD
INPUTS
SHIFT
-CNTL
SCAN
OUTPUTS
(A)
DISPLAY
OUTPUT
(B)DISPLAY
OUTPUT
KEYBOARD/DISPLAY INTERFACE
3-7