Location; Of The Processor - Intel MCS48 User Manual

EXPANDED
MCS-48
SYSTEM
3.2.2
Addressing
External
Data
Memory
External
Data
Memory
is
accessed
with
its
own
two-cycle
move
instructions
MOVX
A,
@R
and
MOVX
@R, A
which
transfer
8
bits of
data
between
the accumulator
and
the ex-
ternal
memory
location
addressed by
the
contents
of
one
of the
RAM
Pointer Registers
RO
or R1. This allows
256
locations
to
be
addressed
in
addition
to
the
resident
locations. Additional
pages
may
be added
by "bank
switching" with extra output
lines of
the
8048.
3.2.3
Examples
of
Data
Memory
Expansion
The accompanying
figure
shows
how
the
8048 can be expanded
using standard
256
X
4
static
RAMs
such as
the 2101-2 or
its
low
power
CMOS
equivalent, the 5101.
An
8212
serves as an address
latch
while
each
4-bit
half of
BUS
is
connected
directly to
a
bidirec-
tional 4-bit
data bus
of
the
memories.
The
WR
output
of
the processor controls the
Read/
Write input
of
the
memories
while the data
bus
output
drivers of
the
memories
are con-
trolled
by RD.
The
chip select
lines of the
memories
are continuously enabled unless
additional
pages
of
RAM
are required. Also
shown
is
the
expansion
of
data
memory
using
the
8155
memory
and
I/O
expanding
device.
Since the
8155 has an
internal 8-bit
address
latch
it
can
interface
directly to
the
8048
with-
out the
use
of
an
external
8212
latch.
The
8155
provides an
additional
256 words
of sta-
tic
data
memory
and
also includes
22
I/O
lines
and
a 14
bit
timer.
See
the following sec-
tion
on
I/O
expansion and
the
8155 data sheet
for
more
details
on
these
additional features.
3.3
Expansion
of
Input/Output
There
are four possible
modes
of I/O
expansion
with the 8048:
one
using a
special
low
cost
expander,
the 8243;
BUS
ALE
8048
RD
WR
A
<c=
8
"T>
<^
V
DI1-8
DS2
R91 , D01
DS1
°*'* D08
MD
I/O 1-4
CS1
2111/2101^
7
5101
OD
R/W
3l
^
1/0 1-4
CS1
. .
2111/2101^^2
07
5101
OD
R/W
+5V
p) r
i . ,
1
3x
BUS
ALE
8048
Wf
RD
c
^
c
c=^>
[TEST
'
INPUTS
AD
-7
ALE 8155
256X8
WR
RAM
RD
IO/M
OO
I/O
-+ TIMER OUT
I/O
8048
INTERFACE TO
256
X
8
STANDARD MEMORIES
3-5