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EXPAND!!D MCS-48 SYSTEM
3.2.2 Addressing
~Xt~rnal
Data Memory
External Data
Memor~
Is
~~cessed
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
~------------~s----------------------------~
ALE
0S2
1m
8212
gg~
8048
MO
I/O 1·4
CS1
A
.~2111/2101
eI2
o 5101
00
R/W
I/O
1-4
CS1
A .A 2111/2101C'S2
o 7 5101
00
R/W
ROt--------------------4----~------------~
~
____
~m~----------------------~----------------~
sus
K 8
"
ALE
8048
WR
RD
PORT
3
TEST
~
INPUTS
18
}
1/0
8048 INTERFACE TO 256 X 8 STANDARD MEMORIES
3·5
ADo·7
ALE 8155
268X8
WR
RAM
iii)
101M
22
"
-y
110
TIMER
IN
TIMER
OUT