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EXPANDED MCS-48 SYSTEM
data sheet
in
Chapter 6 for details. Also
included
in
the
8155
is
a 14-bit program-
mable timer. The clock input to the timer
and the timer
overflow output are available
on external pins. The timer can
be
pro-
grammed to stop on
terminal count or to
continuously reload itself. A square wave or
pulse output on terminal count can also
be
specified.
1/0
Expansion Examples
(See
Also
Chapter 5)
The accompanying figure shows the
expansion of
1/0
using multiple 8243's. The
only difference from a single
8243
system
is
the addition of chip selects provided by
additional
8048
OLitput
lines. Two output
lines and a decoder could also
be
used to
address the four chips. Large numbers of
8243's
would require
achip
select decoder
chip such
as
the
8205
to
save
1/0
pins.
Also
shown
is
the
8048 interface
to
a stan-
dard MCS-80 peripheral;
in
this case, the
8255
Programmable Peripheral Interface, a
40
pin
part which provides three 8-bit pro-
grammable I/O
ports.
The
8255
bus
interface
is
typical of programmable MCS-80 peripher-
als
with
an
8-bit bidirectional data
bus,
a
RD
and
WR
input for Read/Write control, a
CS
PORT
1
8048
(chip select) input used
to
enable the Read/
Write
control logic
and
the address inputs
used
to
select various internal registers.
8048
ALE
Fi15
P20
P21
8048
AD
ViR
BUS
A.,
A,
8255
PROGRAMMABLE
OPTION #,
Ao
A,
PERIPHERAL
INTERFACE
8255
PROGRAMMABLE
PERIPHERAL
INTERFACE
AD
FA
00·7
cs
-=-
OPTION
#2
INTERFACE TO
MCS-SO
PERIPHERALS
PORT
A
PORT
B
PORT
C
PORT
A
PORT
B
PORT
C
PROGI------~-----__+------~---
___
___'
LOW
COST
1/0
EXPANSION
3-8