Intel MCS48 User Manual page 313

8255A/8255A-5
8255A
OPERATIONAL DESCRIPTION
Mode
Selection
There
are
three
basic
modes
of operation
that
can be
select-
ed by the system software:
Mode
—
Basic
Input/Output
Mode
1
-Strobed
Input/Output
Mode
2
—
Bi-Directional
Bus
When
the reset input
goes "high"
all
ports
will
be
set to
the input
mode
(i.e.,
all
24
lines
will
be
in
the high im-
pedance
state).
After the reset
is
removed
the
8255A
can
remain
in
the input
mode
with
no
additional initialization
required. During the
execution
of
the
system program
any
of
the other
modes may
be selected using a single
output
instruction.
This allows a single
8255A
to service
a
variety of peripheral devices with a simple software
maintenance
routine.
lint;.
The modes
for
Port
A
and
Port
B
can be separately defined,
while Port
C
is
divided into
two
portions as required
by
the
Port
A
and- Port
B
definitions. All of the
output
registers, in-
cluding the status
flip-flops, will
be
reset
whenever the
mode
is
changed.
Modes may
be combined
so that their
functional definition
can be
"tailored" to
almost any
I/O
structure.
For
instance;
Group B
can be
programmed
in
Mode
to
monitor
simple switch
closings
or display
compu-
tational
results.
Group
A
could be
programmed
in
Mode
1
to
monitor
a
keyboard
or tape reader
on
an interrupt-driven
basis.
ADDRESS BUS
CONTROL
BUS
Ji
n
Ji
RD.WR
tVDo
A
-A,
CS
8255A i
B
C
A
J
8
WO
J
4
h/o
J
4
M/O
1
8
w
PB
7
-PB„
PC
3
-PC
PC
7
-PC
4
PA
7
-PA„
MODE
1
—
"T_B_
PB
7
-PB
CONTROL CONTROL
PA
7
-PA„
OR
I/O
OR
I/O
MODE
2
—
«
J
B
I
8 [I/O
PB
7
-PB
I/O
A T
8
[BIDIRECTIONAL
J PA,-PA„
Figure
3.
Basic
Mode
Definitions
and Bus
Interface
CONTROL WORD
o
D
7
D
6
D
5
D
4
D
3
D
2
D,
D„
PORT
C
(LOWER)
1
= INPUT
=
OUTPUT
PORTB
1
= INPUT
=
OUTPUT
MODE SELECTION
0=
MODE0
1
=
MODE
1
^\
PORT
C (UPPER)
1
= INPUT
=
OUTPUT
PORTA
1
= INPUT
=
OUTPUT
MODE
SELECTION
00 =
MODE
01 =
MODE
1
IX
=
MODE
2
MODE
SET FLAG
1
=
ACTIVE
Figure
4.
Mode
Definition
Format
The
mode
definitions
and
possible
mode
combinations
may seem
confusing
at first
but after
a cursory review
of
the
complete
device operation a simple,
logical I/O ap-
proach
will
surface.
The design
of the
8255A has taken
into
account
things
such
as
efficient
PC
board
layout,
control signal definition vs
PC
layout
and complete
functional
flexibility
to
support almost
any
peripheral
device with
no
external
logic.
Such
design represents
the
maximum
use
of
the available
pins.
Single
Bit
Set/Reset Feature
Any
of the eight
bits
of Port
C
can be Set or Reset using
a
single
OUTput
instruction. This feature reduces software
requirements
in
Control-based
applications.
8-23
00744A