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8255A18255A·5
CONTROL
WORD
Figure 5.
Bit
Set/Reset Format
Operating Modes
BIT
SET/RESET
1 "'SET
0=
RESET
BIT
SELECT
01234567
0101010180
001100118,
00001'118
BIT
SET/RESET
FLAG
0'"
ACTIVE
MODE 0 (Basic Input/Output). This functional configura·
tion
provides
simple
input
and
output
operations
for
each of the three ports.
No
"handshaking"
is required,
data is
simply
written
to
or
read from a specified port.
.
--'r
C
IR
-
INPUT
t==..t
AR
-
CS,
Al,
AD
----
------
~<
MODE 0 (Basic Input)
~r-
<>
tAW
-I
CS.
A1, AD
OUTPUT
MODE 0 (Basic
Output)
t
Ro
When
Port C
is
being
used
as
status/control
for
Port A or
B.
these
bits
can
be
set
or
reset
by using the
Bit
Set/Reset op·
eration just
as
if
they were data
output
ports.
Interrupt Control Functions
When the 8255A ie programmed
to
operate in mode 1
or
mode
2,
control
signals are provided
that
can be used as
interrupt
request
inputs
to
the
CPU. The
interrupt
reo
quest
signals, generated from port
C,
can be
inhibited
or
enabled by setting or resetting the associated
INTE flip·
flop,
using
the
bit
set/reset
function
of
port
C.
This function allows the Programmer
to
disallow
or
allow a
specific
I/O device
to
interrupt the
CPU
without
affecting
any other device in the interrupt structure.
INTE flip-flop definition:
(BIT-SET)
-INTE
is
SET
- Interrupt enable
(BIT-RESET)
- INTE
is
RESET
- Interrupt disable
Note:
All
Mask
flip-flops
are
automatically
reset
during
mode selection
and
device
Reset.
Mode 0
Basic
Functional Definitions:
• Two 8-bit ports
and
two 4-bit ports.
•
Any
port
can
be
input or output.
• Outputs
are
latched.
• Inputs
are
not
latched.
•
16
different Input/Output configurations
are
possible
in this Mode.
tRR
0
-,
<-
-t
HR
-!
-
tRA------:!
1
I.
toF
l---
'wW---~-
7
f-
I'---tow
'wo--l
twA
-
-
9-21
AFN-00744A-05