Filter
Top Products
8255A18255A·5
(RESET)
Reset. A
"high
on this input clears the control register
and
all ports
(A,
C,
C)
are set
to
the Input mode.
Group A and Group B Controls
The functional configuration
of
each port is program·
med by the systems software. In essence, the
CPU
"out·
puts"
a control word
to
the 8255A. The control word con-
tains information such as "mode",
"bit
set",
"bit
reset",
etc., that initializes the functional configuration
of
the
8255.
-
Each
of the Control blocks (Group A
and
Group 8)
accepts
"commands" from
the
ReadlWrite Control Logic,
receives
"control words" from
the
internal
data
bus
and
issues
the
proper
commands
to its
associated
ports.
Control Group A - Port A
and
Port C upper
(C7·C4)
Control Group B - Port B
and
Port
Clower
(C3-CO)
The
Control
Word
Register
can
Only
be
written into.
No
Read
operation of the Control
wo;:d
Register
is
allowed.
..
..
"
CI
___
--'
Figure
2.
8225A Block Diagram Showing Group A and
Group B
Control Functions
Ports A,
B,
and
C
9-19
The 8255A contains three 8-bit ports
(A,
B,
and
C).
All
can
be
configured in a wide variety
of
functional charac·
teristlcs by the system software but each has
its
own
special features or
"personality"
to
further enhance the
power and flexibility
of
the 8255A.
Port
A.
One 8-bit data output latch/buffer and one 8-bit
data input latch.
Port
B.
One 8-bit data input/output latch/buffer and one
8·bit data input buffer.
Port
C.
One 8-bit data output latch/buffer and one 8·bit
data input buffer (no latch for input). This port can be
divided into
two
4-bit ports under the mode control.
Each 4-bit port contains a
4-blt latch and
It
can be used
for the
control signal outputs and status signal inputs in
conjunction with ports A and
B.
PIN
CONFIGURATION
PIN
NAMES
D7
00
·DATA
BUS
(BI·DIRECTIONALI
RESET
RESET
INPUT
CS
CHIP
SELECT
--
"e
READ
INPUT
--
W"
WRITE
INPUT
AG,A1
PORT ADDRESS
--
PA7·PAD
PORT
A
181Tl
PB7·P80
PORT
BIBITI
Pe7·pca PORT C (BITI
Vee
+5
VOL
15
GNe
'VOLTS
AFNoOO744A-03