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SILICON GATE MOS
8255
MODE
DEFINITION
SUMMARY TABLE
MODE
0
IN
OUT
PAO
IN
OUT
PA1
IN
OUT
PA2
IN
OUT
PA3
IN
OUT
PA4
IN
OUT
PAS
IN
OUT
PAe
IN
OUT
PA7
IN
OUT
PBO
IN
OUT
P81
IN
OUT
PB2
IN
OUT
PB3
IN
OUT
PB4
IN
OUT
PB5
IN
OUT
PBe
IN
OUT
PB7
IN
OUT
PCO
IN
OUT
PC,
IN
OUT
PC2
IN
OUT
PC3
IN
OUT
PC4
IN
OUT
PCS
IN
OUT
PC6
IN
OUT
PC7
IN
OUT
MODE
1
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
INTRB
INTRB
IBFB
OBFB
STBB
ACKB
INTRA
INTRA
STBA
I/O
IBFA
I/O
I/O
ACKA
I/O
OBFA
MODE
2
GROUP
A
ONLY
~
•
•
•
...
~
...
...
•
•
...
~
~
•
...
~
--
--
--
--
--
--
--
--
I/O
I/O
I/O
INTRA
STBA
'IBFA
ACKA
OBFA
MODE 0
OR
MODE 1
ONLY
i
GROUPB
i
GROUP A
I i
GROUP A GROUPS
INPUT CONFIGURATION
Os
0
3
Do
i i
GROUP A GROUP B
(DEFINED BY MODE 0
OR
MODE 1 SELECTION)
OUTPUT CONFIGURATION
0
7
0
6
Os
0
4
0
3
O
2
0
1
Do
Mode 1
Status
Word
Format
Mode 2
Status
Word
Format
allows
the
programmer
to
test
or
verify
the
"status"
of
each
peripheral device and change
the
program flow accordingly..
There
is
no special instruction
to
read
the
status
informa-
tion from
Port
C. A normal read operation
of
Port C
is
executed
to
perform this function.
Reading Port C Status
In
Mode
0,
Port
C transfers
data
to
or
from
the
peripheral
device. When
the
8255
is
programmed
to
function
in
Modes
1
or
2,
Port
C generates
or
accepts "hand-shaking" signals
with
the
peripheral device. Reading
the
contents
of
Port C
Special Mode Combination Considerations
Thereare several combinations
of
modes when
not
all
of
the
bits
in
Port
C are used for
control
or
status.
The
remaining
bits can
be
used
as
follows:
If Programmed
as
Inputs -
All
input lines can be accessed during a normal Port C
read.
If Programmed as
Outputs
-
Bits
in
C
upper
(PC7-PC4)
must
be individually accessed
using
the
bit
set/reset function.
Bits
in
Clower
(PC3-PCO) can be accessed using
the
bit
set/reset function
or
accessed as a threesome by writing
into Port
C.
Source Current Capability on Port B and Port C
Any
set
of
eight
output
buffers, selected randomly from
Ports
Band
C can source
lmA
at
1.5 volts. This feature
allows
the
8255
to
directly drive Darlington
type
drivers
and high-voltage 'displays
that
require such source current.
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