Intel MCS48 User Manual page 367

8279/8279-5
INTERFACE CONSIDERATIONS
Scanned Keyboard Mode,
2-Key
Lockout
There are three possible
combinations
of
conditions
that
can occur
during
debounce
scanning.
When
a key
is
depressed, the
debounce
logic
is
set.
Other depressed
keys are looked
for
during the next
two scans.
If
none
are
encountered,
it
is
a single
key depression
and
the
key position
is
entered into the
FIFO
along with the
status of
CNTL
and
SHIFT
lines.
If
the
FIFO
was
empty,
IRQ
will
be
set to signal
the
CPU
that
there
is
an
entry
in
the FIFO.
If
the
FIFO was
full,
the key
will
not
be entered
and
the error flag
will
be
set.
If
another closed switch
is
encountered,
no entry to the
FIFO can
occur.
If
all
other
keys are released before this
one, then
it
will
be entered
to
the FIFO.
If
this
key
is
released before any
other,
it
will
be
entirely
ignored.
A
key
is
entered
to
the
FIFO
only
once per depression,
no matter
how many
keys
were pressed
along with
it
or
in
what
order they
were
released.
If
two keys
are
depressed within the
debounce
cycle,
it
is
a simultaneous depression. Neither key
will
be recognized
until
one key remains
depressed
alone.
The
last
key
will
be treated as a single
key depression.
Scanned Keyboard Mode, N-Key
Rollover
With N-key Rollover each key depression
is
treated
independently from
all
others.
When
a
key
is
depressed,
the
debounce
circuit
waits 2
keyboard scans and then
checks
to
see
if
the
key
is still
down.
If
it
is,
the key
is
entered
into
the FIFO.
Any number
of
keys can be
depressed and another can be recognized and entered
into
the FIFO.
If
a
simultaneous depression occurs, the
keys are recognized
and entered according
to
the order
the
keyboard scan found them.
Scanned Keyboard
—
Special Error
Modes
For
N-key
rollover
mode
the
user can
program
a special
error
mode. This
is
done
by the "End Interrupt/Error
Mode
Set"
command. The debounce
cycle
and
key-validity
check
are as
in
normal N-key mode.
If
during a single
debounce
cycle
,
two keys
are
found depressed,
this
is
considered
a
simultaneous multiple depression,
and
sets
an
error
flag.
This
flag
will
prevent
any
further writing into
the
FIFO and
will
set interrupt
(if
not
yet set).
The
error flag
could be read
in
this
mode
by reading the
FIFO
STATUS
word. (See
"FIFO
STATUS"
for further details.)
The
error
flag
is
reset
by sending
the
normal
CLEAR command
with
Cf
-
1.
Sensor
Matrix
Mode
In
Sensor
Matrix
mode, the
debounce
logic
is
inhibited.
The
status of
the
sensor switch
is
inputted directly to the
Sensor
RAM.
In this
way
the Sensor
RAM
keeps an image
of
the state of the
switches
in
the
sensor
matrix.
Although
debouncing
is
not provided,
this
mode
has
the
advantage
that the
CPU knows how
long the sensor
was
closed and
when
it
was
released.
A
keyboard
mode
can
only indicate
a validated closure.
To make
the software
easier,
the
designer should
functionally
group
the
sensors by
row
since
this is
the format
in
which
the
CPU
will
read them.
The IRQ
line
goes high
if
any sensor value
change
is
detected
at
the
end
of a
sensor matrix scan.
The IRQ
line
is
cleared by the
first
data read operation
if
the
Auto-
Increment
flag
is
set to zero, or
by the
End
Interrupt
command
if
the
Auto-Increment
flag
is
set to
one.
Note: Multiple:
changes
in
the matrix
Addressed by
(SL0-3
=
0)
may
cause
multiple interrupts. (SLo
=
in
the
Decoded
Mode). Reset
nay
cause
the
8279
to
see
multiple
changes.
Data
Format
In
the
Scanned Keyboard mode,
the character entered
into
the
FIFO corresponds
to the position of
the switch
in
the
keyboard
plus the status of
the
CNTL
and SHIFT
lines (non-inverted).
CNTL
is
the
MSB
of
the character
and
SHIFT
is
the next
most
significant
bit.
The
next
three bits
an;
from
the
scan counter and
indicate the
row the key
was
found
in.
The
last
three
bits
are
from
the
column counter and
indicate to
which
return line the
key
was connected.
MSB LSB
CNTL
SHIFT
I I
SCAN
I
1
1 1
RETURN
.1
,
L.
SCANNED KEYBOARD DATA FORMAT
In
Sensor Matrix mode, the data
on
the return
lines
is
entered
directly
in
the
row
of the
Sensor
RAM
that
corresponds
to
the
row
in
the matrix being scanned.
Therefore,
each
switch postion
maps
directly to a
Sensor
RAM
position.
The SHIFT and
CNTL
inputs are
ignored
in
this
mode. Note
that
switches are not necessarily the only
thing that
can be connected
to
the return lines
in this
mode.
Any
logic that
can be
triggered by the
scan
lines
can
enter data
to
the return
line
inputs.
Eight multiplexed
input ports
could be
tied to
the return
lines
and scanned
by
the 8279.
MSB LSB
RL
7
UL
6
RL
5
RL4 RL3 RL
2
RL1
RLp
J
In
Strobed Input
mode, the data
is
also
entered
to
the
FIFO
from
the return
lines.
The
data
is
entered by the
rising
edge
of a
CNTL/STB
line
pulse.
Data can
come
from
another
encoded keyboard
or
simple switch matrix.
The
return lines
can
also
be used
as
a
general purpose strobed
input.
MSB LSB
I
RL7
I
RL
6
RL
5
RL
4
RL
3
RL
2
RI_1
RL
Display
Left
Entry
Left Entry
mode
is
the simplest display format
in that
each
display position directly
corresponds
to
a byte
(or
nibble)
in
the Display
RAM. Address
in
the
RAM
is
the left-most
display character
and address
15
(or
address
7
in
8
character display)
is
the
right
most
display character.
Entering characters from
position
zero
causes
the display
to
fill
from
the
left.
The
17th
(9th)
character
is
entered back
in
the
left
most
position
and
filling
again
proceeds from
there.
8-77
00742A