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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
Mode.
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.
Sen.or 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
com-
mand
if
the Auto-Increment flag is set
to
one.
Note: Multiple changes in the matrix Addressed
by
(SLo{j
=0)
may cause multiple Interrupts.
(SLo=O
in the Decoded
Mode). Reset may 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
are 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
I
CNTL
!SHIFT!
+TUR~
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 post ion 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
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
CNTLlSTB 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
RL7! RLs I
RLsl
RL41
RL3!
RL21 RL, !
RLo
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 0 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
ze~o
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.
9-75
AFN-00742A-D&