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5. 3. 4.2.
4
The
output of each
display driver
is
multiplexed under
the control of
an
internal clock.
Eight-bit display
data
(for
seven
segments
+
decimal
point
or
eight
LED
indicators) are
put
onto
the device output bus
(pins
1
to 4
and
24
to 27).
A
positive pulse
is
then applied
to
the
enablement
line
of the
device
or
group
of indicators
whicn
is
to
display the data.
The enablement
line
waveforms
consist of
500 ps positive-going pulses
at
approximately
250
pps.
5.3.5
Keyboard
Block
5.
3.5.1
Functional Description
5.
3.5.
1.1
A
simplified
diagram
of the
keyboard block
is
given
in
Figure
5.7.
The
encoding
of the
keyboard data
is
performed
within
the
system
without microprocessor
action.
An
interrupt
request (IRQ)
is
made
to
the
microprocessor
when
encoding
is
complete.
Data
transfer
is
initiated
by
the
KEYBOARD
ENABLE
signal
from
the
microprocessor.
5.
3.5. 1.2
The
32
keys are
divided
into
two
16-key matrices.
When
a
key
is
pressed,
its
position
is
encoded
into a 5-bit
word.
One
bit,
carried
on
the
KEYBOARD EXTEND
line,
indicates the
matrix
in
which
the
key
is
located.
The
remaining
bits
indicate the
position of
the
key
within the matrix.
5.
3.5.
1.3
When
a
key
is
pressed, the
encoder
examines
both matrices simultaneously
and generates a
4-bit
code
representing
the
key
position.
The
same
four
bits
are
generated
regardless of the
matrix
in
which
the
key
is
located.
5. 3.5.
1.4
If
the
key
pressed
is
in
the
extended ke y matrix, one
of the inputs to
the
NAND
gate
is
pulled low.
The
KEYBOARD
EXTEND
line
is
then
set to logic
0.
If
the
key
is
in
the
non-extended matri
x,
the
inputs
to
the
NA
ND
gate are
isolated
from
the
key
line
by one
of the diodes
and
the
KEYBOARD
EXTEND
line
remains
at logic
1.
5-12
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