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Figure
5.7
-
KeyboardhBlock
Diagram
5.3.5.2
Circuit Description
5.
3.5. 2.1
The schematic
is
given on
page
7-15.
The
keys
are divided
into
two
16-key matrices
having
common
row
lines
connected
to
the
encoder
at
IC2-7,
8,
10,
and
11.
The
matrices have separate
column
lines
connected
in
pairs to
IC2-1,
2, 3,
and
4.
5. 3.5.
2.2
The
encoder normally
holds the
row
lines
at logic
0,
When
a
key
is
pressed, the
corresponding
column
line
is
pulled to logic
0.
The
encoder then
scans the
keyboard and
stores a
4-bit
code, corresponding
to
the
row and column
of the key,
in
an
internal
register.
Because
the
column
lines
are
connected
to the
encoder
in
pairs,
it
cannot
find
which matrix
contains the key.
5. 3.
5. 2. 3
The
KEYBOARD EXTEND
line
indicates
which matrix
contains
the
key
that
is
pressed.
The
inputs to
IC2
are
normally
held at logic
1
so
that
SK2
pin
9
is
at
logic
1.
If
a
key
in
the
extended matrix (column
lines
connected
directly to the inputs
of
IClb)
is
pressed,
one
input of
IClb
is
pulled to
logic 0
and
SK2
pin 9 will
go
to logic
0.
The column
lines
of
the
other
matrix
are
isolated
from
the inputs of
IC2 by D6, D7, D9,
and DIO,
so
that the
logic level
at
SK2
pin
9
is
not
changed
when
a
key
in
this
matrix
is
pressed.
5.
3.5.
2.
4
When
the
key-position
code
has
been
stored,
the
encoder
sets
the
KEYBOARD DATA READY
line,
at
SK2
pin
4,
to
logic
1
gi
ving a
microprocessor
interrupt.
The
microprocessor
sets
IC2-13
to
logic
0
using the
KEYBOARD
ENABLE
line,
and
the
encoder
puts the 4-bit
code onto
t
he
bus.
The
microprocessor
reads the
code and
the state of the
KEYBOARD
EXTEND
line
to
determine which key
has
been
pressed.
5-13
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