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2-1
A-D Key
Identification
Circuit
The key identification
circuit
used in this chassis
uses a switched
resistive
ladder
network
in
an
A-D
conversion
circuit to generate
and send a voltage
to the
CPU when a key is pressed.
The
CPU
uses
this
voltage
to determine
which
key was
pressed.
This
resistive
circuit
eliminates
the
need
for encoder/decoder
devices,
simplifying
design
and
adding
to the
reliability of the
TV.
The table shows the voltages
input to CPU pin 34 and 35,
when a given key is pressed.
Q
K1
5"
SW
CPU
35 4
34 +
47kzizEa47k
[:1
15'
K3
&o
15'
6.8k
K'
Kll
6.8'
—
—
4.7'
K5
~2
4.7'
—
2.7'
K6
K13
2.7'
—
—
1.8'
~7
K14
1.8'
—
—
1.5'
K8
K15
1.5'
—
—
$
150
Inputvoltageto pin 35
I
Kev I
flangeof volta9es I Function
OFF
K1
K2
K3
K4
K5
K6
K7
KR
lessthan0.6V
0.6V- 1.2V
1.2V- 1.8V
1.8V- 2.4V
2.4V- 3.OV
3.OV - 3.7V
3.7V - 4.3V
4.3V - 4.9V
morethan4.9V
NoKey
Pos.+
Pos.-
vol.+(TuSlow)
Vol.
-(Tu Slow)
Function
PreseUMemory
Colour S ystem
SIFSvstem
i
10k
Input voltage to pin 34
-,
Key
Range ofVoltages
Function
OFF
lessthan0.6V
60prog.positions
Sw
0.6V- 1.2V
30prog.positions
K9
1.2V- 1.8V
TVIAV
K1O
1.8V-J2.4V
n/a
Kll
2.4V--3.OV
nla
K12
3.OV - 3.7V
nla
K13
3.7V- 4.3V
nla
K14
4.3V- 4.9V
nla
K15
morethan4.9V
nla
When the supply voltage is 5.oV.
AA1-A
-8-
AA1-A
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