12
KEY INPUT CIRCUIT ON CPU BOARD
34
EPSON
S5U1C17801T1100 HARDWARE MANUAL
(SOFTWARE EVALUATION TOOL FOR S1C17801)
12
Key Input Circuit on CPU Board
Switches connected to the SVT
17801
(S
1
- S
6
) are connected with the AD input port (AIN
7
) and input port (P
31
)
on the S
1
C
17801
as shown in the figure
12
.
1
.
AV
DD
AIN7
S1C17801
P31
S1
S2
S3
S4
S5
S6
5.1K
5.1K
3
4
3
4
3
4
3
4
3
4
3
4
1
2
1
2
1
2
1
2
1
2
1
2
5.1K
5.1K
5.1K
5.1K
5.1K
Fig.
12
.
1
Key Input Connection Circuit on CPU Board
In this circuit, the press of a switch is detected by the port (P
31
) status and the pressed switch is determined by
detecting an input value of AD (AIN
7
).
The mechanism of detecting the switch status by the port (P
31
) status is as follows: If all switches are open, the
port (P
31
) is in HIGH state as it is pulled up by the
35
.
7
K
Ω
resistor (
5
.
1
K
Ω
x
7
) as shown in the figure
12
.
1
. If
any of the switches is pressed, on the other hand, the pull-down resistor value becomes smaller than the pull-up
resistor value, and this turns the port (P
31
) LOW. The mechanism of this circuit enables the press of any switch to
be detected by using interrupt or other method.
Figure
12
.
1
also indicates that the pressed switch can be determined by the variance of ratio between the pull-
up resistance value and the pull-down resistance value, because the ratio changes depending on the switch being
pressed. The change can be detected by an AD (AIN
7
) input value. Figure
12
.
2
shows the equivalent circuit of the
figure
12
.
1
.
AV
DD
R1
R2
V
Fig.
12
.
2
Equivalent Circuit of Single Key Input
The following equation is satisfied in this equivalent circuit:
V = R
2
/(R
1
+R
2
)
×
AV
DD
"V" value for each switch is obtained by resistance values of R
1
and
R
2
that are obtained when the switch is pressed. Thus the pressed
switch can be determined by comparing the "V" value with the value
actually entered into AD (AIN
7
).
Summary of Contents for S5U1C17801T1100
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