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3. TECHNICAL BRIEF
3.8 Keypad Switches and Scanning
The key switches are metal domes, which make contact between two concentric pads on
thekeypad layer of the PCB when pressed. There are 21 switches (Normal Key 21EA, No
sidekey), connected in a matrix of 5 rows by 5 columns and additional GPIO 35 for KEY_ROW5,
as shown in Figure 3-27, except for the power switch, which is connected independently.
Functions, the row and column lines of the keypad are connected to ports of AD6527. The
columns are outputs, while the rows are inputs and have pull-upresistors built in.
When a key is pressed, the corresponding row and column are connected together, causing the
row input to go low and generate an interrupt. The columns/rows are then scanned by AD6527 to
identify the pressed key.
K
E
Y
R
3
K
E
Y
_
R
O
W
0
6
R
3
K
E
Y
_
R
O
W
1
6
R
3
K
E
Y
_
R
O
W
2
6
R
3
K
E
Y
_
R
O
W
3
6
V
A
3
3
0
A
V
C
L
R
3
K
E
Y
_
R
O
W
4
6
R
3
3
9
K
E
Y
_
C
O
L
0
R
3
4
0
K
E
Y
_
C
O
L
1
R
3
4
1
K
E
Y
_
C
O
L
2
R
3
4
2
K
E
Y
_
C
O
L
3
R
3
4
3
K
E
Y
_
C
O
L
4
V
A
3
3
1
A
V
L
C
P
O
W
E
R
K
1
0
8
0
V
A
3
1
1
A
V
L
C
1
4
S
0
2
0
5
0
1
1
7
8
0
V
A
3
0
2
A
V
L
C
1
4
S
0
2
0
5
0
4
2
4
8
0
V
A
3
0
3
7
A
V
L
C
1
4
S
0
2
0
5
0
3
0
8
0
V
A
3
0
4
S
T
A
R
1
4
S
0
2
0
5
0
A
V
L
C
1
4
S
0
2
0
5
0
3
5
8
0
V
A
3
0
5
A
V
L
C
1
4
S
0
2
0
5
0
6
8
0
V
A
3
0
A
V
L
6
8
0
6
8
0
6
8
0
6
8
0
1
4
S
0
2
0
5
0
Figure 3-27 Keypad Switches and Scanning
V
B
A
T
R
3
0
1
E
N
D
1
0
K
R
3
0
9
E
Y
6
8
0
2
3
5
6
8
9
0
S
H
A
R
P
6
V
A
3
0
7
V
A
C
1
4
S
0
2
0
5
0
A
V
L
C
1
4
S
0
2
0
5
0
A
V
- 40 -
C
L
R
L
E
F
T
U
P
R
I
G
H
T
D
O
W
N
3
0
8
V
A
3
0
9
L
C
1
4
S
0
2
0
5
0
A
V
L
C
1
4
S
0
2
0
5
0
M
E
N
U
S
E
L
S
E
N
D
S
I
M
_
D
A
T
A
V
A
3
1
0
A
V
L
C
1
4
S
0
2
0
5
0
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