Download Input Switches

Vcc
Keyboard Matrix of Switches
(cont.)
Keyboard Matrix of Switches
12
00
01
02
10
11
12
74LS151 8-to-1
Input
Multiplexer
A
06
07
I0
I1
17
I2
Scanned Switch
Data
I3
To Input Port
I4
Z
I5
I6
B
70
71
77
I7
E
S2 S1 S0
• A keyboard is an array of switches arranged
in a two-dimensional matrix.
• A switch is connected at each intersection of
vertical and horizontal lines.
• Closing the switch connects the horizontal
line to the vertical line.
• 8*8 keyboard can be interfaced directly into
8-bit output and input ports at point A and B.
Vcc
O0
O1
E3
E2
O2
E1
74LS138 3-of-8
Decoder
S2, 2008
O3
O4
O5
A2
O6
A1
O7
A0
Select Input From
Output Port
Scan Input From Output Port
COMP9032 Week8
9
S2, 2008
COMP9032 Week8
Keyboard Matrix of Switches
(cont.)
Ghosting
• Software can scan the key board by
outputting a three-bit code to the decoder and
then scanning the multiplexer to find the
closed switch or switches.
Vcc
R3
– The combination of the two 3-bit scan codes
(A2A1A0 and S2S1S0) identifies which switch is
closed. For example, the code 000000 scan
switch 00 in the upper left-hand corner.
• The diode prevents a problem called
ghosting.
S2, 2008
COMP9032 Week8
10
R2
R1
Col 0 Col 1 Col 2
00
01
02
10
11
12
20
21
22
Row 0 (Pulled low, error)
Row 1 (Pulled low, OK)
Row 2 (High, OK)
Low
(Scanned column)
11
S2, 2008
COMP9032 Week8
12