Temasek Polytechnic
Microcontroller Technology
EMC2001
Keypad and Sensor Board

4x4
Matrix
Keypad
Touch
Sensor

Key Encoder

The keypad and sensor board houses mainly the 4x4 Matrix Keypad, MM74C992 16 Key
Encoder and QProx QT113 Touch Sensor.
The 4x4 Matrix Keypad is an input device that can be used to interface to microcontroller
(for entry of password for example).
The MM74C992 16 Key Encoder provides all the required logic to encode an array of keypad switches. This requires an external capacitor to implement key board scanner.
A Data Available output goes to a high level when a valid keyboard entry has been made.
The Data Available output returns to a low level when the entered key is released, even if another key is depressed. The Data Available will return high to indicate acceptance of the new key after a normal debounce period.
The use of this encoder drastically reduces the number of I/O pins that would be required without it. Without it, 8 - 16 I/O lines would be required; with it, the number of I/O lines drops to 5 or 6.

D
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1

C
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1

B
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1

A
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1

Key
1
2
3
F
4
5
6
E
7
8
9
D
A
0
B
C

Table 1

D

RC3

C

RC2

B

RC1

A

RC0

DA

RD0

OE

Key Encoder

PIC18F4520

Figure 1
The way the 74C922 encoder works is very simple: When a key has been pressed, the
Data Available pin goes high and the key value (0 – 15) is output from D – A. The logic levels of D – A and the corresponding key values are given in Table 1. For example, when the key 7 is pressed, the logic levels of D - A will be 1000. By connecting these signals to the microcontroller and implementing a look-up technique, all key presses can
be