The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 7
Figure 7-5. You can use a 74HC74 D-type flip-flop to latch a switch press. A
second port pin clears the flip-flop.
Adding a Keypad
Keypads offer more options than individual switches or pushbuttons, but at lower cost and
smaller size than a full keyboard. Examples of products that may use keypads include
electronic locks, EPROM programmers, and many test instruments. On some devices, the
keys have custom legends that describe the specific functions of the keys, but generic
numbered keypads are also useful.
Keypad Types
Some keypads have attached cables that terminate in a connector. Others have headers to
which you can solder wires or connect your own cable.
Different keypads follow different decoding schemes for detecting which key is pressed.
Some have a dedicated connector pin for each key and a single common pin to which the
pins connect when a key is pressed (Figure 7-6). You can wire and access these like a series
of individual switches, with a pull-up or pull-down resistor at each switch.
Other keypads use matrix encoding, where the switch connections are arranged in a
rectangular array. Figure 7-7 illustrates a typical hex keypad that uses matrix encoding. There
are four rows (Y) and four columns (X) to which the switches connect. Each key corresponds
to a hexadecimal digit.
116 The Microcontroller Idea Book