The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Switches and Keypads
7
Switches and Keypads
Most microcontroller projects will include switches, a keypad, or some other way of
allowing users to control the circuits inside. The control might involve flipping a switch
begin an operation, pressing a key to to select an option, or entering a number for the program
to use in its operations. For simple tasks, you can use toggle, slide, or pushbutton switches.
Other projects might call for a keypad with an array of switches, with each labeled with a
number, letter, or other description. This chapter shows how to add each of these to your
system.
Simple Switches
Figure 7-1 shows two single-pole, single-throw toggle or slide switches connected to an
input port. Each has contacts that connect when the switch is closed, and open when the
switch is open.
In (A), when the switch is open, the pull-up resistor brings the input high. When the switch
closes, the input connects to ground and reads low. Switch (B) is the reverse: when the switch
is open, the pull-down resistor brings the input low. When the switch closes, it connects to
+5V and reads high.
You can connect a switch to an unused bit on the 8052-BASIC’s Port 1, to an input on an
8255 PPI, or to a input on a 74LS244 buffer. The 8052-BASIC has internal pullups that
bring the inputs to +5V when they are not being driven by another source. So, if you connect
The Microcontroller Idea Book 109