The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Switches and Keypads
Figure 7-2. Use a rotary switch to allow users to choose from among several
options.
where address is the location of the byte to be read in memory and bit is the bit number of
the switch.
Detecting a Switch Press
Momentary switches are useful when you want to get the computer’s attention. For example,
you might have a program that normally displays the current temperature and time, but
switches to a setup routine when you press a switch. On a normally open pushbutton
(momentary) switch, the contacts close when you press the switch, then open as you release
it. In a normally closed momentary switch, the contacts open when you press, and close on
release.
Using Interrupts
An easy way to detect a switch press is with an external interrupt. The 8052-BASIC
automatically detects the switch press and branches to an interrupt-handling subroutine.
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