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44-Pin Demo Board Lessons
3.2.6
Lesson 6: Switch Debouncing
Mechanical switches play an important and extensive role in practically every com-
puter, microprocessor and microcontroller application. Mechanical switches are inex-
pensive, simple and reliable. However, switches can be very noisy electrically. The
apparent noise is caused by the closing and opening action that seldom results in a
clean electrical transition. The connection makes and breaks several, perhaps even
hundreds, of times before the final switch state settles.
The problem is known as switch bounce. Some of the intermittent activity is due to the
switch contacts actually bouncing off each other. Imagine slapping two billiard balls
together. The hard non-resilient material doesn’t absorb the kinetic energy of motion.
Instead, the energy dissipates over time and friction in the bouncing action against the
forces push the billiard balls together. Hard metal switch contacts react in much the
same way. Also, switch contacts are not perfectly smooth. As the contacts move
against each other, the imperfections and impurities on the surfaces cause the electri-
cal connection to be interrupted. The result is switch bounce.
The consequences of uncorrected switch bounce can range from being just annoying
to catastrophic. For example, imagine advancing the TV channel, but instead of getting
the next channel, the selection skips one or two. This is a situation a designer should
strive to avoid.
Switch bounce has been a problem even before the earliest computers. The classic
solution involved filtering, such as through a resistor-capacitor circuit, or through re-set-
table shift registers (see Figure 3-4 and Figure 3-5). These methods are still effective
but they involve additional cost in material, installation and board real estate. Debounc-
ing in software eliminates these additional costs.
FIGURE 3-4:
FILTERING DEBOUNCE SOLUTION
+V
R2
R1
Filtered
Switch
SW
C 1
Output
FIGURE 3-5:
SHIFT REGISTER DEBOUNCE SOLUTION
+V
R1
D
Qn
Filtered
Switch
SW
CLK
CLR
Output
Debounce
Clock
? 2007 Microchip Technology Inc.
DS41296B-page 23
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