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Maxim > Design Support > Technical Documents > Application Notes > Microprocessor Supervisor Circuits > APP 287
Keywords: switch debounce, overvoltage, ESD protection, touchscreen monitors, overvoltage protection,
touch screen displays, push button, pushbutton
APPLICATION NOTE 287
Switch Bounce and Other Dirty Little Secrets
Sep 01, 2000
Abstract: Maxim provides a simple solution to reduce the effects of switch ringing. These devices are
applicable for applications with push-button panels, touch screen displays, and simple push-button
devices. These devices also provide overvoltage and ESD protection. The MAX6816/MAX6817/MAX6818
integrates numerous functions into a single IC, thus eliminating numerous discrete components.
Switches can do some really odd things. Most engineers learn this dirty little secret soon after
connecting a switch or a relay to a digital system. Switches don't make and break cleanly on the time
scales of digital systems. Instead, a typical switch makes multiple transitions during the tens of
milliseconds required to open or close, due to effects that include age, operating inertia, mechanical
design, and the microscopic condition of the switch-contact surfaces. Commonly called "switch bounce,"
this behavior is an inescapable fact of life.
After connecting a standard switch to a digital counting circuit, you can observe several counts on
opening and several counts on closing (Figures 1 and 2). This erratic action can wreak havoc on data,
because the exact number of counts does not necessarily repeat in the long term. Switch bounce is not
consistent from unit to unit, lot to lot, or even over the life of an individual switch. Membrane switches
and some other types don't appear to bounce when new, but all mechanical switches bounce sometimes.
Nothing can ensure that another switch of the same type will act the same way, or that a particular
switch will remain bounce-free as it ages.
Figure 1. This rising-edge switch bounce for a small pushbutton switch shows an approximate 5ms
bounce interval that includes 10 transitions. Like a bouncing ball, the switch-action frequency increases
toward the right.
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