Debouncing Keyboard Switch Inputs

Any mechanical contact will actually make and break many, many times on each closure. This is because the mass of the moving contact, combined with the spring inherent in all metals, will make it literally bounce off after the initial strike, and several times thereafter.

Jack G. Ganssle did some experiments and captured the waveforms of a bunch of switches, disproving many popular myths about switch bounce. In particular, elastomer button outputs don't "bounce" so much as very slowly rise and fall. The "bounce" on such elastomer button outputs can be eliminated with a Schmitt trigger between the button and the digital logic. "A Guide to Debouncing" http://www.ganssle.com/debouncing.pdf Jack G. Ganssle 2004, 2008

Debouncing Inputs with PICs

Debouncing Inputs with SXs

See also:

• http://www.all-electric.com/schematic/debounce.htm How to debounce with hardware.
• http://drmarty.blogspot.com/2009/05/best-switch-debounce-routine-ever.html "The Best Switch Debounce Routine Ever" by Marty Hauff
• Scott Dattalo: Tweaking a PIC for debounce,

Archive:

• List post "My First Original Circuit!" A Lockout to allow only the first switch pressed to send a signal.