Electronic – Button debounce ringing

buttoncapacitancedebounceswitches

I have a circuit where I am using a tactile switch to pull 3.3v down to ground and investigating debouncing the button. My relevant part of the circuit is

The 100nF cap does work here my signal has ringing that operates in the undefined region and causes a few false clicks. Increasing the capacitance to 1uf makes it worst. I tried using 10nF which was better. It reduced the ringing time but did not remove it.
Image of scope trace below for 10nF.

This ringing is not really a switch bounce contact problem is it.
What causes this ringing. There is capacitance on the power rail could it be that ?

My best results are with removing the caps entirly.

I know I can use software to solve this issue so it is not a big issue but I am using this as a learning exercise.

My easyeda design:

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Best Answer

The ringing is caused by poor layout (possibly breadboarding is the worst culprit) AND the fact that your button's switch-contact is shorting out a charged capacitor - the instantaneous current flow is (practically) tens of amps and that surge (along with poor wiring and layout creating excessive loop inductance) causes the ringing.

Try putting a 10 ohm resistor in series with each capacitor.

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Electronic – How is this Pushbutton Debounce code

I can't put code in a comment, hence an answer. My 'framework' for simple embedded systems is the main loop with polling. To minimize current consumption the main loop can wait let's say 50 ms in idle mode. I don't know which uC you use, I am familiar with PICs, which can awake from a sleep by an interrupt.

 set up an interrupt to wake me from sleep each 50 ms
 down_counter = 0
 for(;;){
    sleep();
    if( key down ){
       down_counter++;
       if( down_counter == 20 ){
          (start of) long_down detected
       }
    } else {
       if( down_counter > 1 && down_counter < 20 ){
          (end of) short press detected
       }
       down_counter = 0;
    }
 }

Electronic – Time parameters of pressing a tact button by people

I am not sure how fast. It's likely few hundered miliseconds.

2) Here are some nice resources about the topic. It highly depends on the type of switch and how it's built, but 10's of miliseconds is the norm.

http://www.ganssle.com/debouncing.htm

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CC0QFjAA&url=http%3A%2F%2Fwww.picotech.com%2Fexperiments%2Fswitch_bounce%2Fmeasuring-switch-bounce.html&ei=lPG6UOeaOsG82wXgrYGYDQ&usg=AFQjCNHB902_WMeWA-7odiuq6gCt1rEciA&cad=rja

http://www.eng.utah.edu/~cs5780/debouncing.pdf

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