Debounce Trigger Switches in Extreme Conditions

[TheAnalogKid83]

Hi I am curious about the best way to debounce a trigger switch in hardware.

These switches will be used in extreme conditions(-40 to 85, very strict EMI restrictions, etc.)

My initial idea is to just put a simple low pass filter (RC circuit) on the input from the switch to the microcontroller pin. I'd do some research to figure out what is a good time constant for the application.

Is this acceptable, or do I need to do something more active like schmitt triggers? I'd like to throw in a resistor/cap and call it a day, but I must do a very effective design.

I've no experience, so anyone that can tell me some advice would be appreciated.

 

[TheAnalogKid83]

also, I should say that I'd like to avoid software debouncing code as much as possible if the hardware solution is cheap and easy enough.

 

[berkeman]

The traditional switch debounce in hardware uses a SPDT switch and cross-coupled NAND gates. Have you seen this circuit? I have to run, but I can google it if you can't find it. (It's also shown in Horowitz and Hill's Art of Electronics in figure 8.42 [1st edition]).

 

[TheAnalogKid83]

Spdt??

 

[berkeman]

Spdt??

SPDT = Single Pole, Double Throw

Switches are often classified in terms of the number of "poles" and the number of "throws". Check out the table at the start of this wikipedia article about switches:

http://en.wikipedia.org/wiki/Switch

And here's a diagram of how to use a SPDT switch and cross-coupled NAND gates to debounce the switch (see Figure 3):

http://www.elexp.com/t_bounc.htm

 

[minorwork]

Schmitt trigger might be what you need.

 

[NoTime]

Schmitt trigger might be what you need.

That would help with an RC network, but in general no.

 

[alvaros]

I would use an RC filter + schmitt trigger.

RC alone will fall the rise/down time of the signals and digital circuits need a minimum rise/down time to work properly.

 

[TheAnalogKid83]

thank you very much for the help. it looks like the CPU I'm using has a KP module interface with debouncing internal, but I was still curious as to the best way to do it in hardware. Thanks.