Michael Park wrote:

> As I prepare to try my hand at debouncing a switch input, I'm curious to
> know if anyone has characterized mechanical bounce in terms of typical
> frequencies and durations.  If that information is not directly
> available, my question then becomes one of pragmatics: What sampling
> frequency have you successfully used, and how many samples did you need
> before declaring a switch settled?
> Scott Dattalo, on his amazing debounce technique[1], talks about the
> "vague terms" surrounding debouncing in general.  I'd like to get a feel
> for the numbers that real-world practitioners use.

Hmmm. If Scott has said anything about a PIC, it is gospel, but heregoes another approach :
If you have a timed interrupt, it becomes extremely easy to debounce
switches. In your ISR, you sample the input. If it is 'true', increase a
counter, say InputDebounce. If it is 'false', clear it. If it reaches a pre-set
value, you set flag , say InputDetected. The next time the input is sampled,
it increases InputDebounce to one over the pre-set limit, and must not be
increased again. Since the flag will only be set if InputDebounce = pre-set
value, you don't have to worry about repeated triggers.

Interrupt Service Routine :
********************
Check input
Is input true ?
    Yes : Is InputDebounce<Pre_set ?
          Yes : Increase InputDebounce
                Is InputDebounce=preset ?
                      Yes : set InputDetected
    No : Clear InputDebounce


Main Routine :
**************
Your code....
Is InputDetected true ?
    Yes : Service the input
          InputDetected = false
Your code....

Why go to all this trouble (it is a lot of code!) ?
1) You can debounce many inputs with different debounce periods simultaneously.
2) Your main routine is held up with little delays that screw up your timing.
3) You can use this technique to debounce keypads with nice and exotic options like multiple keys
etc.

Drawbacks :
1) One byte and one bit  RAM needed per input.
2) You need a TMR0 interrupt (not avaiable on all PICs)

--
Friendly Regards

Tjaart van der Walt
mailto:tjaart@wasp.co.za

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