> -----Original Message-----
> From: Olin Lathrop [SMTP:olin_piclist@EMBEDINC.COM]
> Sent: Wednesday, September 03, 2003 1:12 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC:] How to monitor a dozen shafts?
>
> Michael Rigby-Jones wrote:
> > The period of just one
> > tooth on the 16 tooth wheel at 200RPM would be a whopping 188ms,
>
> This is WAY off, observe:
>
> (200rev/min / 60sec/min) = 3.333Hz shaft speed
>
> 3.333Hz * 16 teeth/rev = 53.33Hz tooth frequency
>
> 1 / 53.33Hz = 18.8mS tooth period
>
Granted, I screwed up with that one..not sure how I got a factor of ten out.


> > so you
> > wouldn't need to poll the inputs particularly quickly to get a
> > reasonable precision.
>
> Except that this is based on a value that is off by an order of magnitude.
>
> > (the CCP module is great for arbitrary rate interrupts
> > with no timer loading overhead)
>
> Yes, a CCP module can be used to generate a periodic interrupt.  However,
> my first choice for this is timer 2 since it's less useful for other
> stuff.
>
> > By running a counter for each
> > input and storing the value on a rising (or falling) edge you can
> > easily and quickly make a multi channel counter.
>
> But he wants speed, not total elapsed revolutions.
>
And speed (or at least period, from which speed can be derived) is what he
would get.  The counter would counter the number of interrupts between
rising edges, not the total number of rising edges.

Mike






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