> Now, since I want to smooth out any jitter in the tach pulse signal, I
> average out the most recent 20 pulses. I have a CCP1 interrupt function
> that populates a 20-element array with the value of the CCP1 capture
> register.  Once that array is full, I average the values in the array and
> use that number as the pulse width.  You could adjust or eliminate this
> based on how many teeth you have on your tach pickup, how fast you want it
> to respond to changing RPM, etc.

I see no reason why this won't work as you intended, but I think there are
better strategies.  Why is a box filter better than a single or double pole
filter?  The latter require much less state and are easy to compute,
especially if the filter fractions are 2**-N.  This can usually be the case
when all you want to do is get rid of some high frequency noise.  If you are
going to use a box filter, why not 2**N samples so that the divide to find
the average turns into a right shift?

Another observation is that you are doing the filtering on the periods.  The
speed value probably has a more predictable dynamic range, making single
pole low pass filtering easier there using fixed point.

In short, I would convert each period to its speed value, then low pass
filter the speed values.  If you don't need a new speed reading as each
tooth goes by, you can set up the timer 1 prescaler to give you only every
Nth pulse.  I doubt you really need 1 part in 6M.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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