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Thread: Challenge! (Everyone loves a...)
picon face BY : Drew Vassallo email (remove spam text)

>What I found for a tach functionality for an on-board data logger for my
>boats is to use a fixed period of reporting, and have it just count pulses
>that period.  There was too much variation rotation-to-rotation for the
>method to be as useful as I would have liked.
>My operating range was about 600-60000 RPM.

This sounds similar to my operating range.  What did you use for your time
period?  And did you resolve it into RPM?  If so, your multiplier is your
error.  i.e., if you had a time period of 16 bits at 4MHz, you'd have 6
pulses in 0.065535 seconds, resolved to RPM is 5493 RPM.  However, if you
were almost at 7 pulses, which would be 6408 RPM, you'd be off 14%.  Plus,
if you had this on a display, it would vary between these two values
constantly.  This is what I wanted to avoid.  The trick is to get the time
frame as high as possible to lower the multiplier.

Your resolution at a 16-bit time counter is 60/.065535 RPM or 915.  Not good
enough at speeds of 2000 rpm, ~ 50% error and low resolution.

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