>> I'm currently building an RPM measuring device but I have a decision
>> that
>> I may need professional assistance with. I will be measuring the
>> period of
>> a single rotation and will thereafter convert this value to a RPM
>> equivalent. Implementing the tedious conversion formula on assembly
>> language will prove to be a challenging and time consuming task.

No, it's just a division.

You also probably don't need to do the division every period.  There will
some jitter on the periods, so you want to do a little low pass filtering.
In a lot of cases you can design the system to use period directly instead
of rotation speed.  For example, if you are just looking for speed
thesholds, you can do that just as well with period thresholds.

If you really need speed, such as when displaying RPM to a user, then you
can do the conversion in the display routine.  The individual periods are
still measured and low pass filtered (ususally in the CCP interrupt
routine), then the latest official period is left lying around where the
foreground code can grab it at any time.  The display routine would only run
every 250 to 500 ms anyway, so you do the divide then.

>> The problem is that the
>> period is exponentially related to the RPM.

No, it's just the reciprocal.


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