If you use a table for this, get a spreadsheet and list all possible periods
you can measure and the corresponding RPM.  I guess you'll do this for 256
values for now.  Now look at adjacent RPM values in the table and you'll see
that at one end of the table, there is a larger jump from one RPM value to
the next.  Can you live with that resolution?  If not, then you'll need to
use a larger table, and that takes a bit more work, and a lot more space.

Personally, I would (and have already) done the math.  Yes it does take a
bit of work, but it's not really complex.



8valves wrote:
> 
> Howzit guys
> 
> 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. Instead
> what I'd like to do is use the period value as an index into a look up
> table that will hold the corresponding RPM values. The problem is that the
> period is exponentially related to the RPM. How do I go about conditioning
> the period value as the index into the table? Would you guys suggest that
> I instead implement the formula? If so, does anybody have asm routines to
> handle these big numbers?
> 
> Thanks guys
> 

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