> It looks like you're measuring period instead of frequency

I am. Division of TMR1 into 4,000,000 (the number of 250ns in
1 second) returns a hertz result directly

eg if TMR1 capture is 0x3918 (ie d14616 * 250ns = 3654us), then
dividing 4,000,000 by 14616 = 273Hz

> Another approach would be to actually measure the frequency
> 
> Frequency counters are relatively slow compared to period counters

That's what I've done on another project (variable-pitch propellor
controller for light planes)

Calculations are made on a running average of the propellor rpm and
the result determines the direction and duration of the blade motor to
meet the set rpm target


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