Using the hall sensor from a bldc motor to get rpm on arduino

[Benta]

BTW: the Hall sensors do not "control the speed of the motor", they control the commutation of the stator.

Of course hall sensors can be used for speed loop, too, why not; that's pretty usual thing to do. Using them primarily for commutation does not exclude other types of uses for the same data.

Please do not misrepresent what I wrote. The speed loop has nothing to do with commutation, and of course you can use the Hall sensor information for the speed loop as well. I never said otherwise.
Is this kind of hyperbole your standard form?

[trilerian]

Any chance you're using voltage to control the speed of the fan?  The pulse Vmax is the same as the voltage driving the fan (at least the one I played with on my scope did this.)  You may be generating runt pulses.

No, the signal is coming from a 540 sized motor for RC racing.  The voltage driving the motor will be around 8.0v.

I put together a code with interrupts, but it was just as slow to start recording as the first way.  Ultimately I don't start getting readings until 30-40Hz.  I changed the interrupts to gather more and less counts for the average, but then accuracy goes out the window.  It is kind of funny really, the dyno works best at 10Hz sampling with whatever software it is written with.  Using interrupts I calculated that 10Hz gave good results for the arduino. 

Regardless, using either method works to get the rpm with ok accuracy.  It just doesn't start outputting data quickly enough to use it for my original intent.  But I will use it for another project. 

[Siwastaja]

BTW: the Hall sensors do not "control the speed of the motor", they control the commutation of the stator.

Of course hall sensors can be used for speed loop, too, why not; that's pretty usual thing to do. Using them primarily for commutation does not exclude other types of uses for the same data.

Please do not misrepresent what I wrote. The speed loop has nothing to do with commutation, and of course you can use the Hall sensor information for the speed loop as well. I never said otherwise.
Is this kind of hyperbole your standard form?

I frankly did not understand what you meant by "the Hall sensors do not "control the speed of the motor", they control the commutation of the stator.", and I still don't. Quite obviously, hall sensors do not directly and literally control either the speed or commutation of the motor, but their data can be used as inputs for circuits that control commutation, speed or both. If we assume by "control" OP means "using as an input for control", then your remark is off because hall sensors can equally well "control" both commutation and speed, so that kind of "not X but Y" when both X and Y are true would be wrong.

That is the assumption I made; Instead of asking for clarification as I should have, I elaborated on that assumed meaning, on the basis that OP might have misunderstood it the same way as I did, in order to be helpful and correct the misunderstanding, even if it was not what you meant. Sorry 'bout that. Now it's your turn to explain A) what you actually meant, B) why do you think it's helpful.

[Benta]

I frankly did not understand what you meant by "the Hall sensors do not "control the speed of the motor", they control the commutation of the stator.", and I still don't.

I don't see any ambiguity at all. On BLDCs the stator (in classic PMDCs "anchor") is fixed, no matter whether it's an inrunner or an outrunner. The PMs move.
That the Hall sensors can be used for an additional speed control loop is obvious, but not their prime purpose.