1) https://noctua.at/pub/media/wysiwyg/...hite_paper.pdf

2) @ these very low frequencies, the use of a divider per 2 ( so ... to measure the rotation period ! ) and using interrupts to drive a custom counter is to consider ...

aside that that wouldn't interfere much with the general timing ...

Now, being too accurate to measure a PC fan speed is somewhat a nonsense ... ( except for some geeks ... )

Alain