Andre Abelian wrote:
> Lets say to measure each period I still need to setup a fixed time
> right?

No, you set up a free running counter that is incremented at a fixed known
rate.  On smaller PICs like a PIC 16, this will be timer 1.  The CCP module
then captures a snapshot of timer 1 whenever a pulse comes in.  In the CCP
interrupt, you subtract the new reading minus the previous reading to get
the timer counts between them.

Most engines idle around 700 RPM, or 12 Hz.  Let's say you want this
tachometer to work down to 4 Hz.  That means timer 1 must not overflow
faster than 4 Hz, which means it must be clocked no faster than 262 KHz.  S=
o
clocking timer 1 at 250 KHz works just fine.  At 8000 RPM =3D 133 Hz, that
still gives you a resolution of over 1 part in 1800.  Even if you clocked
the timer off of a 32768 Hz watch crystal, you still get 1 part in 246
resolution at 8000 RPM.  With a little low pass filtering, you sortof
combine period of recent pulses and get a bit more resolution.


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