> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Andre Abelian
> Sent: 14 February 2011 17:49
> To: Microcontroller discussion list - Public.
> Subject: [PIC] RPM measurement question
>=20
> Hi all,
>=20
> this is for VW bug engine.
> I have square wave pulse. I use function generator to generate it.
every
> 4th
> pulse is
> 1 cycle. I am using pic12f683 capture input to read this pulse and I
setup
> to
> read every 4th pulse. some thing
> basic I am not picturing right. I need to find out how fast this
engine
> (pulse)
> is running so engine is based on RPM
> revolution per minute. in this case I have to setup a time and count
> pulses to
> determine the speed. Since it is RPM
> counting 1 minute makes no sense so my question is how fast I should
count
> pulses to determine RPM? another words
> lets say RPM shows 1000 revolution per minute if break it down will be
in
> 1
> second 16.666 revolution so
> should I count for 1 second and expect 16 or 17 counts?
>=20
> I do not know for some reason I am not picturing  it right...
>=20
> any suggestion ?

This depends entirely on your application.  If this going to be for a
tachometer, then one second update rate will not be anything like fast
enough, and you won't get very good resolution at low engine speeds.

Instead of counting the number of pulses within a fixed gate period,
also consider measuring the period of the pulse and taking the
reciprocal.  This gets you a new frequency measurement for every pulse,
at the cost of a (possibly large) division operation.

Regards

Mike

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