Finally I solved this, Library from the same person call FreqPeriod.
I can measure from 14.11 till 133.33 HZ without a problems!.

In my case.  it will be  from 850 till  8000 RPM Two stroke cylinder motor.

Thanks Dave!

I have no person that I can discuss this problems...

1. I'm planning to use NXP LPC2478 processor for RPM counter. It has 32 bit timer and hardware allowed to fill up to 4 MHZ  fill signal. So  I can use 555  timer to  form proper signal from Hall sensor. I can easy take care about 5 microsecond open gate ... But I come up the Arduino device and decided to switch to it . Many reasons for that .. free developer tools and a lot of sensors and shields...

I have short conversation with guy that deal with dynamometer and he measure RPM using extra Hall sensor for crankshaft teeth.

It has 32 Teeth ( I will measure tommorow), so I can get more accuracy  measurement of RPM.

Some people advise me that I can measure   acceleration and find out how cylinder 1 and cylinder 2 works  using measure the period between teeth.

Sorry I forgot to mention that the project page

http://code.google.com/p/arduino-telematic-box/

Regards
Igor

Thanks for reply

1. library http://interface.khm.de/index.php/lab/experiments/arduino-frequency-counter-library/

2. What do mean display every second? This is the data logger....

3. And Interrupts  is what for ?

The problem I think that I can not measure 14.1 Hz  in 100ms period ...

Probably use lm2917 freq to vol  converter and measure using ADC ?

Hi!

I'm going to use the Arduino as a data logger.
I need to count RPM 2 stroke 2 cylinder engine (650-8500) each 100 ms.

The frequency range will be from   10.83 HZ   to 141.6 HZ.

The question is If I will use FreqCounter library with 100 ms  can I measure decimal point ?

How else I can measure RPM  each 100 ms ? 10 RPM accuracy is not a problem

Thanks
Igor  

Thanks. I'll use pin2.

Thanks! I know that as I post. I'm asking may be there is a simplest way, like read some register which source is used... Or may be I'm wrong - may be proccessor don't know which source it is used.

I think this question is a common for using Arduino as a monitoring controller

Thanks!

Arduino use 12V for main  power from power supply. To USB port just hookup +5V backup battery. So wheck when power down Arduino still working without interuption. Work fine.
But
how to find out  in my program when 12V lost (220V down)?

I know I can use any pin hookup to 12V( using resitors) and messure, but
may be more easy way to do that ?

I hope it should be like Interupt that happend when Arduino switch from one power to backup and  back