Chris

Interesting. I have a tacho application in mind, and was wondering about
whether to time the interval between adjacent pulses, or to count a
number of pulses over a fixed time. You've gone for the latter - are
there pros and cons for the two options?

Regards

John Midgley

>-----Original Message-----
>From:  Chris Eddy [SMTP:ceddy@NB.NET]
>Sent:  01 June 2000 05:45
>To:    PICLIST@MITVMA.MIT.EDU
>Subject:       Re: Truck tachometer
>
>Jacky;
>
>For tachometer measurement, I suggest using the interrupt on port B0 feature.
>The software can increment a count every time a pulse arrives, and at the
>same
>time timer 0 or timer 1 is running a crystal time.  At some predetermined
>time
>increment, the counts and time are computed to arrive at RPM's.  RPM's are
>really RPS, so you divide counts by time period, scale it to arrive at the
>correct units, and you have your RPM's.  I would go one step further and fold
>each new reading into a mild low pass filter so that you get a smooth result.
>
>Just a side note, if the alternator is belt driven, you may not get a precise
>result.  The belt might have 'slippage' under various RPM's and alternator
>load.  Picking up the ignition pulse is more exact.
>
>Chris Eddy
>
>Jacky Joulin wrote:
>
>> Hello ,
>> I need realize an application to read truck speed motor ( a tachometer ).