>I am working on a pulse meter
>I want to use a 16f84 or 16f627-628
>Supouse that the normal pulse is beetwen min.0 and max.240
>So in that case i have a frequency beetwen 0-4 pulse/sec.
>How can i measure the time beetwen 2 pulses using the rb0/int interrupt
>and 4MHz clock.
>I need to have a number beetwen 0-240 after to interrupts

This is hilarious.  I was just looking for a better method to calculate RPM
last week.  Basically the same thing.  I ended up not using the 'time
between pulse' method because of the calculations involved.  I used "pulses
per a period of time" multiplied by a constant to bring it to RPM, but since
I had so many pulses  to count, the error was reduced.  You couldn't do that
here because of the low number of pulses.

If you want to time the pulses, just set up a 16-bit counter on TMR0 as
described below.  Then, take 60,000,000 and divide it by the timer value
between pulses to get 'pulses per minute'.  There are routines out there for
division:

http://204.210.50.240/techref/microchip/math/basic.htm

1) enable TMR0 interrupt on Tcy.  Clear TMR0 and counter on first pulse
2) every TMR0 overflow, increment the 16-bit counter
3) for a range of 0-4 pulses per second, or .25 seconds per pulse, you'll
have an upper byte value <4 and a middle byte value 0-255.  The
remaining TMR0 is your lowest byte.  Then you just perform the division to
get your 'pulses per minute'.

--Andrew
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