Im trying to do the same thing with some added difficulties. Im trying to
determine the rpm on a motor bike, which normally rev's much higher than a
car.

Added complexity : some motor bikes fire the spark twice for every 4-stroke
cycle, or once per rev ( on the combustion stroke AND on the exhaust
troke  - well my 6 cyclinder does anyway ).

I Also came up with the idea of not trying to display the rpm in realtime
but rather at about 4hz. updating the display faster than that is pointless.
And then use a rule of averages for calc the rpm for that period. Makes
things a litte less jittery. I also only display down to 100 rpm. On a bike
that can rev at 14,000 rpm , not seeing the 10's and 1's isnt all that
important.

I have a working analogue version that uses a PLL and a decade counter that
drives a 3 1/2 digit 7seg led.  That had a display freq of 2hz, so the pll
multiplied the rpm pulse to reflect x amount of pulses per .5 second =
actual rpm. counting and displaying it was easy then.

> > I'm building a device that's sampling the RPM of an engine.
> > The incoming signal is therefore in the range of max 350 Hz, and it
> > is connected to RB0.
>