H,

it is easy:

A cascaded 12-stage binery counter (4040) begins to run
The counter stops by a marke, what comes one time pro RPM
The value of the counter must be served (zB. 75HCT573)
Now to the begin, to the begin, to the begin.

To show the value there are 2 possibilitys:
I calculate with a 8052 and show with LEDs or LCDs

The other way is: I calculated before (with basic or per hand) and
write the BCD-values in a eprom, the adresses of the eprom gives the counter.

This solutions are exact and better than I work with IRQ.


With regards Manfred