Switch at cetain frequency

I want to design a crank disconnect and overspeed tripping circuit for a small engine, whose speed signal will be taken from a magentic Pickup. Depending on the number of teeths of the flywheel the crank disconnet speed will be around 500Hz and Overspeed will be around 3000Hz. I have gone through some of the tutorials and found that timer0 of the PIC can handle the frequency detection. I have 16F84A and 16F877A PICs. Any suggestions?
Thanks

You could set up an external interrupt that will trigger each time the magnetic pickup is triggered, the interupt routine would read the running timer's value and compare it to the timer's value the last time the interrupt ran, then you just work the math out from there as to the rotation speed.

Using an interrupt is a good way to allow the PIC to do other things.
Else just using an internal countdown to do the same task. If it reaches zero, then jump to whatever routine or output logic you need.

There's always a 555 timer if you want to do it the hard way

A little **broken link removed** could be used to turn the frequency into an analog voltage -- then you could just read an analog voltage when you wanted the engine speed and have no need for interrupts or timers. Just an example of the hardware/software tradeoffs made in designs...

PIC is the best way as you can add a delay for say 500mS to detect overspeed before the action takes place. 'F84 is all you need - use up the old one's first!