This is a possibility with a very small amount of code.
Basically chip counter measures frequency, that then is supplied to a DDS which generates 4X the
frequency, then is divided by 2 to produce an exact 50% duty cycle 2X freq output. System pretty
much does not care about input duty cycle as its edge detection for freq measurement.
Additionally there is an over RPM limit which is capable of shutting off ignition if it has a control input,
for motor protection.
Lastly there is a V proportional to freq output should that be an easier display interface. There are
OpAmps onchip if you need to scale the V to a specific range, or easier than that a couple of
additional lines of code to take care of that. Done with 12 bit DAC onchip.
Also onchip is a LCD controller in case you want to drive a LCD or LED or OLED type of display.
Right hand window shows many resources left in chip to use on other stuff. Like use its A/D to measure
V, other sensors like oil/temp/level for display in case you wanted to grow functionality of design.
Design tool (PSOC Creator) and compiler free, board for this $ 10 or $ 15. Depending on what
you want overall system to do.
I will try out above system in a couple of hours or so after I get some other work done.
Regards, Dana.
Post Followup : Seems to be working fine.