Speaking of analog frequency counters...
Check this out.. specificaly for VR sensors, I think that would be the best way to read the turbine shaft speed for now at least... just need an analog input.. RPM accuracy with 10 bits would be +-150 RPM, but seriously if it's +-1000 (there's other inaccuracies) it'll be close enough to do what I need it to.
www.ti.com/lit/ds/symlink/lm2907-n.pdfBest part of it is it doesn't take many CPU cycles to figure it out, and you could have the identical circuit with a different scaling factor for RPM as well.. the decoding would just be analoginput*scalefactor = RPM, and it could be read at any time, it'll be spike-free since it's analog, which naturally incorporates averaging, and doesn't need interrupts (you said you really like to save that one pin with an interrupt didn't you?)
There's always more than one way to skin a cat, and looking at all the options (maybe trying them all?) is good.
Benefit to the binary counter method? very high accuracy at high RPM's, but had a long delay at low RPM, doesn't take too may CPU cycles and doesn't need an interrupt though, but must poll the pin at at least double the rate of your peak input frequency... Only requires one CHEAP chip ($.58) and one digital input pin
Benefit to the LM2907 analog? Can read the voltage any time you want the RPM, requires virtually no CPU time, easy to scale, does require an analog input (we're going to be needing a LOT of those, maybe more than are available), Suffers from poor accuracy with 1024 bits + LM2907 non-linearity.
'94 dually, 67/67 HE351VE, NV5600, ~600hp
'93 ECLB 47RH, new toy truck, H pump project, 1000hp goal, 300K miles
93 XCLB auto, bone stock, 350K miles
93 XCLB 5spd, bone stock, 100K miles