In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Tinker wrote:

Just throwing in my 2 cents on this one. The common place to read ignition pulses is from the negative side of the ignition coil as this is where is normaly switched. Optoisolators are a wonderful thing, thanks to Mr Schwab for turning me on to them. Anyway, at this point in the electrical system you'll get a division of the engine rpms, 4 cylinders typically fire twice pre ervolution, 8 cylinders 4 times, and singles are a different animal because they are generally once per due to a crankshaft mounted ignition sensor.

So on a 4 cylinder engine, reading one high and one low period will give you the time for half of a crank revolution, and most likely will take a full revolution to complete, Bean could chime in on this. Is the SX capable of catching the high and the low imediately following each other or will it need an extra transition to set up for the next capture?

Engines with individual coils per cylinder could go either way, or you could end up reading the time for 2 revolutions for each ignition signal, some motorcycles may fire 4 times per crank revolution but I believe the typical number to be 2.

RPM's is always dependant on engine and ignition design, distributor car engines have a 2:1 gearing on the distributor, crank triggers dont, motorcycles and single cylinders being primarily crank triggered, and cars depend on the vintage.

None of this is set in stone, it just depends on what your working on, and some of this information may not be perfectly accurate because most of my experience is with motorcycles and older cars. GAF would be the expert in this area. Some day I'll find the spell checker in this thing, till then please excuse the typos.

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