Put a mag sensor on the ring gear.
Increment a counter memory location each time
A tooth went by. In effect the computer will
then know about what the e-shaft angle is relative
to the crank angle sensor or home sensor. Resolution with
100 ring gear teeth will then be about 3.6 degrees.

The same signal can be used to build a tach in software.

For an example:
At 2000 RPM the CAS signal comes in at  2000/60 or 33.333 pulses
per second.

With 100 teeth you will get 3333 pulses per second from the ring gear.

To convert that to RPM the real time clock must up date
at 2000/3333 which equals .6 seconds.

I used this trick in my road test system back in the early 70's.
http://www.rotaryeng.net/IEEE-road-test-sys.pdf

Where do we get the .6 second pulse to up date the real
time clock? Guess what? A 555 oscillator will do the job.
It can have a variable resister for calibration reasons.
If there are more or less teeth on the flywheel than 100 the pot
can be adjusted to adjust the real time clock pulse.

This can be done in software using a free running counter
but it is more complicated and not as easily adjusted as
merely using a 555 osc. This device is called a RTC or
Real Time Clock. See the Xplain RTC jpg.

This basically is how the 2nd gen, 3rd gen and RX8 EFI's come up
with RPM. The second gen had 2 mag pickups in the CAS
which looked like a cut down distributor. The 3rd gen had 2 wheels
on the end of the shaft and two mag pick up just like what we
are doing here almost. Instead of the second wheel we are using
the starter ring gear.

The RX8 uses one pick up and one complex wheel to
determine the difference between the ignition trigger (called home) and
the RPM or crank angle position. This is basically called a missing
pulse detector. Saves a buck or two in hardware at the expense
of more complicated software however we are not making 100,000
cars a year so another CAS sensor is not going to break us.

Minor adjustment to the 555 oscillator can accommodate the
the 2nd gen system or the 3rd gen system. Take your pick however
the ring gear pickup trick is the most accurate and the fastest
tach up date signal. Mazda does not use it as the number of
teeth on the ring gear can vary depending on auto tranny
or manual tranny.

The CAS trigger (or home)  is set at 100 degrees before top dead center.
The real ignition pulse is counted from there depending on the RPM.
At low RPM it at 22 degrees BTDC as the 2 ms dwell time is not significant.
At high RPM more advance is used to compensate for the 2 ms dwell time
to come out at 22 BTDC.


Paul Lamar