Just guessing here, but ...

[Dale Ulan]

>    Well, not on BOSCH boxes, where timing is done by counting
>    flywheel teeth after a ref. point ... and sometimes further
>    controlled by a sub-tooth time period (on the 60-2 tooth wheels)

And on older GM boxes (with distributors), they just use the time
between TDC pulses, with a small non-linearity adjustment for timing
for the engine's speed variation from cylinder to cylinder. You can
do this with an output compare and input capture register on an HC11:

    TIC - last time -> cyl time
    TIC -> last time
    TIC + cyl time * (90 deg - ignition timing -
		      dwell time) / 90 degrees -> TOC
    TIC + cyl time * (90 deg - ignition timing) / 90 degrees -> nextTOC

That's just the math. The code is a bit more complicated, to compensate
for error conditions that happen frequently...

-Dale