TattleTales

[Orin Eman]

> Spark control is a little more picky.  It pretty much requires knowing where
> the crank is to within a degree or less  (Is that right?).    So I assuming
> that I (at this point) dont really want to add any additional processors  I
> need to figure out how to get the tattle tale to know where the crank is.
> If I have it count the teeth on the crank wheel and leave a gap at some
> point, then it'll know that after it sees so many peeks,  (it can stop
> counting at a given number) that it has reached the gap in the wheel,  then
> it knows position, then if it divides the degrees on the crank that have
> teeth by the time it took to count it'll have RPM.  Then it can start a
> timer to fire an injector after a certain amount of time ( based on rpm and
> load and...)
> Now the problem I see here is that all these things have to happen in a
> pretty little amount of time.  I really have no idea whether the tattle tale
> can work that fast  and still keep track of injection duties.

Here are some numbers for you from my car:

Flywheel teeth: 135
Max RPM: 6700
Resolution of flywheel teeth: 2.67 degrees, 1.33 if you use
BOTH edges of the flywheel signal.

(Anyone got a simple circuit to produce a pulse on both edges of
a square wave?)

Time for one 'half-tooth' at 6700 RPM: about 33 micro-seconds.
That's how long you have to react to fire the coil or injector
and get 1.33 deg resolution (a constant delay is OK BTW).

> Maybe having a dedicated PIC working realtime and letting the tattle tale
> provide timing numbers or injector durations is the solution.  As Robert
> Harris suggested in a recent post.

I can monitor the above with a PIC running at 4MHz, ie. 1 micro-second
per instruction.  Interrupt driven, I can just respond to an ignition
event in the same 'half-tooth' that it occurred.

Driving the ignition would be easier using the Capture/Compare modules,
driving the counter off the flywheel teeth.  I'm not using the
CCM for the ignition signal since I'm using it to capture duty cycles.

HTH, Orin.