749 and RPM values.

[Shannen Durphey]

Seems like most of the code questions on this list have turned toward
tuning questions, or CFC hazing with the Freon thread. ; )

I've been (veeery) slowly picking away at some 749 syty code.  This p4
measures engine speed by counting the time between reference pulses in
increments of 1/65535 of a second and placing that value where it can be
accessed later for further calculations.  One of those calculations is to
multiply the 16 bit counter value by an 8 bit value representing the
number of cylinders, to get a value equal to rpm.  Because this value is
corrected for the number of cylinders, it's the same value for any
engine.  In other words, 2500 rpm for a 3 cyl is the same value as 2500
rpm for an 8 cyl.

Quite a while back, I made up a chart showing reference pulses, the value
stored by the ref. pulse timer, and the hex value obtained when the
counter value is multiplied by number of cylinders (knumcyl in the p4
doc).  What's interesting to me is that even though the p4 docs do not
state there's a knumcyl value for a 5 cyl, the rpm calculations at least
will support one.  All may not be lost for the Volvo guys. ; )

I've looked at some sections of the 165 and 7730 hac's, and quick checks
suggest that gm might be using the same code.  Good old gm likes to use
tried n true methods.  

What use is this chart?  For me it's been helpful to substitute known
values while hunting and pecking away at various bits of code.  Hopefully
it will help someone else.  I've uploaded it to incoming as "multcyl.txt".

Shannen
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