GM P4 $400x regs

[Ludis Langens]

trinity at golden.net (Mike) wrote:
> I was actually referring to that 6.25mS timer value. It seems very familiar
> in the P4 system but I can't remember why. Interestingly, 6.25mS is the
> period of a 160Hz clock (and the slow ALDL rate is 160Hz...but that's
> another neat coincidence).

That's not a coincidence!  160 Hz is the Chevy Heartbeat of America. 
160 Hz allows 16 tasks to be run 10 times a second.

The old ECMs use the heartbeat interrupt to also send one bit out the
ALDL interface.  At the start of the interrupt handler, they trigger a
digital one shot for the desired pulse width.  During the interrupt
handler, the next pulse width gets loaded into the one shot.

It looks like the very oldest ECMs did 100% software bit banging to
generate the ALDL protocol.  All it would take is clearing an output at
the start of the interrupt handler, and then setting it at one of two
later points.  No additional precise timer is needed.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/