Piggy backing 165 ECM chips

[Marteney, Steven J.]

1)	Leaving a ground untied on an IC (aka EPROM) is a bad idea.  I
wouldn't recommend it.
2)	Asynchronously switching between two EPROMs can cause problems.  You
may end up disobeying output setup and hold times.  Since the MPU gets its
opcodes from EPROM as well as data, you could end-up crashing the MPU (sort
of a blue screen of death, only automobile related ;-).  Also, if you switch
the chips the ONLY thing can change are static data tables and values, NO
CODE CHANGES!  I would think the best (only) way to do on-the-fly EPROM
switching would be to synchronously multiplex the data outputs from the two
chips and let them both run at the same time always.
3)	Be careful taking the simplistic approach of "just switch the upper
address lines" on a larger EPROM.  A download of the Fairchild 27cXXX family
of EPROMS reveals some interesting pin changes (VCC moves around, pins take
on two functions) as you move up in size.  Research!
4)	Always remember to de-bounce any kind of "switch" you may make.
De-bounce circuits can be a PITA but I remember seeing a de-bounce IC from I
believe Maxim that works well.  (Used it about a year ago.  A lot simpler
than caps, invertors, feedback circuits...)

HTH.  Just my, oh, about $1.25 worth of opinions ;-)
Steve


		-----Original Message-----
		From:	Roger Heflin [mailto:rah at horizon.hit.net]
		Sent:	Tuesday, May 16, 2000 1:05 PM
		To:	GM-ECM
		Subject:	Re: Piggy backing 165 ECM chips



		On Tue, 16 May 2000, Rick Lindstedt wrote:

		> I have a thought and would like to know the consensus of
"more" 
		> knowledgeable one out there on this..
		> 
		> I would like to have a chip for motor and a chip for
nitrous on my car... 
		> can I use the ground pin on the chip to switch back and
forth between the 
		> two? and if I can ..is it possible to do this on the fly
as well, ie car 
		> running???
		> 
		> or is there a better method to this??
		> 
		> Thanks for any input or suggestions..

		That probably won't work.  Other input/outputs on the off
chip may act
		as enough ground to cause trouble.

		There should be a OE pin (output enable) if one was to put a
gate
		between the two OE pins on the chips, and the OE from the
computer one
		could switch one or the other.   Also if one was to put a
chip that
		has twice the space in it, and switch the top address line
from 1 to 0
		would give you two full chip spaces.   If you used a chip
bigger than
		2x you would have even more.   Now doing this on the fly may
be a
		bigger issue.

		I was thinking to find a computer output (or 2-3) and find
one
		computer input, and use the computer input to signal enable
nitrous,
		and use the 2-3 outputs to trigger different stages.   This
would take
		some code addition, but it realisticly does not sound that
bad to
		implement, and eliminated some of the extra hardware (TPS
switch) and
		puts them under computer control.    I am going to need to
figure out
		if there are any extra outputs/inputs and maybe reuse some
unused or
		less useful ones.   That way when the computer signals the
nitrous it
		can also boost the fuel (may require bigger injectors,
though I
		figure I have somewhere around 100 extra hp in my current
		injectors) and reduce spark.

					Roger
		
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