switching between EPROMS

[Donald Whisnant]

> From: Warren Crowther <warren at advgroup.co.nz>
> Date: Wed, 28 Aug 1996 21:06:11 +1200
> Subject: Re: switching between EPROMS
> 
> >     If you switch it with the engine turned OFF, I don't think you'd have 
> >     a problem.
> 
> This is a simple solution, and will work, but..
> 
> I'd prefer to do it with the engine ON!!!  I don't see any reason why it
> cannot be done, provided the switch over is not done at a critical time, and
> that the only difference between the EPROMS is the data: ie. the fuelling,
> ignition timing, etc.  Also it should be done quick enough so that the micro
> does not miss an external data fetch (anyone know what the micro will do in
> this case??  will it hang or retry? ).
> 
> Warren
> 


Warren...

It will hang...  The micro doesn't have any way of knowing what is
"supposed"
to be out there... So during the switch, if it is in the middle of a
cycle,
if the data values going to the micro get goofy, the micro will end up
executing
the "goofy code" and not what it should...  If you are real lucky and
happen to hit it on a "inconsequental" instruction, nothing will happen
except
maybe a slight glitch...  At worst case, it will lockup and totally
stop...

What I'm planning to do (eventually) is to use a couple of EEPROMs that
I
can program with my PC --- one EEPROM just has a "selector" byte telling
the little bit of logic I have weither to take the values from EPROM or
EEPROM.
With a little bit of addition, a switch could be added along with a
small
circuit that would synchronize the micro's access to the rom so that the
switch won't cause a problem...  The logic could be as simple as a
flip/flop.
i.e. the "switch" goes into the data input on the flip/flop.  The
flip/flop
is clocked by the end of the E-clock cycle of the micro.  And the output
of
the flip/flop is sent to the enable of the rom's (i.e. either a upper
bit
if you are using a 2x sized rom or to a circuit that will "and" the
enables with the CPU enables) ...  Sounds simple enough to implement...

Good luck...
Donald Whisnant
dewhisna at ix.netcom.com