switching EPROMs

[Krister Wikstrom]

>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

Most systems have no indication on failing data fetch, they just read in
an incorrect value, propably FF or 00, and just do what that value suggests.
If a code fetch fails, anything can happen. Remember that the same chip has
also the code, so you'll get code fetches on it all the time.

There might be some unexpected problems, if the software expects some
previously read values - for example if it fetches a 16-bit value, and
you happen to make the change just after the first byte...

I've been also thinking of swithing EPROMs on the fly, and I made a simple
circuit (that is not tested yet...), which uses a latch to do the switch
at the rising edge of CE. This way it will have some time before the next
fetch can happen. I have OR-gated the CE of the EPROMs, and this will put
some delay on the signal, but in my case it was of no importance, and
you can always compensate by using a bit faster EPROM!

Note, that on some microcontroller systems CE could be directly connected
to ground. On these you could use the OE instead.


  Krister Wikstrom
  kwi at mamma.icl.fi