[Coco] EEPROM replacement for 27C256

[Gene Heskett]

On Sunday 28 June 2015 17:42:30 Darren A wrote:
> On Sun, Jun 28, 2015 at 11:55 AM, Gene Heskett  wrote:
> > Another piece of trivia folks.
> >
> > As long as the data buss is the same width AND on the same series of
> > pins, it doesn't matter one bit if data pin d0 is connected to the
> > data line on the board named d0. Ditto for d1-d7 (or d15 for 16 bit
> > wide chips) Because any sucn accidental or on purpose scrambling
> > simplifies the board layout, scrambling of the data written, it will
> > be unscrambled by the same "re-arrangement" on the subsequent read. 
> > You will get back the same data you wrote in any event.
> >
> > Exactly the same situation exists for the address lines, the
> > operative again is that the same group of pins are being used.  The
> > a0 thru a15 (or a23, a31 in the case of wider chips) can be fed to
> > the chip in any sequence.
> >
> > As long as those conditions are met, it will work as expected.
> >
> > I can think of a flash eprom that would need exact matching because
> > they often have to erase and rewrite a whole 4k block to change one
> > bit.  The cocosdc might, probably is subject to that exception, but
> > Darren can correct me.
>
> Scrambling the data and/or address lines is great when dealing with
> RAM but I never do that with EEPROM or Flash.  As you mention Gene,
> those devices often utilize sector or paging schemes for
> erasure/programming, so you'd probably want to have at least the
> address lines wired correctly.  There are also Software Data
> Protection schemes to think about and whether or not you want to write
> to the device outside of the circuit (use a chip programmer).
>
> - Darren

In those exceptions, definitely true.

Cheers, Gene Heskett
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