Date   : Fri, 12 Nov 2010 21:16:20 +0000
From   : list-a_cloud9.bbc-micro@... (Theo Markettos)
Subject: ROMs

In article <4CDD7737.5010501@...> you wrote:
> If you were hoping for an in-situ Flash to reprogram, you can get little 
> Flash devices in a ROM style pinout (such as the AMD 28F256 in PDIP), 
> but the question remains - how do you get the data onto it in the first 
> place? You can, with the Am28F256 part, program them in a standard EPROM 
> programmer [*], which is cool, but in this respect it doesn't offer much 
> over a traditional EPROM, save for the lack of UV erasure.

The Am28F256 and equivalent M28F256 from ST:
http://www.datasheetcatalog.org/datasheets/560/163745_DS.pdf
http://www.datasheetcatalog.org/datasheets/1150/222636_DS.pdf

come in 32 pin DIP packages.  That means you can fit them to a 28 pin ROM
socket and have 4 'legs over the side'.  The normal VCC pin is N/C, so on
the extra 4 pins you provide 5V, /WE and 12V programming voltage.  All of
those are easily available as flying leads on a Beeb.

Then you erase or program the device by writing some magic values to it to
select write mode.  If you don't get the command sequence right, the write
is ignored just like an EPROM.  When you're done you flip the device into
Read mode and then use it as normal.

That means you just need a custom version of *SRLoad and you can load
non-volatile sideways RAM images into it.

Intel also do a 64KB 28F512 on the same principle:
http://www.romservice.ru/datashee/290204.PDF
or a 128KB 28F010:
http://www.datasheetcatalog.org/datasheet/Intel/mXvsrwv.pdf

They'll need an address line or few bodging from the Beeb's ROM selector,
but means you can have 8 ROMs in one flash chip.

Theo