Subject: Re: Hardware Project
From: dempson@actrix.gen.nz (David Empson)
Date: Fri, Nov 13, 1998 6Ç10 
Message-id: <1digo03.118jc48dwt1g5N@dempson.actrix.gen.nz>

Will Smith <root@ordinateur.ctron.com> wrote:

> On 12 Nov 1998, David Wilson wrote:
> 
> > Disadvantages:
> >     I never bothered to upgrade it to v2.x as I would have to
> >     program another EPROM - if I built this again today I might use
> >     EEPROM or Flash memory to ease the upgrading.
> 
> i don't know much about EEPROMs.  if a patch needed to be done on an OS on
> the EEPROM, could it be done?

In general, yes.

EEPROM means "Electrically Eraseable Programmable Read Only Memory".

In a traditional EPROM, you have to remove the chip from the board and
place it under an ultraviolet light of the appropriate wavelength for 20
or 30 minutes in order to erase the chip.  You can then reprogram the
EPROM and insert it into the board.

With an EEPROM, the chip (or part of the chip) is erased by applying a
voltage to a specific pin.  The chip can then be reprogrammed.

In some cases, the EEPROM requires a high voltage (e.g. +12V) to be
erased or programmed, but most modern ones are able to operate from a
+5V supply, so they can be reprogrammed in-circuit.

Yet another variant is Flash ROM.  I've never been entirely clear on the
difference between Flash ROM and EEPROM, as they seem to overlap in
features.  Flash is generally characterised by being able to be erased
and reprogrammed in sectors, while EEPROM is more often bulk erased and
reprogrammed one byte at a time.

It is probably just a difference in the underlying architecture that
results in the different names.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand