can one write to (save to) eprom memory (or some type of eprom) in a
similar way as one can to RAM? That is, can a program save data
directly to eprom and do it multiple times or is the writing-to
capability limited in some way (e.g you need to do it with some
special kind or eprom burner or can do it just once?

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Request for Question Clarification by maniac-ga on 20 Jul 2004 17:29 PDT

Hello Myq,

The answer is generally
  "Yes, but..."

A program can save to EPROM's - for an example - think of the BIOS on
your computer and the settings you can make and save. Those get saved
in some kind of static storage (such as an EPROM or battery backed
storage). However, the specifics will vary with the kind of device
chosen.

Do you 
 - have a specific part or type of device in mind?
or do you
 - want a summary of device types / how to update?
Please clarify your question.

Thanks
  --Maniac

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Clarification of Question by myq-ga on 21 Jul 2004 01:33 PDT

I would be saving a file (repeatedly) of a few hundred KB. I think a
summary of the different types is what I want. What would be the
factors in preferring an eprom over ram (since ram can also be made
non-volatile by use of an embedded battery)

Hello Myq,

Since you mention a relatively large size and "files" in your
clarification, let me start with flash memory since that appears to be
the best fit.

A very brief summary with a lot of good links is at
  http://en.wikipedia.org/wiki/Flash_memory
Some of the links at the end describe various file systems suggested for flash.

In a little more detail
  http://computer.howstuffworks.com/flash-memory.htm/printable
which also refers to
  http://computer.howstuffworks.com/rom.htm/printable
has a good introduction into how flash memory works and how it
compares to EPROMs. Since you are interested in relatively large
updates (not a few word / byte updates) a flash memory appears to be a
good fit for your application.

As an example of flash memory and a LOT of techincal information, see:
  http://www.intel.com/design/flash/
or more specifically
  http://www.intel.com/design/flcomp/prodbref/298044.htm
and related links that describe a product line with 32 Mbit or more
storage, 100,000 cycle operation, and 20 year data retention. Software
is also available for this in Windows CE; it may also be supported by
Linux.
The interactive demonstration at
  http://www.intel.com/design/flash/leftnav/demos.htm
has some marketing fluff, but also has some good introductory material
as well as describing the differences between the models Intel
provides.

To compare, here are also some related technologies and articles:

http://www.erg.abdn.ac.uk/users/gorry/eg2068/course/mem.html
part of course materials - a brief summary of different types of
memory including EPROM and EEPROM / flash.

http://www.mod.uk/issues/dstis4/solidstate.htm
a MOD summary of upcoming technologies for solid state memories and
storage. They are looking forward for some magnetic storage
alternatives that may remove the limits on data updates.

http://www.extremetech.com/resource_center/0,1598,s=1124,00.asp
A set of links to other articles summarizing different memory
technologies. The non volatile memory description (may have to scroll
down for the link) has brief definitions of several technologies.

Search phrases included:
  flash memory
  describe eprom memory
  describe non volatile memory
  static memory

If any part of this answer is incomplete or unclear, please make a
clarification request. Good luck with your work.

  --Maniac

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Request for Answer Clarification by myq-ga on 22 Jul 2004 14:05 PDT

Thanks for the answer, but I did not think a few hundred KB was a
'relatively large' filesize in these days when 256MB RAM is considered
low.
I was interested in the comparison between RAM and eprom, really,
rather than Flash Memory etc.
For example, one point of comparison would be the price. How much
would , say, of 500KB of Eprom cost?
Another would be whether cheap RAM can be made effectivel non-volatile
by having an extra power source (a cell hidden from the user, like a
computer BIOS does) or would the power consumption of 500KB of Eprom
be too much and require too much battery power to remain non-volatile
for extended periods (many months)

---

Clarification of Answer by maniac-ga on 22 Jul 2004 19:33 PDT

Hello Myq,

When usually dealing with EEPROMs, the write rates are VERY SLOW. Not
anywhere close to the times for data rates for RAM. See the Intel
references for examples but using the J3 flash specification, it
indicates about 200 microseconds to write 32 bytes. Note also in the
other references that flash is generally faster than EEPROMs for
multiple byte updates (due to the design).

Well - let's look at a few of today's prices at www.outpost.com

 - PC 100 DIMM - 256 Mbyte is $75 (Kingston)
 - a Compact Flash - 256 Mbyte is $35-40, less with discount
 - a memory stick - 256 Mbyte is $65 to $90
and so on.

Not sure why, but Outpost only listed EPROM programmers - about $220
each. No prices for EPROM's themselves. Let's try another site, this
time JDR (www.jdr.com)

  $3 for an 8K EEPROM, $9 for a 32K EEPROM, $30 for an 128K EEPROM

Note the difference in units - Kbytes, not Mbytes. A considerable
increase in price for the capability.

You can certainly do a circuit with a volatile RAM w/ battery backup.
A motherboard battery typically lasts a few years (say 3-5). However
with the price ranges I mention above, the flash will cost less for a
similar amount of storage.

  --Maniac

great answer

myq-ga:

Note the two points if you are talking about EPROM devices:

1. EPROM can't be erased electrically - it need an untraviolet light
over the little window (usually under a sticker) to erase.  You can
write to different locations while it is in the circuit, but to reuse
a given location, you'll have to take it out, erase it, and put it
back (or erase it while powered off in the circuit, with ultraviolet
light).  The programming ("writing") may also involve different
voltages than the typical read voltage levels, depending on the
device.

2. The number of write cycles are limited (older: 10,000 cycles,
newer: around 100,000 cycles).  So they are unsuitable for repeated
writes.  (Same limitation holds for EEPROMs which are "electrically"
erasable, without needing UV light).

THe speed limits as described in the other reply holds as well.

CMOS RAM with battery back up can be used for may write cycles and may
be more suitable for your purpose.

Regards.