Re: [Re: Using 28F256 in a 747 instead of 2732A??????????]

[Dig <turbodig@usa.net>]

Bob Valentine <bob@tecmark.com> wrote:
> At 06:12 PM 2/2/00 -0600, you wrote:
> >In that case you'd need a 28F32 or 28Fxxx where xxx is the appropriate
> >size. :)
> >Some 29Cxxx are also Flash, but a different brand.
> >I think 28Fxxx are usually Intel, 29Cxxx are AMD or Atmel or something
> >like that.
> 
> I was about to ask a simular question about using EEPROM instead of EPROM,
> but that seems to have been answered - just saved me from buying a eraser.
> <grin>
> 
> What are the differences (or effects/problems) of using flash vs. eeprom?


As I recall, speed and write methodology are the primary differences.
A flash device provided greater speed (and life cycle), but the EEPROM
wasn't subject to the flash's page/chunk writing mechanism.

I did a lot of work with Flash and EE designing UNIX device drivers for
a solid-state "hard drive" we were designing. The early designs were EE,
and were plagued with poor yeild problems. You'd always get a bad chip
out of the 20 or so on the card. (~12M of data) The later versions were
flash, and they seemed to yield better.

I'm sure that technology has evolved a lot in the 5-6 years since that
card startted production. At the time, a 1M flash chip was the state of
the art. The rules probably have changed.

For my money, the EEPROM would be a far better choice for the type of 
work we'd be doing with it, since you only have to re-rout 3 pins to make
it work. I like the xicor parts, if for no other reason than that I have
a sleeve of 'em sitting in my desk! <g>

http://www.xicor.com/pdf_files/x28hc256.pdf

For purposes of comparison...
http://www.fairchildsemi.com/ds/NM/NM27C256.pdf

It's a non-erase-before-write part, which means you can just write to
'em any old time. They're fairly easy to do logic for too, there's not
anything tricky about the cycle setups. The unlock cycle is fairly long,
but for our purposes you could just leave it unlocked from the initial
program.

The real tragedy is that the '749 doesn't have a WE to the MEMCAL that
I know of (anybody got a MEMCAL pinout?). You could proably pull one 
from the bus somewhere (off the slide connector terminals?) and run it
to one of the unused pins on the end with the ESC substrate. 

I need to look at the CS on the EPROM on the 749... see if it toggles
with chip accesses or whether they just pulled it low.

I've got a design cooking for a parallel-port based writer for the EE,
using a few '374 latches and buffers, using the CE to determine if the 
EPROM was "in use". If not, isolate it from the bus and write the 
latched in data. If the CS is pulled low, though, it'll require decode
logic to generate a CS.

I'm thinkin' with surface-mount stuff (including the EEPROM), a guy
could do this in the form-factor of the average sammich-adapter board.

Add some decent PC software, and you could have a plug-in "DFI" for
anything that took a 66-pin MEMCAL. 

Now if I only had time to work on all this...

Later,

Dig
turbodig@usa.net 


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