8 Jun
2008
8 Jun
'08
3:34 p.m.
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________________________________
On Sunday 08 June 2008, Andrew Lynch wrote:
During the design, I wrestled with Flash ROM versus
EPROM for quite a
bit.
I went with EPROM since they are unchangeable in circuit and have
larger capacity for the same 32 pin socket.
My assumption was with cheap and plentiful EPROM programmers (search
for Willem on eBay ~$20) that practically everyone interested would
have one.
Flash ROM has its advantages but requires more advanced software for
in circuit reprogramming. The software would have to be written too.
Flash should actually be easier than an EPROM to program, especially
in-circuit, as it's designed to do that.
If you're worried about accidental erasure, just throw a switch or
jumper on the board to disable the /WE input to the flash.
Pat
--
Purdue University Research Computing --- http://www.rcac.purdue.edu/
The Computer Refuge --- http://computer-refuge.org
________________________________
-----REPLY-----
Hi Pat,
Well, in theory you may be right but this SBC has already been designed and
manufactured.
The PCB exists and is shipping now so it is a bit late for the initial
design considerations.
I understand the appeal of Flash ROM but the parts are more expensive and
many can be corrupted in circuit with errant software.
Even with code protection modes, there is no guarantee of non-corruption on
Flash ROMs.
At some point you'll have to lower the protection if you ever want to write
to that part of the chip.
In circuit reprogramming ability opens the door for those sorts of bugs to
creep in to the code.
EPROMs cannot be corrupted in circuit, short of a broken component, AFAIK.
Some Flash ROM chips do support code protection but not all.
Really, any part you select for this job has its costs and benefits. It all
gets down to trade offs.
One of my primary goals of the SBC is to keep the overall cost to a minimum
while providing something useful.
Generally speaking, EPROM is cheaper than Flash ROM and also has larger
capacity. The price is requiring external programming and erasing.
Incorporating EPROM is also requires less complex circuit and the software
is easier to write and debug.
There is no chance of "bricking" the SBC due to a bug in the CBIOS code
either.
At least in this case, I am also more familiar with the technology and I was
able to reuse my prototype design almost as is.
Not having to write special software was a big plus too. In short, EPROM
seemed like a hands down winner.
What caught me by surprise was the larger than expected demand for the
pre-programmed EPROMs.
Even that isn't really much of a problem since if I can't find a source for
low cost used parts, I can just buy a tube or so of new ones from Jameco.
Brand new, they only cost $6.25 each when you buy 10 or more.
I can just raise the price of the EPROM but I would rather keep it as low as
possible.
Thanks for the help and comments. Please let me know if anyone gets any
other ideas.
Have a nice day!
Andrew Lynch
PS, I did some investigating and experimenting with the 29F020 style chips.
It turns out making a shim socket for getting them to work is trivial. I
have several compatible chips on hand. It still doesn't add in circuit
reprogramming but even that would be fairly easy with some cuts and jumpers
and appropriate software.