I'm a bit confused as to whether I should use EEPROM or FLASH memory for
storing "constants". The 18F452 data sheet says some  things that I don't
really understand.

Section 6.8 says:
" The data EEPOM is a high endurance byte-addressable array that has been
optimized for the storage of frequently changing information (.e.g. program
variables or other data that are updated often). Frequently changing values
will typically be updated more often that specification D124. { This is the
# of EEPROM write cycles before a refresh is required.}  If this is not the
case, an array refresh must be performed. For this reason, variables that
change infrequently (such as constants, IDs, calibration, etc.) should be
stored in FLASH program memory."

Then it says in a note:

"If data EEPROM is only used to store constants and/or data that changes
rarely, an array refresh is likely not required"

This suggests to me that if I need to write to the EEPROM more than the
D124 limit (e.g. 1 million cycles - think this means per cell), I can do so
provided that I do a refresh of the EEPROM before the D124 limit. Is this
correct?

Now it also suggests that infrequently changing data should be stored in
FLASH rather than EEPROM if refresh is going to be needed. I gather that if
I'm not going to write often enough to have to do a refresh that I can
store calibration data, etc in EEPROM along with "changing" data.

OK. Here is what I'm trying to do. I've a program that writes checkpoint
data to EEPROM every few minutes, and uses different blocks of EEPROM each
time to maximize the number of times I can write. I figure I can run the
program for about 4 years before reaching the minimum 1 million cycle limit.

I also want the user to be able to enter some setup data (calibration data,
for example) and save it - this would typically only be done one or twice
over the same 4 years.

It SEEMS from the data sheet that in this instance I can safely store my
calibration data in EEPROM. Is this correct?

A question about refresh. The refresh routine they show appears to read
then rewrite each EEPROM cell with the same data. If my app was going to
exceed the D124 spec, can I just do a refresh every D124 (e.g. 1 million)
cycles, thus essentially having EEPROM with unlimited endurance?

If refresh indeed refreshes EEPROM, why the suggestion not to use it for
infrequently changing data?

Any insight anyone can offer will be greatly appreciated.

Thanks


Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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