Robert James Kaes wrote:

> 
> I thought I understood the refresh requirement, but I guess
> I was unclear about the cycle limit.  The way I read the
> datasheet was that if I refreshed the EEPROM before the
> cycle limit, it would basically reset the cycle count to zero.

Yes, but you must sum all writes to any cells to get
the D124 param.

> However, it would appear that this is mistaken.  The cycle
> limit is a "hard" limit.  Is this correct?  For example, if the
> data sheet says that the cycle limit before refresh is 1M
> cycles, then the EEPROM can _only_ handle 1M cycles
> before it starts to fail.

Still, a *total*  of 1M writes to *all* cells (= the D124 param)

> So, in my application where the EEPROM value is updated once 
> every two seconds,..

BUT !!
Here you talk about 1M writes to *one single* cell !! That is
something completely different and is not directly connected
to the refresh issue. The max writes to *one single* cell
is specified in the D120 param. And it doesn't matters how
much you "refresh" in this case, that single cell is "worn out"
anyway after D120 writes.

> Is my understanding of how the EEPROM work correct now?  

Only you know... :-) :-)

> There is no way for an EEPROM to work "forever"?

It depends on your definition of "forever"...
But why do you think that Microchip have put the
D120-D124 params into the data sheet in the first place ?

To sum it up,
If you are only using *one single* EEPROM cell, you will be able
to write D120 times to it. You never have to do any refresh (there
is nothing to refresh, since you don't use any other cells...).

Another way to solve this could be to :

- Put a large cap as a "power supply backup" at the PIC. Maybe
  also a diod, so the main supply might drop before
  the PIC gets powered down.

- Keep your EEPROM parameter current in a RAM location
  as long as the power supply is "good".

- Monitor the main power supply, and if it drops below
  some limit, quickly copy the RAM location to the EEPROM.

Make sure that the CAP is large enought to be able to power
the PIC during the EEPROM write.

Or, as Bob wrote, take a look at the Ramtron devices.

Regards,
Jan-Erik.



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