> In message  <19970922.144517.6878.3.mkeitz@juno.com> PICLIST@MITVMA.MIT.EDU
>  writes:
> > On Mon, 22 Sep 1997 13:24:04 GMT Mike Watson <mike@D-M-G.DEMON.CO.UK>
> > writes:
> >
> > >Is there not a danger with this method that the pointer location
> > >could get corrupted if it is being written at the same time that
> > >the power is being removed?
> > >
> > >Have you a mechanism to deal with that?
> >
> > It is important that the new pointer is written *after* the data.  If
> > power fails while writing to the new data block, the pointer will still
> > point to the old one, so the incompletely written data will not be used.
> > If the new data block is written but the power fails while writing the
> > pointer, both blocks of data will be valid (one old, one new).  The read
> > routine must mask the pointer so it always reads one or the other data
> > block.  If the pointer is bad the worst that could happen is the older
> > data will be retreived, the same result as if the new data were not
> > completely written.
> >
> > All this is based on the assumption that cutting the power off in the
> > middle of an EEPROM write will only affect the byte being written.
> >
>
> Is this a good assumption to make? Does it hold good with the
> on board C84 EEProm as well as external IIC EEProm and FRAM?
>
> This has more than academic interest for me as I am inheriting a
> project which exhibits ee-memory loss.
>
>
> Cheers,
>
> Mike Watson
>
>
>
> --
> Mayes uk
>
Hi
Like somebody said; *my penny's worth*
I also had some values to save to EEPROM in a 16c84 in case of power
loss. I fitted a BIG capacitor over the supply line. Then I placed a
diode in the positive line on the supply side of the cap. Before that
was a potential divider and a Zener for protection. The output of
this devider went to a input portpin to cause a interupt. After
first setting all drive ports to input, there was ample time to store
to eeprom. The rest of the remaining time was spend reading the
supply-fail pin.

Regards
CC