Official word from Atmel is that the 100 year retention period begins with the first write to each EEPROM location.
This would suggest that to persist EEPROM data as far into the future as possible that you should....
- Ensure that the data block is not all
1 bits using escaping or bracketing if necessary.
- Write the block of data at block location 0 on year 0.
- Write a copy of the the data at block location 1 on year 100.
- Write a copy of the the data at block location 2 on year 200.
To read the stored data out, start at the top of the EEPROM address space and read blocks backwards as long as you see only
1 bits. When you see a block of data that has at least one
0 bit in it, use that data.
Note that if you need to update the data, you can update in place at the same location for the first 100 years and first 100,000 write cycles before moving on to virgin EEPROM locations.
Quoted conversation with Atmel below...
Created By: Abirami Sivakumar (12/21/2018 6:16 AM) Hello Josh,
Yes your understanding is correct.
The erased condition of EEPROM is 0xFF. From the factory it is
completely erased and manufactured. So if you are writing some value
to the particular location, the corresponding value will remain after
Best Regards, Abirami Sivakumar. Created By: Josh Levine (12/13/2018
12:46 PM) This makes sense. So a device received from the factory has
EEPROM filled with 0xff.
If on year 1 I write a 0x00 to a location in the first block, then I
would expect that 0x00 to be readable at year 100 .
If on year 100 I write a 0x00 to a location in the second block, then
I would expect that 0x00 to be readable at year 200.
Is this understanding correct?
Would you expect an programmed EEPROM location with a 0xff from the
factory to remain readable as an 0xff indefinitely?
-josh Created By: Abirami Sivakumar (12/13/2018 6:58 AM) Hello Josh,
Thanks for contacting Microchip Technical Support Team
Data retention time starts from first time to the EEPROM location.
Hope this clarifies. Please let us know if you need further
Best Regards, Abirami Sivakumar.