Can I improve EEPROM lifetime by not changing individual bits?

Question

In my current project I need to store (among other things) the current elapsed time in hours to retrieve it in case of power loss. Since I am using a Arduino Nano I would ideally like to use the built in EEPROM without additional hardware.

The values are stored every hour and their order will always be:

0 → 1 → 2 → 3 → 4 → 5 → 6 → 0 → ....

If I write that on EEPROM per 0->0 full cycle the last bit will change 7 times and the other bits are pretty much idle. That's why I thought of distributing the changes more evenly by using instead the sequence

0 → 1 → 3 → 7 → 15 → 31 → 63 → 0 → ....

This would change every bit only once during a full 0->0 cycle. However after reading this post I think I need to work with zeros rather than ones, so

255 → 254 → 252 → 248 → 240 → 224 → 192 → 255

Now, I would like to clarify if

1. the strategy works altogether, e.g. does this spread of changes to different bits indeed improve EEPROM life expectancy?
2. I can use the EEPROM library function EEPROM.update() to realize the strategy?

Remark: I asked the same question on arduino.stackexchange but was redirected over here.

Remark2: While the specified 100000 cycles would roughly translate to 10 years of service, the question is still interesting for me since I would like ideally to switch to 5 or 10 minute intervalls (from hours)

Remark3: Since my EEPROM is not really used that much I could simply use 7 bytes to realize the strategy, however if possible I would like to understand the possibilities of a solution using just one byte for the sake of understanding how EEPROM works and a sense of "cleanliness".

Answer 1

It depends which code you use to handle the EEPROM memory. Arduino code seems to always perform an erase/write cycle to the location you want to write, so writing bits low one at at time will just erase them high and then low again for each write.

The ATMega328 is capable of separate erase and write functions, so you might want to use that feature to write bits of single memory location to low state and then erase them high when needed.

The ATMega328 EEPROM has a page size of 4 bytes according to the datasheet, which may not be visible to you as an user, but it still means that the chip cannot internally erase single bytes, but always a group of 4 bytes. So even if you wrote to four consecutive addresses, a naive code would erase the 4-byte block four times for each of the individual byte writes, writing back the bytes that need to be preserved. So it still makes sense to switch to more sophisticated code that handles erases and writes separately when needed.

Answer 2

You don't have to do anything, according to the data sheet, because that guarantees:

The EEPROM has an endurance of at least 100,000 write/erase cycles.

"elapsed time in hours" is written once per hour, so that's 10⁵/24/365.25 years, more than 11 years of continuous operation. So, don't do anything.

Anyways, granularity for EEPROM write access is a single byte – so you could as well just use 6 bits of that to signal

0. 0 (0b00000000 freshly erased, now set the first bit)
1. 1 (0b00000001 1. bit set, now set the second bit)
2. 3 (0b00000011 2. bit set, now set the third bit)
3. 7 (0b00000111 3. bit set, now set the fourth bit)
4. 15 (0b00001111 4. bit set, now set the fifth bit)
5. 31 (0b00011111 5. bit set, now set the sixth bit)
6. 63 (0b00111111 6. bit set, now erase)

That will remove 6 out of 7 of your erase cycles, so it would probably increase your EEPROM lifetime sixfold.