Re: [EE] High reliability store and recall of data in EEPROM

0x01 XOR 0xFE  should be 0xFF, shouldn't it? ;)

A long (long) time ago, I had to do something similar with a PIC16F877 
after learning how unreliable the EEPROM within the PIC was.  I was 
storing odometer data, and a big part of that each data value required 
multiple bytes (4 IIRC, as I was storing up to 1,000,000 with 0.01 
precision). I used an external serial EEPROM.

With ideas/help from this list way back then, what I ended up doing was 
this:
- Group EEPROM memory into *many* sets of (IIRC) 6-bytes each. One byte 
represented "dirty" (with a key value to represent "clean"), 4 were the 
data bytes, and one was a checksum
- Mark set as dirty (all were initialized to dirty from the start anyway)
- Write 4 data bits and checksum
- Read back 4 data bits and checksum and if all match, then mark the 
dirty bit as clean.
- Increment set index for next value.

Worked beautifully. Some of these ideas may be useful for you saving 
multiple bytes for each value.

Cheers,
-Neil.




On 12/6/2024 4:49 PM, David VanHorn wrote:
> I have a need to store some data in non-volatile memory, with the caveat
> that corrupt data MUST NOT occur.  Things go "big badda boom" otherwise.
>
> Offhand, I came up with storing each byte as a four byte array (I have TONS
> of space available)
> If I'm storing 0x01 then it would look like this:
>
> 1       2        3       4
> 0x01, 0xFE, 0x01, 0x55
>
> Xor 1 and 2, result must be zero.
> Xor 3 and 2, result must be zero.
> compare 1 and 3 result must be zero (not sure this is needed)
> check 4, must be 0x55 otherwise a wild write has happened in EE.
> Finally the values allowed will be sparse, and likely only a single bit
> will be set, so I will check that the value I read is one of the allowable
> values.
>
> If any of those checks fails, we render safe and halt.
>
> Is there a more "rock solid" method?    What would be done in an automotive
> environment for something like throttle position?   What would be best
> practice for medical life safety?

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