Flash ECC algorithm on STM32L1xx

How does the flash ECC algorithm (Flash Error Correction Code) implemented on STM32L1xx work?

Background: I want to do multiple incremental writes to a single word in program flash of a STM32L151 MCU without doing a page erase in between. Without ECC, one could set bits incrementally, e.g. first 0x00, then 0x01, then 0x03 (STM32L1 erases bits to 0 rather than to 1), etc. As the STM32L1 has 8 bit ECC per word, this method doesn't work. However, if we knew the ECC algorithm, we could easily find a short sequence of values, that could be written incrementally without violating the ECC.

We could simply try different sequences of values and see which ones work (one such sequence is 0x0000001, 0x00000101, 0x00030101, 0x03030101), but if we don't know the ECC algorithm, we can't check, whether the sequence violates the ECC, in which case error correction wouldn't work if bits would be corrupted.

[Edit] The functionality should be used to implement a simple file system using STM32L1's internal program memory. Chunks of data are tagged with a header, which contains a state. Multiple chunks can reside on a single page. The state can change over time (first 'new', then 'used', then 'deleted', etc.). The number of states is small, but it would make things significantly easier, if we could overwrite a previous state without having to erase the whole page first.

Thanks for any comments! As there are no answers so far, I'll summarize, what I found out so far (empirically and based on comments to this answer):

• According to the STM32L1 datasheet "The whole non-volatile memory embeds the error correction code (ECC) feature.", but the reference manual doesn't state anything about ECC in program memory.
• The datasheet is in line with what we can find out empirically when subsequentially writing multiple words to the same program mem location without erasing the page in between. In such cases some sequences of values work while others don't.

The following are my personal conclusions, based on empirical findings, limited research and comments from this thread. It's not based on official documentation. Don't build any serious work on it (I won't either)!

• It seems, that the ECC is calculated and persisted per 32-bit word. If so, the ECC must have a length of at least 7 bit.

• The ECC of each word is probably written to the same nonvolatile mem as the word itself. Therefore the same limitations apply. I.e. between erases, only additional bits can be set. As stark pointed out, we can only overwrite words in program mem with values that:

  • Only set additional bits but don't clear any bits
  • Have an ECC that also only sets additional bits compared to the previous ECC.

• If we write a value, that only sets additional bits, but the ECC would need to clear bits (and therefore cannot be written correctly), then:

  • If the ECC is wrong by one bit, the error is corrected by the ECC algorithm and the written value can be read correctly. However, ECC wouldn't work anymore if another bit failed, because ECC can only correct single-bit errors.
  • If the ECC is wrong by more than one bit, the ECC algorithm cannot correct the error and the read value will be wrong.

• We cannot (easily) find out empirically, which sequences of values can be written correctly and which can't. If a sequence of values can be written and read back correctly, we wouldn't know, whether this is due to the automatic correction of single-bit errors. This aspect is the whole reason for this question asking for the actual algorithm.

• The ECC algorithm itself seems to be undocumented. Hamming code seems to be a commonly used algorithm for ECC and in AN4750 they write, that Hamming code is actually used for error correction in SRAM. The algorithm may or may not be used for STM32L1's program memory.

• The STM32L1 reference manual doesn't seem to explicitely forbid multiple writes to program memory without erase, but there is no documentation stating the opposit either. In order not to use undocumented functionality, we will refrain from using such functionality in our products and find workarounds.