Century "untouchable"

[jerome13013]

Hi,

Within DS3231.cpp, there is no possibility to update the "century" bit at adress 0x05h, bit 7.
You can read it with getMonth(&century) but using decToBcd(month) in the setMonth(month) routine doesn't allow to write it with for example setMonth( 0x80h OR month ).

A standalone function setCentury(bool Century) could be useful to complete the library :)

Yes... I know... the "century" bit is not very useful ;)

Best regards

[IowaDave]

The datasheet says very little about the Century bit, only that, "The century bit (bit 7 of the month register) is toggled when the years register overflows from 99 to 00."

There is no mention that the bit plays any role in managing other time or date registers within the DS3231.

It might have only informational value. In that case, the bit might be just as usefully maintained in a program variable, or in EEPROM.

[IowaDave]

@awickert @jerome13013 I propose closing this Issue as "wontfix" for the following reason:

Even though the Century bit is technically writeable, no function of the DS3231 is affected by writing any value to it. The bit operates to provide information in one direction only: to the user. Its value is potentially highly significant. It signals that the 8-bit value in the "year" register, 0x06, has rolled over from 99 to 00. Perhaps that information should not be easy to erase?

[awickert]

@IowaDave this sounds sensible. The Century bit will remain unwritten, with no function to do this. The library will deprecate in Year 2100 unless amended before then. We will see if this chip is still produced at that point. Fortunately, this will not be a difficult fix when we have to make it.

[awickert]

Reopening, per @IowaDave, who may add documentation towards this (see #103).

[jnuernberg]

While skimming over the documentation, I saw, that this is already documented fairly well towards the end of the Time Retrieval Documentation. @awickert and @IowaDave, I would think this suffices as documentation. What are your thoughts?

Cheers, Jacob

[IowaDave]

I think it would benefit readers to offer a little bit more complete explanation. At February 11, 2026 I am working on edits to the DateTime documentation. A forthcoming PR will propose some additional language clarifying that writing the century bit will not be implemented. The bit is not meaningful for current purposes of this Library because year values are limited to the range 2000-2099 in the present design.

[IowaDave]

At February 13, 2026 I submitted PRs to modify language in the Time-Set.md and the Time-Retrieval.md documentation explaining that the Library does not implement a setCentury() method because, for practical purposes, the Century bit in the DS3231 hardware has value only as an output and not as an input. In other words, programs should treat it as read-only.

Will close this issue as won't-do if and after those two PRs have been merged. I am leaving it to other developers to review the PRs and, if they so choose, to merge them.