I'm having troubles deciphering the datasheet for the DS3231 RTC. The below table shows that 0x02 bit 6 is the flag to operate in 12- or 24-hour mode. But which one correspond to the bit being low or high? I assume the line above the 24 must tell it, but I guess I lack knowledge of the usual conventions.

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    \$\begingroup\$ 24 is 0, 12 is 1. The bar is denoting the active-low logic. \$\endgroup\$
    – Eugene Sh.
    Jun 18, 2018 at 21:19

2 Answers 2


A bit (or a digital signal) is described as \$X/\overline Y\$. The part without the bar is true when the bit is set. The part with a bar is true when the bit is cleared.


\$12/\overline {24}\$ means 12-hour time when this bit is 1, and 24 hour time when this bit is 0.

\$\overline {AM} / PM\$ means AM when the bit is 0, and PM when this bit is 1.

\$R/\overline W\$ in figs. 2,3,4 in the DS3231 datasheet means Read when the is 1 and Write when the bit is 0.


The bar means invert. Basically, that annotation means you want to set it (to 1) for 12 hour mode or clear it (to 0) for 24 hour mode. Same thing for AM/PM - set for PM, clear for AM.


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