AN923 Datasheet, PDF (2/7 Pages) STMicroelectronics – Managing century information using serial real-time clocks

Devices with no century data
Devices with no century data
AN923
M48T02/12, M48T08/08Y/18, M41TC8025
A few devices have only a single byte of year information. The BCD year register represents
a 2-digit number in the range 00 to 99. The result is that the software must interpret what the
2-digit number means. For example, 85 might be interpreted as 1985, or 2085, or 2185. So
the software must maintain the upper two digits of the year, outside of the RTC.
The reader should note that the upper two digits of the 4-digit year are considered the
century value. So, for the devices listed above, the software must maintain the century
information in non-volatile memory such as flash or EEPROM, and increment the century
value whenever the year value, in the RTC, rolls over from 99 to 00.
Devices with one bit of century data
M41ST85W, M41T0, M41T00, M41T00AUD, M41T00CAP, M41T00S,
M41T11, M41T56, M41T80, M41T81, M41T81S, M41T94, M48T35AV,
M48T35/Y, M48T58
Most ST clock devices include at least one bit of century information. When enabled (CEB is
set), the century bit will toggle at the end of every century, at midnight of December 31 of the
year ending in 99. User software must interpret the bit's meaning. For example, users can
let 0 represent 2000-2099 and 1 represent 2100-2199, but other mappings may also be
used. By adding more bits in software, the century information can be extended to whatever
resolution is desired.
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