AN923 Datasheet, PDF (3/7 Pages) STMicroelectronics – Managing century information using serial real-time clocks

AN923
Devices with two bits of century data
Devices with two bits of century data
M41ST87W, M41T60, M41T62, M41T63, M41T64, M41T65, M41T66,
M41T82, M41T83, M41T93
Many of ST's newer RTCs include 2 century bits (CB1, CB0) which function as a 2-bit binary
counter that increments at the end of each century. The user may arbitrarily assign the
meaning of CB1:CB0 to represent any century value, but the simplest way of using these
bits is to extend the year register by mapping them directly to bits 9 and 8 (with the year
register comprising bits 7:0). Higher order century bits can be maintained in the application
software.
Figure 1. Two-bit binary counter (century bits CB1:CB0)
Example: 16-bit year value
CB1:CB0
MAINTAIN
LOWER 8 BITS
CB1:CB0 Century
ADDITIONAL
CONTAINED IN
00
01
00
2000 -2099
01
2100 - 2199
YEAR BITS IN
SOFTWARE
YEAR REGISTER
(07h)
10
2200 - 2299
0000000000000000
11
2300 - 2399
11
10
LET CB1:CB0 REPRESENT
BITS 9 AND 8 TO EXTEND
THE YEAR REGISTER
In this example, CB1:CB0 represent the two lower bits of the century byte.
Devices with a full byte of century data
M48T201V/Y, M48T37V/Y
Lastly, a few ST TIMEKEEPER devices include a full byte of century information. In addition
to the byte of year data, there is another, upper byte for the century, thus comprising a full
4-digit (16-bit BCD) year parameter. These devices will automatically track the century
through the year 9999. However, to go beyond that, user software would still need to add a
bit to support the years 10000-19999.
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