Datasheet
DocID012578 Rev 16 43/64
M41T82, M41T83 Clock operation
The M41T8x only checks the battery when powered by V
CC
. It does not check the battery
while in backup mode. Thus, users are advised that during long periods in backup mode,
the battery can drop to a level at which timekeeping may fail or data becomes corrupted. If,
at power-up, a battery low is indicated, data integrity should be verified.
Forcing a battery check
If it is desired to check the battery at an arbitrary time, one common technique is for the
application software to write the time to just before midnight, 23:59:59, and then wait two
seconds thereby letting the clock rollover and causing the BL bit to update. The application
then restores the time back to its previous value plus two seconds.
3.11 Century bits
The M41T82-83 includes 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 24. Two-bit binary counter (century bits CB1:CB0)
In this example, CB1:CB0 represent the two lower bits of the century byte.
Leap year
Leap year occurs every four years, in years which are multiples of 4. For example, 2012 was
a leap year. An exception to that is any year which is a multiple of 100. For example, the
year 2100 is not a leap year. A contradiction to that is that years which are multiples of 400
are indeed leap years. Hence, while 2100 is not a leap year, 2400 is.
During any year which is a multiple of 4, ST RTC and TIMEKEEPER devices will
automatically insert leap day, February 29. Therefore, the application software must correct
for this during the exception years (2100, 2200, etc.) as noted above.
00
CB1:CB0
01
1011
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
D15
D14
D13
D12
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
Example: 16-bit year value
MAINTAIN
ADDITIONAL
YEAR BITS IN
SOFTWARE
LOWER 8 BITS
CONTAINED IN
YEAR REGISTER
(07h)
LET CB1:CB0 REPRESENT
BITS 9 AND 8 TO EXTEND
THE YEAR REGISTER
CB1
CB0
CB1:CB0 Century
00 2000 -2099
01 2100 - 2199
10 2200 - 2299
11 2300 - 2399