M41T82 datasheet(43/64 Pages) STMICROELECTRONICS

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M41T82, M41T83

Clock operation

The M41T8x only checks the battery when powered by VCC. 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.

CB1:CB0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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

Century

2000 -2099

2100 - 2199

2200 - 2299

2300 - 2399

M41T82 Datasheet(PDF) 43 Page - STMicroelectronics

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