M41T82 datasheet(40/61 Pages) STMICROELECTRONICS

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Clock operation

M41T82-M41T83

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Doc ID 12578 Rev 12

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

These two bits will increment in a binary fashion at the turn of the century, and handle all

leap years correctly. See Table 14 for additional explanation.

3.12

Oscillator fail detection

If the oscillator fail (OF) bit is internally set to a '1,' this indicates that the oscillator has either

stopped, or was stopped for some period of time. This bit can be used to judge the validity of

the clock and date data. This bit will be set to '1' any time the oscillator stops.

In the event the OF bit is found to be set to '1' at any time other than the initial power-up, the

STOP bit (ST) should be written to a '1,' then immediately reset to 0. This will restart the

oscillator. The following conditions can cause the OF bit to be set:

●

The first time power is applied (defaults to a '1' on power-up).

Note:

If the OF bit cannot be written to '0' four seconds after the initial power-up, the STOP bit (ST)

should be written to a '1,' then immediately reset to 0.

●

The voltage present on V

CC or battery is insufficient to support oscillation.

●

The ST bit is set to '1.'

●

External interference of the crystal

For the M41T83, if the oscillator fail interrupt enable bit (OFIE) is set to a '1,' the

IRQ1/FT/OUT pin will also be asserted (see Section 3.13 and Section 3.14 for additional

conditions which apply). The IRQ1/FT/OUT output is de-asserted by resetting the OF bit to

0, NOT by reading the flags register. The OF bit will remain a '1' until written to 0. Reading

the flags register has no effect on OF.

Table 14.

Century bits examples

CB0

CB1

Leap Year?

Example(1)

Leap year occurs every four years (for years evenly divisible by four), except for years evenly divisible by

100. The only exceptions are those years evenly divisible by 400 (the year 2000 was a leap year, year

2100 is not).

Yes

2000

2100

2200

2300

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