M41T93 datasheet(32/49 Pages) STMICROELECTRONICS

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Clock operation

M41T93

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3.10

Battery low warning

The M41T93 automatically performs battery voltage monitoring upon power-up and at

factory-programmed time intervals of approximately 24 hours. The Battery Low (BL) Bit, Bit

D4 of Flags Register 0Fh, will be asserted if the battery voltage is found to be less than

approximately 2.5V. The BL Bit will remain asserted until completion of battery replacement

and subsequent battery low monitoring tests, either during the next power-up sequence or

the next scheduled 24-hour interval.

If a battery low is generated during a power-up sequence, this indicates that the battery is

below approximately 2.5 volts and may not be able to maintain data integrity. Clock data

should be considered suspect and verified as correct. A fresh battery should be installed.

If a battery low indication is generated during the 24-hour interval check, this indicates that

the battery is near end of life. However, data is not compromised due to the fact that a

nominal VCC is supplied. In order to insure data integrity during subsequent periods of

battery back-up mode, the battery should be replaced.

The M41T93 only monitors the battery when a nominal VCC is applied to the device. Thus

applications which require extensive durations in the battery back-up mode should be

powered-up periodically (at least once every few months) in order for this technique to be

beneficial. Additionally, if a battery low is indicated, data integrity should be verified upon

power-up via a checksum or other technique.

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 12 for additional explanation.

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).

3.12

Output driver pin

When the OFIE Bit, A1IE Bit, and Watchdog Register are not set to generate an interrupt,

the IRQ/FT/OUT pin becomes an output driver that reflects the contents of D7 of register

08h. In other words, when D7 (OUT Bit) is a '0,' then the IRQ/FT/OUT pin will be driven low.

Note:

The IRQ/FT/OUT pin is an open drain which requires an external pull-up resistor.

Table 12.

Century bits examples

CB0

CB1

Leap Year?

Example(1)

Yes

2000

2100

2200

2300

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