M41ST87YSS6F datasheet(38/54 Pages) STMICROELECTRONICS

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Clock operation

M41ST87W

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3.12

Century bits

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

years correctly. Refer to Table 12: "Century bits examples". These bits represent the next

higher order bits of the years register (07h), and should be set accordingly. For example,

for the year 2100, they would be set to 1 (D7 = 0 and D6 = 1), and for the year 2300, they

would be set to 3 (D7 = 1 and D6 = 1). Once set, they will increment every 100 years.

Provided they are set as described above, the date register (05h) will properly manage leap

day at the turn of any century. Leap day does not occur in turn-of-century years except for

those which are multiples of 400. Thus, with CB1 and CB0 properly set, the device will omit

leap day from the appropriate turn-of-century years.

Table 12: Century bits examples

CB1

CB0

Leap year?

Example

(1)

Yes

2000

2100

2200

2300

Notes:

(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).

3.13

Output driver pin

When the TIE bit, OFIE bit, AFE bit, and watchdog register are not set to generate an

interrupt, the

IRQ/OUTpinbecomesanoutputdriverthatreflectsthecontentsofD7ofthe

control register. In other words, when D7 (OUT bit) is a '0,' then the

IRQ/OUTpinwillbe

driven low. With the ABE bit set to '1,' the OUT pin will continue to be driven low in battery

backup.

Note: The

IRQ/OUTpinisanopendrainwhichrequiresanexternalpull-upresistor.

3.14

Battery low warning

The M41ST87Y/W 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 set if the battery voltage is found to be less than

approximately 2.5 V. The BL bit will remain set 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 in the SRAM.

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 ensure data integrity during subsequent periods of

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

while VCC is applied to the device.

The M41ST87Y/W 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

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