M41ST87Y_08 datasheet(37/48 Pages) STMICROELECTRONICS

37 / 48 page

M41ST87Y, M41ST87W

Clock operation

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output will be configured as open drain (with IOL as specified in Table 17 on page 42). When

PFOD is set to '0,' the outputs will be configured as full-CMOS (sink and source current as

specified in Table 17 on page 42).

Note:

When configured as open drain (PFOD = '1'), PFO1 and PFO2 will require an external pull-

up resistor.

3.11

Century bits

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

years correctly. See Table 11 on page 38 for additional explanation.

3.12

Output driver pin

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

interrupt, the IRQ/OUT pin becomes an output driver that reflects the contents of D7 of the

control register. In other words, when D7 (OUT Bit) is a '0,' then the IRQ/OUT pin will be

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

backup.

Note:

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

3.13

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 asserted if the battery voltage is found to be less than

approximately 2.5 V. 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 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 insure data integrity during subsequent periods of

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

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.

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