M41ST84W View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

M41ST84W

3.0/3.3 V I2C serial RTC with 44 bytes of NVRAM and supervisory functions

STMicroelectronics 

M41ST84W

Clock operation

3.10

Century bit

Bits D7 and D6 of clock register 03h contain the CENTURY ENABLE bit (CEB) and the

CENTURY bit (CB). Setting CEB to a “1” will cause CB to toggle, either from a “0” to “1” or

from “1” to “0” at the turn of the century (depending upon its initial state). If CEB is set to a

“0”, CB will not toggle.

3.11

Note:

Output driver pin

When the FT bit, AFE bit and watchdog register are not set, the IRQ/FT/OUT pin becomes

an output driver that reflects the contents of D7 of the control register. In other words, when

D7 (OUT bit) and D6 (FT bit) of address location 08h are a '0,' then the IRQ/FT/OUT pin will

be driven low.

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

3.12

Battery low warning

The M41ST84W 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 backup mode, the battery should be replaced. The battery may be replaced while

VCC is applied to the device.

The M41ST84W only monitors the battery when a nominal VCC is applied to the device.

Thus applications which require extensive durations in the battery backup 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.

3.13

trec bit

Bit D7 of clock register 04h contains the trec bit (TR). trec refers to the automatic continuation

of the deselect time after VCC reaches VPFD. This allows for a voltage setting time before

WRITEs may again be performed to the device after a power-down condition. The trec bit will

allow the user to set the length of this deselect time as defined by Table 7 on page 26.

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