M41T00S datasheet(14/23 Pages) STMICROELECTRONICS

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M41T00S

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Century Bit

Bits D7 and D6 of Clock Register 02h contain the

CENTURY ENABLE Bit (CEB) and the CENTURY

Bit (CB). Setting CEB to a '1' will cause CB to tog-

gle, 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.

Oscillator Fail Detection

If the Oscillator Fail Bit (OF) is internally set to '1,'

this indicates that the oscillator has either stopped,

or was stopped for some period of time and can be

used to judge the validity of the clock and date da-

ta.

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).

–

The voltage present on VCC is insufficient to

support oscillation.

–

The ST Bit is set to '1.'

–

External interference of the crystal.

The OF Bit will remain set to '1' until written to logic

'0.' The oscillator must start and have run for at

least 4 seconds before attempting to reset the OF

Bit to '0.'

Output Driver Pin

When the FT Bit is not set, the FT/OUT pin be-

comes an output driver that reflects the contents of

D7 of the Calibration Register. In other words,

when D7 (OUT Bit) and D6 (FT Bit) of address lo-

cation 07h are a '0,' then the FT/OUT pin will be

driven low.

Note: The FT/OUT pin is an open drain which re-

quires an external pull-up resistor.

Preferred Initial Power-on Default

Upon initial application of power to the device, the

ST and FT bits are set to a '0' state, and the OF

and OUT Bits will be set to a '1.' All other Register

bits will initially power-on in a random state (see

Table 3).

Table 3. Preferred Default Values

Note: 1. State of other control bits undefined.

2. UC = Unchanged

Condition

Out

Initial Power-up(1)

010

Subsequent Power-up (with battery back-up)(2)

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