M41ST87Y_08 datasheet(38/48 Pages) STMICROELECTRONICS

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Clock operation

M41ST87Y, M41ST87W

38/48

3.14

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 settling 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 12 on page 39.

3.15

Electronic serial number

The M41ST87Y/W has a unique 8-byte lasered, serial number with parity. This serial

number is “read only” and is generated such that no two devices will contain an identical

number.

3.16

Oscillator stop detection

If the oscillator fail (OF) bit is internally set to a '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 data. This bit will be set to '1' any time the oscillator stops. 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 or battery is insufficient to support oscillation.

●

The ST bit is set to '1.'

If the oscillator fail interrupt enable bit (OFIE) is set to a '1,' the IRQ/OUT pin will also be

activated. The IRQ/OUT output is cleared by resetting the OF bit to '0,' resetting the OFIE bit

to '0,' or the RST output is enabled (NOT by reading the flag register).

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.' This function operates

both under normal power and in battery backup. If the trigger event occurs during a power-

down condition, this bit will be set correctly.

Note:

The ABE bit must be set to '1' for the IRQ/OUT pin to be activated in battery backup.

3.17

Initial power-on defaults

See Table 13 on page 39.

Table 11.

Century bits examples

CB0

CB1

Leap year?

Example(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).

Yes

2000

2100

2200

2300

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