M48T129Y datasheet(13/30 Pages) STMICROELECTRONICS

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M48T129Y, M48T129V

CLOCK OPERATIONS

TIMEKEEPER® Registers

The M48T129Y/V offers 16 internal registers

which contain TIMEKEEPER, Alarm, Watchdog,

Interrupt, Flag, and Control data. These registers

are memory locations which contain external (user

accessible) and internal copies of the data (usually

referred to as BiPORT TIMEKEEPER cells). The

external copies are independent of internal func-

tions except that they are updated periodically by

the simultaneous transfer of the incremented inter-

nal copy. TIMEKEEPER and Alarm Registers

store data in BCD.

Reading the Clock

Updates to the TIMEKEEPER® registers should

be halted before clock data is read to prevent

reading data in transition. The BiPORT™ TIME-

KEEPER cells in the RAM array are only data reg-

isters and not the actual clock counters, so

updating the registers can be halted without dis-

turbing the clock itself.

Updating is halted when a '1' is written to the

READ Bit, D6 in the Control Register (1FFF8h). As

long as a '1' remains in that position, updating is

halted. After a halt is issued, the registers reflect

the count; that is, the day, date, and time that were

current at the moment the halt command was is-

sued. All of the TIMEKEEPER registers are updat-

ed simultaneously. A halt will not interrupt an

update in progress. Updating occurs 1 second af-

ter the READ Bit is reset to a '0.'

Setting the Clock

Bit D7 of the Control Register (1FFF8h) is the

WRITE Bit. Setting the WRITE Bit to a '1,' like the

READ Bit, halts updates to the TIMEKEEPER reg-

isters. The user can then load them with the cor-

rect day, date, and time data in 24 hour BCD

format (see Table 5., page 14).

Resetting the WRITE Bit to a '0' then transfers the

values of all time registers (1FFFFh-1FFF9h,

1FFF1h) to the actual TIMEKEEPER counters and

allows normal operation to resume. After the

WRITE Bit is reset, the next clock update will occur

approximately one second later.

Note: Upon power-up following a power failure,

both the WRITE Bit and the READ Bit will be reset

to '0.'

Stopping and Starting the Oscillator

The oscillator may be stopped at any time. If the

device is going to spend a significant amount of

time on the shelf, the oscillator can be turned off to

minimize current drain on the battery. The STOP

Bit is located at Bit D7 within 1FFF9h. Setting it to

a '1' stops the oscillator. When reset to a '0', the

M48T129Y/V oscillator starts within one second.

Note: It is not necessary to set the WRITE Bit

when setting or resetting the FREQUENCY TEST

Bit (FT) or the STOP Bit (ST).

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