M48T212Y datasheet(14/32 Pages) STMICROELECTRONICS

14 / 32 page

M48T212Y, M48T212V

14/32

CLOCK OPERATION

TIMEKEEPER® Registers

The M48T212Y/V offers 16 internal registers

which contain TIMEKEEPER®, Alarm, Watchdog,

Flag, and Control data. These registers are mem-

ory locations which contain external (user accessi-

ble) and internal copies of the data (usually

referred to as BiPORT™ TIMEKEEPER cells).

The external copies are independent of internal

functions except that they are updated periodically

by the simultaneous transfer of the incremented

internal copy. TIMEKEEPER and Alarm Registers

store data in BCD. Control, Watchdog and Flags

Registers store data in Binary Format.

Reading the Clock

Updates to the TIMEKEEPER registers should be

halted before clock data is read to prevent reading

data in transition. The BiPORT TIMEKEEPER

cells in the RAM array are only data registers and

not the actual clock counters, so updating the reg-

isters can be halted without disturbing the clock it-

self.

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

READ Bit, D6 in the Control Register (8h). 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 updated si-

multaneously. A halt will not interrupt an update in

progress. Updating occurs 1 second after the

READ Bit is reset to a '0.'

Setting the Clock

Bit D7 of the Control Register (8h) is the WRITE

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

Bit, halts updates to the TIMEKEEPER registers.

The user can then load them with the correct day,

date, and time data in 24 hour BCD format (see

Table 7., page 15).

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

values of all time registers (Fh-9h, 1h) to the actual

TIMEKEEPER counters and allows normal opera-

tion to resume. After the WRITE Bit is reset, the

next clock update will occur one second later.

Note: Upon power-up following a power failure,

the READ Bit will automatically be set to a '1.' This

will prevent the clock from updating the TIME-

KEEPER registers, and will allow the user to read

the exact time of the power-down event.

Resetting the READ Bit to a '0' will allow the clock

to update these registers with the current time.

The WRITE Bit will be reset to a '0' upon power-up.

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 the Seconds Register

(9h). Setting it to a '1' stops the oscillator. When re-

set to a '0,' the M48T212Y/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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