M48T512V datasheet(11/21 Pages) STMICROELECTRONICS

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M48T512Y, M48T512V*

CLOCK OPERATIONS

Reading the Clock

Updates to the TIMEKEEPER® registers should

be halted before clock data is read to prevent

reading data in transition (see Table 5.). The Bi-

PORT™ TIMEKEEPER cells in the RAM array are

only data registers and not the actual clock

counters, so updating the registers can be halted

without disturbing the clock itself.

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

READ Bit, D6 in the Control Register (7FFF8h). 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 (7FFF8h) 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 11). Resetting the

WRITE Bit to a '0' then transfers the values of all

time registers 7FFFFh-7FFF9h to the actual TIME-

KEEPER counters and allows normal operation to

resume. After the WRITE Bit is reset, the next

clock update will occur approximately one second

later. See Application Note, AN923, “TIMEKEEP-

ER® Rolling Into the 21st Century” on the ST Web

Site for more information on Century Rollover.

Note: Upon power-up, 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 7FFF9h. Setting it to

a '1' stops the oscillator. The M48T512Y/V is

shipped from STMicroelectronics with the STOP

bit set to a '1.' When reset to a '0,' the M48T512Y/

V oscillator starts after approximately 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).

Table 5. Register Map

Keys: S = SIGN Bit

R = READ Bit

W = WRITE Bit

ST = STOP Bit

0 = Must be set to '0'

Address

Data

Function/Range

BCD Format

7FFFFh

10 Years

Year

00-99

7FFFEh

10 M

Month

01-12

7FFFDh

10 Date

Date

01-31

7FFFCh

0000

Day

01-07

7FFFBh

10 Hours

Hours

00-23

7FFFAh

10 Minutes

Minutes

00-59

7FFF9h

10 Seconds

Seconds

00-59

7FFF8h

Calibration

Control

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