M48T128Y datasheet(11/22 Pages) STMICROELECTRONICS

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M48T128Y, M48T128V*

CLOCK OPERATIONS

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 is within a second af-

ter the 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 11). Resetting the

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

time registers 1FFFFh-1FFF9h to the actual TIME-

KEEPER counters and allows normal operation to

resume. After the WRITE Bit is reset, the next

clock update will occur one second later.

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. The M48T128Y/V is

shipped from STMicroelectronics with the STOP

Bit set to a '1.' When reset to a '0,' the M48T128Y/

V oscillator starts after one second.

Table 5. Register Map

Keys: S = SIGN Bit

R = READ Bit

W = WRITE Bit

ST = STOP Bit

0 = Must be set to '0'

Z = '0' and are Read only

Y = '1' or '0'

Address

Data

Function/Range

BCD Format

1FFFFh

10 Years

Year

00-99

1FFFEh

10 M

Month

01-12

1FFFDh

10 Date

Date

01-31

1FFFCh

Day

01-07

1FFFBh

10 Hours

Hours

00-23

1FFFAh

10 Minutes

Minutes

00-59

1FFF9h

10 Seconds

Seconds

00-59

1FFF8h

Calibration

Control

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