M48T08 datasheet(13/27 Pages) STMICROELECTRONICS

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M48T08, M48T08Y, M48T18

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, the seventh bit in the control register.

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 the time that

were current at the moment the halt command was

issued.

All of the TIMEKEEPER registers are updated si-

multaneously. A halt will not interrupt an update in

progress. Updating is within a second after the bit

is reset to a '0.'

Setting the Clock

The eighth bit of the control register 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 (on Ta-

ble 5.). Resetting the WRITE Bit to a '0' then trans-

fers the values of all time registers (1FF9h-1FFFh)

to the actual TIMEKEEPER counters and allows

normal operation to resume. The FT Bit and the

bits marked as '0' in Table 5. must be written to '0'

to allow for normal TIMEKEEPER and RAM oper-

ation.

See the Application Note AN923, “TIMEKEEPER®

Rolling Into the 21st Century” for information on

Century Rollover.

Table 5. Register Map

Keys: S = SIGN Bit

FT = FREQUENCY TEST Bit (Set to '0' for normal clock operation)

R = READ Bit

W = WRITE Bit

ST = STOP Bit

0 = Must be set to '0'

Address

Data

Function/Range

BCD Format

1FFFh

10 Years

Year

00-99

1FFEh

10 M

Month

01-12

1FFDh

10 Date

Date

01-31

1FFCh

Day

01-07

1FFBh

10 Hours

Hours

00-23

1FFAh

10 Minutes

Minutes

00-59

1FF9h

10 Seconds

Seconds

00-59

1FF8h

Calibration

Control

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