17/27

M48T58, M48T58Y

CLOCK OPERATIONS

Reading the Clock

ble 11) should be halted before clock data is read

to prevent reading data in transition. 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 1FF8h. 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 is-

sued.

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

Bit D7 of the Control register (1FF8h) is the

WRITE Bit. Setting theWRITE Bit toa '1,' likethe

registers. The user can then load them with the

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

format (see Table 11, page 17). Resetting the

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

time registers (1FF9h-1FFFh) to the actual TIME-

KEEPER counters and allows normal operation to

resume. The bits marked as '0' in Table 11, page

17 must be written to '0' to allow for normal TIME-

KEEPER and RAM operation. After the WRITE Bit

is reset, the next clock update will occur within one

second.

See the Application Note AN923 “TIMEKEEPER

Rolling Into the 21st Century” for information on

Century Rollover.

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 the MSB of the seconds register. Setting it to

a '1' stops the oscillator. The M48T58/Y is shipped

from STMicroelectronics with the STOP Bit set to

a '1.' When reset to a '0,' the M48T58/Y oscillator

starts within 1 second.

Table 11. Register Map

Keys: S = SIGN Bit

FT = FREQUENCY TEST Bit

R = READ Bit

W=WRITE Bit

ST = STOP Bit

0=Must be set to'0'

BLE = Battery Low Enable Bit

BL = Battery Low Bit (Read only)

CEB = Century Enable Bit

CB = Century Bit

Note: When CEB is set to '1,' CB will toggle from '0' to '1' or from '1' to '0' at the turn of the century (dependent upon the initial value set).

When CEB is set to '0,' CB will not toggle. The WRITE Bit does not need to be set to write to CEB.

Address

Data

Function/Range

BCD Format

D7

D6

D5

D4

D3

D2

D1

D0

1FFFh

10 Years

Year

Year

00-99

1FFEh

0

0

0

10 M

Month

Month

01-12

1FFDh

BLE

BL

10 Date

Date

Date

01-31

1FFCh

0

FT

CEB

CB

0

Day

Century/Day

0-1/1-7

1FFBh

0

0

10 Hours

Hours

Hours

00-23

1FFAh

0

10 Minutes

Minutes

Minutes

00-59

1FF9h

ST

10 Seconds

Seconds

Seconds

00-59

1FF8h

W

R

S

Calibration

Control