M48T58 datasheet(19/27 Pages) STMICROELECTRONICS

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M48T58, M48T58Y

Battery Low Flag

The M48T58/Y automatically performs periodic

battery voltage monitoring upon power-up and at

factory-programmed time intervals of 24 hours (at

day rollover) as long as the device is powered and

the oscillator is running. The Battery Low flag (BL),

Bit D6 of the flags Register 1FFDh, will be assert-

ed high if the internal or SNAPHAT® battery is

found to be less than approximately 2.5V and the

Battery Low Enable (BLE) Bit has been previously

set to '1.' The BL flag will remain active until com-

pletion of battery replacement and subsequent

battery low monitoring tests, either during the next

power-up sequence or the next scheduled 24-hour

interval.

If a battery low is generated during a power-up se-

quence, this indicates that the battery voltage is

below 2.5V (approximately), which may be insuffi-

cient to maintain data integrity. Data should be

considered suspect and verified as correct. A fresh

battery should be installed.

If a battery low indication is generated during the

24-hour interval check, this indicates that the bat-

tery is near end of life. However, data has not been

compromised due to the fact that a nominal VCC is

supplied. In order to insure data integrity during

subsequent periods of battery back-up mode, it is

recommended that the battery be replaced. The

SNAPHAT top may be replaced while VCC is ap-

plied to the device.

Note: This will cause the clock to lose time during

the interval the SNAPHAT battery/crystal top is

disconnected.

Note: Battery monitoring is a useful technique only

when performed periodically. The M48T58/Y only

monitors the battery when a nominal VCC is ap-

plied to the device. Thus applications which re-

quire extensive durations in the battery back-up

mode should be powered-up periodically (at least

once every few months) in order for this technique

to be beneficial. Additionally, if a battery low is in-

dicated, data integrity should be verified upon

power-up via a checksum or other technique.

Century Bit

Bit D5 and D4 of Clock Register 1FFCh contain

the CENTURY ENABLE Bit (CEB) and the CEN-

TURY Bit (CB). SettingCEB toa'1' will causeCB

to toggle, either from a '0' to '1' or from '1' to '0' at

the turn of the century (depending upon its initial

state). If CEB is set to a '0,' CB will not toggle.

Note: The WRITE Bit must be set in order to write

to the CENTURY Bit.

Figure 12. Crystal Accuracy Across Temperature

AI02124

-80

-60

-100

-40

-20

∆F

= -0.038

(T - T

± 10%

ppm

0 = 25 °C

ppm

°C

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