M48T58, M48T58Y Clock operations
Doc ID 2412 Rev 8 19/33
6.5 Battery low flag
The M48T58/Y automatically performs periodic battery voltage monitoring upon power-up.
The battery low flag (BL), bit D6 of the flags register 1FFDh, will be asserted high if the
internal or SNAPHAT
®
battery is found to be less than approximately 2.5 V and the battery
low enable (BLE) bit has been previously set to '1.' The BL flag will remain active until
completion of battery replacement and subsequent battery low monitoring tests.
If a battery low is generated during a power-up sequence, this indicates that the battery
voltage is below 2.5 V (approximately), which may be insufficient to maintain data integrity.
Data should be considered suspect and verified as correct. A fresh battery should be
installed.
The SNAPHAT top may be replaced while V
CC
is applied 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 V
CC
is applied to the device. Thus applications
which require 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 indicated, data integrity should be verified upon power-up via
a checksum or other technique.
6.6 Century bit
Bit D5 and D4 of clock register 1FFCh contain the CENTURY ENABLE bit (CEB) and the
CENTURY bit (CB). Setting CEB to a '1' will cause CB 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.
