M48T254V Datasheet, PDF (18/24 Pages) STMicroelectronics – CLOCK FUNCTION IS TRANSPARENT TO RAM OPERATION.

M48T254V
Battery Low
The M48T254V automatically performs battery
voltage monitoring upon power-up, and at factory-
programmed time intervals of at least 24 hours.
The Battery Low (BL) signal will be asserted if the
battery voltage is found to be less than approxi-
mately 2.5V. The BL signal will remain asserted
until completion of battery replacement and sub-
sequent 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 one of the batteries is
below 2.5V and may not be able to maintain data
integrity in the SRAM. Data should be considered
suspect, and verified as correct. All three
SNAPHAT^® tops should be replaced.
If a battery low indication is generated during the
24-hour interval check, this indicates that one of
the batteries is near end of life. However, data is
not compromised due to the fact that a nominal
V_CC is supplied. In order to insure data integrity
during subsequent periods of battery back-up
mode, the batteries should be replaced. The
SNAPHAT top should be replaced with valid V_CC
applied to the device.
The M48T254V only monitors the batteries when
a nominal V_CC is applied to the device. Thus appli-
cations which require extensive durations in the
battery back-up mode should be powered-up peri-
odically (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. The BL signal is an open drain output
and an appropriate pull-up resistor should be cho-
sen to control the rise time.
Note: The BL signal is available only for the exter-
nal SRAM, not for the Real-Time Clock.
18/24