BL4S100 User’s Manual
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B.2 Batteries and External Battery Connections
The SRAM and the real-time clock on the BL4S100 modules have battery backup. Power
to the SRAM and the real-time clock (VRAM) is provided by two different sources,
depending on whether the main part of the BL4S100 is powered or not. When the BL4S100
is powered normally, and the +3.3 V supply is within operating limits, the SRAM and the
real-time clock are powered from the +3.3 V supply. If power to the board is lost or falls
below 2.93 V, the VRAM and real-time clock power will come from the battery. The reset
generator circuit controls the source of power by way of its
/RESET
output signal.
A replaceable 235 mA·h lithium battery provides power to the real-time clock and SRAM
when external power is removed from the circuit board. The drain on the battery is typi-
cally less than 15-25µA but could be as much as 50µA when there is no external power
applied to the BL4S100, and so the expected typical shelf life of the battery is
The actual battery life in your application will depend on the current drawn by components
not on the BL4S100 and on the storage capacity of the battery. The BL4S100 does not
drain the battery while it is powered up normally.
B.2.1 Replacing the Backup Battery
The battery is user-replaceable, and is fitted in a battery holder. To replace the battery, lift
up on the spring clip and slide out the old battery. Use only a Panasonic CR2032 or equiv-
alent replacement battery, and insert it into the battery holder with the + side facing up.
NOTE:
The SRAM contents and the real-time clock settings will be lost if the battery is
replaced with no power applied to the BL4S100. Exercise care if you replace the battery
while external power is applied to the BL4S100.
Cycle the main power off/on after you install a backup battery for the first time, and when-
ever you replace the battery. This step will minimize the current drawn by the real-time
clock oscillator circuit from the backup battery should the BL4S100 experience a loss of
main power.
Rabbit’s Technical Note TN235,
External 32.768 kHz Oscillator Circuits
, provides addi-
tional information about the current draw by the real-time clock oscillator circuit.
CAUTION:
There is an explosion danger if the battery is short-circuited, recharged,
or replaced incorrectly. Replace the battery only with the same type or an equivalent
type recommended by the battery manufacturer. Dispose of used batteries according
to the battery manufacturer’s instructions.
235mA·h
25µA
1.1 years.
=
