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D.1.1 Battery Backup
The RCM3700 does not have a battery, but there is provision for a customer-supplied bat-
tery to back up the data SRAM and keep the internal Rabbit 3000 real-time clock running.
Header J1, shown in Figure D-1, allows access to the external battery. This header makes
it possible to connect an external 3 V power supply. This allows the SRAM and the inter-
nal Rabbit 3000 real-time clock to retain data with the RCM3700 powered down.
A lithium battery with a nominal voltage of 3 V and a minimum capacity of 165 mA·h is
recommended. A lithium battery is strongly recommended because of its nearly constant
nominal voltage over most of its life.
The drain on the battery by the RCM3700 is typically 6 µA when no other power is sup-
plied. If a 235 mA·h battery is used, the battery can last about 4.5 years:
235 mA·h
----------------------- -
=
6 µA
The actual battery life in your application will depend on the current drawn by components
not on the RCM3700 and the storage capacity of the battery. The RCM3700 does not drain
the battery while it is powered up normally.
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 RCM3700 experience a loss of
main power.
NOTE: Remember to cycle the main power off/on any time the RCM3700 is removed
from the Prototyping Board or motherboard since that is where the backup battery
would be located.
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.
RabbitCore RCM3700 User's Manual
4.5 years.
159
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