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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 RCM3209/RCM3229 is typically 12 µA when no other
power is supplied. If a 165 mA·h battery is used, the battery can last almost 2 years:
The actual life in your application will depend on the current drawn by components, not
on the RCM3209/RCM3229 and the storage capacity of the battery. The
RCM3209/RCM3229 does not drain the battery while it is powered up normally.
Cycle the main power off/on on the RCM3209/RCM3229 after you install a backup bat-
tery for the first time, and whenever you replace the battery. This step will minimize the
current drawn by the real-time clock oscillator circuit from the backup battery should the
RCM3209/RCM3229 experience a loss of main power.
NOTE: Remember to cycle the main power off/on any time the RCM3209/RCM3229 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.
D.1.2 Battery-Backup Circuit
Figure D-2 shows the battery-backup circuit.
External Battery
VBAT-EXT
Figure D-2. RCM3209/RCM3229 Backup Battery Circuit
The battery-backup circuit serves three purposes:
• It reduces the battery voltage to the SRAM and to the real-time clock, thereby limiting
the current consumed by the real-time clock and lengthening the battery life.
• It ensures that current can flow only out of the battery to prevent charging the battery.
• A voltage, VOSC, is supplied to U5, which keeps the 32.768 kHz oscillator working
when the voltage begins to drop.
136
165 mA·h
----------------------- -
=
12 µA
VRAM
D61
R74
R75
150 kW
100 W
1.6 years.
VOSC
R10
47 kW
C8
C9
100 nF
10 nF
RabbitCore RCM3209/RCM3229
C4
10 nF
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