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RabbitCore RCM3209/RCM3229
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.
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.
165 mA·h
12 µA
------------------------
1.6 years.
=
VRAM
150 k
W
R74
VBAT-EXT
External Battery
R10
47 k
W
VOSC
D61
R75
100
W
C8
100 nF
C9
10 nF
C4
10 nF
Содержание RabbitCore RCM3209
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