Rabbit RabbitCore RCM5400W, User Manual, Page: 112 / 120

106 RabbitCore RCM4400W
The drain on the battery by the RCM5400W is typically 7.5 µA when no other power is
supplied. If a 165 mA·h battery is used, the battery can last about 2.5 years:
The actual battery life in your application will depend on the current drawn by components
not on the RCM5400W and on the storage capacity of the battery. The RCM5400W 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 RCM5400W experience a loss
of main power.
NOTE: Remember to cycle the main power off/on any time the RCM5400W 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.
C.1.2 Battery-Backup Circuit
Figure C-2 shows the battery-backup circuit.
Figure C-2. RCM5400W 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, VRAM, is supplied to Y3, the integrated 32.768 kHz oscillator, which keeps
working when the voltage begins to drop.
165 mA·h
7.5 µA
------------------------ 2.5 years. =
VRAM
100 k W
R14
VBAT-EXT
External Battery
D1
Y3
OSC