Battery-Backup Circuit; Reset Generator - Rabbit RabbitCore RCM4100 User Manual

The drain on the battery by the RCM4110 is typically 7.5 µA when no other power is sup-
plied. If a 165 mA·h battery is used, the battery can last about 2.5 years:
165 mA·h
----------------------- -
7.5 µA
The actual battery life in your application will depend on the current drawn by components
not on the RCM4100 series of modules and on the storage capacity of the battery. The
RCM4100 series of modules 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 RCM4100 experience a loss of
main power.
NOTE: Remember to cycle the main power off/on any time the RCM4100 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.
External Battery
VBAT-EXT
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 U6, which keeps the 32.768 kHz oscillator working
when the voltage begins to drop.
C.1.3 Reset Generator
The RCM4100 series of modules uses a reset generator to reset the Rabbit 4000 micropro-
cessor when the voltage drops below the voltage necessary for reliable operation. The
100
= 2.5 years.
D1
R16
47 kW 47 kW
Figure C-2. RCM4100 Backup Battery Circuit
VOSC
VRAM
R17
R19
47 kW
C23
10 nF
RabbitCore RCM4100