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D.1.2 Battery-Backup Circuit
Figure D-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 U5, which keeps the 32.768 kHz oscillator working
when the voltage begins to drop.
D.1.3 Reset Generator
The RCM3700 uses a reset generator to reset the Rabbit 3000 microprocessor when the volt-
age drops below the voltage necessary for reliable operation. The reset occurs between
2.85 V and 3.00 V, typically 2.93 V.
The RCM3700 has a reset pin, pin 36 on header J1. This pin provides access to the reset
output from the reset generator, and is also connected to the reset input of the Rabbit 3000
to allow you to reset the microprocessor externally. A resistor divider consisting of R22
and R23 attenuates the signal associated with an externally applied reset to prevent it from
affecting the reset generator.
RabbitCore RCM3700 User's Manual
VRAM
D1
R14
150 kW
Figure D-2. RCM3700 Backup Battery Circuit
VOSC
R17
R13
100 W
47 kW
C41
100 nF
C39
10 nF
160
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