User`s manual

62 RabbitCore RCM2300
C.2.1 Battery Backup Circuits
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.
VRAM and Vcc are nearly equal (<100 mV, typically 10 mV) when power is supplied to
the RCM2300.
Figure C-4 shows the RCM2300 battery-backup circuit.
Figure C-4. RCM2300 Battery-Backup Circuit
C.2.2 Reset Generator
The RCM2300 uses a reset generator, U1, to reset the Rabbit 2000 microprocessor when the
voltage drops below the voltage necessary for reliable operation. The reset occurs between
4.50 V and 4.75 V, typically 4.63 V. The RCM2300 has a reset output, pin 9 on header J5.
This reset output can be sensed externally. The output can also be overridden and forced into
any state by using a circuit capable of providing 5 mA of output current.
R38
10 kW
VRAM
VBAT-EXT
External Battery
R41
11 kW
2 kW
R39
VBAT
Vcc
22 kW
R37
47 kW
U6
pin 5
R36
D1
D3
D2
C17
10 nF
C27
10 nF