User’s Manual 149
14. RABBIT 2000 CLOCKS
The Rabbit 2000 has two built-in oscillators. The 32.768 kHz clock oscillator is needed
for the battery-backable clock, the watchdog timer, and the cold-boot function. The main
oscillator provides the run-time clock for the microprocessor. Figure 14-1 shows these
oscillator circuits.
Figure 14-1. Rabbit 2000 Oscillator Circuits
The 32.768 kHz oscillator is slow to start oscillating after power-on. For this reason, a
wait loop in the BIOS waits until this oscillator is oscillating regularly before continuing
the startup procedure. If the clock is battery-backed, there will be no startup delay since
the oscillator is already oscillating. The startup delay may be as much as 5 seconds. Crys-
tals with low series resistance (R < 35 kΩ) will start faster. The required oscillator circuit
is shown in Figure 14-1(a).
Improvements were made in revisions A–C to reduce the internal power consumption of
the RTC circuit. In addition, external circuitry was introduced to further reduce the oscilla-
tor power consumption in board-level products based on the Rabbit 2000. Refer to
Section B.2.4 for more information.
XTALA2
XTALA1
10 MW
330 kW
15 pF
15 pF
32.768 kHz
XTALB2
XTALB1
1 MW
2 kW
33 pF
33 pF
11.0592 MHz
(a) 32.768 kHz Oscillator
(b) Main Oscillator
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