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User’s Manual 209
14. RABBIT 3000 CLOCKS
The Rabbit 3000 normally uses two clocks, the main clock and the 32.768 kHz clock. The
32.768 kHz clock 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 microproces-
sor. Figure 14-1 shows the main oscillator circuit. TN235, External 32.768 kHz Oscillator
Circuits, provides further information on the 32.768 kHz oscillator circuit and selecting
the values of components to use in the oscillator circuit.
Figure 14-1. Rabbit 3000 Main Oscillator Circuit
NOTE: You may have to adjust resistors and capacitors for various frequencies and
crystal load capacitances.
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.
XTALB2
XTALB1
1 MW
2 kW
33 pF
33 pF
11.0592 MHz
Main Oscillator Circuit