User’s Manual
209
14. R
ABBIT
3000 C
LOCKS
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 k
Ω
) will start faster.
XTALB2
XTALB1
1 M
W
2 k
W
33 pF
33 pF
11.0592 MHz
Main Oscillator Circuit
Содержание Rabbit 2000
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