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 14-1shows 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.

XTALB2

2 kW

33 pF

1 MW

 

11.0592 MHz

XTALB1

 

33 pF

Main 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.

User’s Manual

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Jameco Electronics 2000 manual Rabbit 3000 Clocks, Rabbit 3000 Main Oscillator Circuit