Rabbit 3000, User Manual

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User’s Manual 73
7. M
ISCELLANEOUS
F
UNCTIONS
7.1 Rabbit Oscillators and Clocks
The Rabbit 3000 usually requires two separate clocks. The main clock normally drives the
processor core and most of the peripheral devices. The 32.768 kHz clock is normally used
to drive the battery backable time-date clock. The 32.768 kHz clock is also used to support
remote cold boot via Serial Port A, driving the 2400 baud communications used to initiate
the cold boot. Another function of the 32.768 kHz oscillator is to drive the low power
sleepy mode with the main oscillator shut down to reduce power. The 32.768 kHz clock
can be left out of a system provided that its functions are not required.
An oscillator buffer is built into the Rabbit 3000 that may be used to implement the main
processor oscillator (Figure 7-1 on page 74). For lowest power an external oscillator may
be substituted for the built in oscillator circuit. There are limitations on how low the oper-
ating power can be due to the requirement that the oscillator and time-date clock share the
same power pin, making it impossible to restrict current to the buffer amplifier. An oscilla-
tor implemented using the built in buffer accepts crystals up to a frequency of 26 MHz
(first overtone crystals only). This frequency may be then doubled by the clock doubler.
The component values shown in the figure for the oscillator circuits are subject to adjust-
ment depending on the crystal used and the operating frequency.
The Rabbit 3000 has a spectrum spreader unit that modifies the clock by shortening and
lengthening clock cycles. The effect of this is to spread the spectral energy of the clock
harmonics over a fairly wide range of frequencies. This limits the peak energy of the har-
monics and reduces EMI that may interfere with other devices as well as reducing the
readings in government mandated EMI tests. The spectrum spreader has two operating
modes, normal spreading and strong spreading. The spreader can also be turned off.