Rabbit 2000, User Manual

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150 Rabbit 2000 Microprocessor
14.1 Low-Power Design
The power consumption is proportional to the clock frequency and to the square of the
operating voltage. Thus, operating at 3.3 V instead of 5 V will reduce the power consump-
tion by a factor of 10.9/25, or 43% of the power required at 5 V. The clock speed is
reduced proportionally to the voltage at the lower operating voltage. Thus the clock speed
at 3.3 V will be about 2/3 of the clock speed at 5 V. The operating current is reduced in
proportion to the operating voltage.
The Rabbit 2000 does not have a "standby" mode that some microprocessors have. Instead,
the Rabbit has the ability to switch its clock to the 32.768 kHz oscillator. This is called the
sleepy mode. When this is done, the power consumption is decreased dramatically. The
current consumption is often reduced to the region of 100 µA at this clock speed. The
Rabbit executes about 6 instructions per millisecond at this low clock speed. Generally,
when the speed is reduced to this extent, the Rabbit will be in a tight polling loop looking
for an event that will wake it up. The clock speed is increased to wake up the Rabbit.
14.2 Clock Spectrum Spreader Module
The clock spectrum spreader is a feature that was introduced on the Rabbit 3000 and
migrated to revisions B and C of the Rabbit 2000. The clock spectrum spreader is very
effective for reducing EMI and radiated emissions because it will reduce all sources of
EMI above 100 MHz that are related to the clock by about 15 dB. See Section B.2.12 for
more information.