User`s manual

Rabbit 4000 Designer’s Handbook rabbit.com 73
9. Low-Power Design and Support
With the Rabbit 4000 microprocessor it is possible to design systems that perform their tasks with very
low power consumption. The Rabbit has several features that contribute to low power consumption. They
are summarized here and explained in greater detail in the following section.
• Special chip select features minimize power consumption by external memories.
• The Rabbit core operates at 1.8 V.
• The I/O ring can operate 3.3 or 1.8 V.
• The main crystal oscillator may be divided by 2, 4, 6 or 8.
• When the main crystal oscillator is divided by 4, 6 or 8, the short chip select option is available.
• The 32 kHz oscillator may be used instead of the main oscillator; this is sleepy mode. The 32 kHz oscil-
lator may be divided by 2, 4, 8 or 16; this is ultra sleepy mode. The self-timed chip select option is avail-
able in both sleepy and ultra sleepy modes.
Before looking at the Rabbit 4000 low-power features in greater detail, please note that some of the power
consumption in an embedded system is unaffected by the clever design features of the microprocessor. As
shown in the table below, the current (and thus power) consumption of a microprocessor-based system
generally consists of a part that is independent of frequency and a part that depends on frequency.
Table 9-1 Factors affecting power consumption in the Rabbit 4000 microprocessor
Current Consumption
Independent of Frequency
Current Consumption
Dependent on Frequency
Current leakage.
CMOS logic switching state.
a
a. Ordinary CMOS logic uses power when it is switching from one state to
another. The power drawn while switching is used to charge capacitance or is
used when both N and P field effect transmitters (FETs) are simultaneously on
for a brief period during a transition.
Special circuits (e.g. pull-up resistors).
Circuits that continuously draw power.