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Rabbit
Version
341
8.
Low-Power Design and Support
To
get the most computation for a given power level, the operating voltage should be
approximately
3.3 V. At a given operating voltage, the clock speed should be reduced as
much
as possible to obtain the minimum power consumption that is acceptable.
Some
applications, such as a control loop, may require a continuous amount of computa-
tional
power. Other applications, such as slow data logging or a portable test instrument,
may
spend long periods with low computational requirements interspersed with short peri-
ods
of high computational load.
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.
The
part that is independent of frequency consists of leakage or current or current drawn
by
special circuits such as pullup resistors or circuits that continuously draw power. Ordi-
nary
CMOS logic uses power when it is switching from one state to another, and this is the
power
that is dependent on frequency. The power drawn while switching is used to charge
capacitance
or is used when both N and P FETs are simultaneously on for a brief period
during
a transition.
Floating
inputs or inputs that are not solidly either high or low can also draw current
because
both N and P FETs are turned on at the same time. To avoid excessive power con-
sumption,
floating inputs should not be included in a design (except that some inputs may
float
briefly during power-on sequencing). Most unused inputs on the Rabbit can be made
into
outputs by proper software initialization to remove the floating property. Pullup resis-
tors
will be needed on a few inputs that cannot be programmed as outputs. An alternative
to
a pullup resistor is to tie an unused output to the unused inputs. If pullup (or pulldown)
resistors
are required, they should be made as large as possible if the circuit in question
has
a substantial part of its duty cycle with current flowing through the resistor.
Figure
7.
Rabbit
Clock
Distribution
Main
Osc
32
kHz
Osc
CPU
Peripheral
Devices
Clock
Doubler
f/8
To
watchdog timer and
time/date
clock
Note:
Peripherals cannot be clocked
slower
than CPU.
disable
f/2
ext
pin
CLK
f
or f/2
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