Rabbit 2000 - 8 Low-Power Design and Support

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