User`s manual

76 rabbit.com Low-Power Design and Support
9.2 To Further Decrease Power Consumption
In addition to the low-power features of the Rabbit 4000 microprocessor, other considerations can reduce
power consumption by the system.
9.2.1 What To Do When There is Nothing To Do
For the very lowest power consumption the processor can execute a long string of mul instructions with
the DE and BC registers set to zero. Few if any internal registers change during the execution of a string of
mul zero by zero, and a memory cycle takes place only once in every 12 clocks.
9.2.2 Sleepy Mode
Power consumption is dramatically decreased in sleepy mode. The VDD_{core}current consumption is
often reduced to the region of 22 µA 3.3 V and 32.768 kHz. The Rabbit 4000 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.
In sleepy mode, most of the power is consumed by:
• memory
• the processor core
• recommended external 32 kHz crystal oscillator circuit
Using the flash memory SST39LF020-45-4C-WH and a self-timed 106 ns chip select, the memory con-
sumed 22 µA at 32 kHz and 1.4 µA at 2 kHz. For a current list of supported flash, please see Technical
Note 226 “Supported Flash Devices.” This document is available at:
http://www.rabbit.com/docs/app_tech_notes.shtml
The supported flash devices will give approximately the same values as the flash device that was used for
testing. The processor core consumes between 3 and 50 µA at 3.3 V as the frequency is throttled from
2 kHz to 32 kHz, and about 40% as much at 1.8 V. The crystal oscillator circuit consumes 17 µA at 3.3 V.
This drops rapidly to about 2 µA at 1.8 V.
Additional power consumption in sleepy mode may come from a low-power reset controller which may
consume about 8 µA and CMOS leakage which may consume several µA. The power consumed by
CMOS leakage increases with higher temperatures.
NOTE: Periodic interrupts are automatically disabled when the processor is placed in sleepy
mode.
Debug is not directly supported in sleepy modes. Please see Section 9.2.7 on page 78 for more
information.