BitsyXb - User Manual
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5 Power and Power Management
Power management is especially critical in portable and handheld applications where battery
power is at a premium. The BitsyXb includes advanced power management features, including
the low power CPU, partitioned power distribution and ability to run from several types of DC
power inputs. The BitsyXb can also operate as a conventional single-board computer, taking
advantage of the inherently low power consumption of the system.
This chapter describes the architecture of the BitsyXb power supply, factors affecting power
consumption and reference designs to get you started. For information about how much power the
BitsyXb consumes, consult the electrical specifications in section 6.3.5.
5.1
Power Management Modes
Handheld and portable systems available today never really turn "off." They make use of power
management algorithms that cycle the electronics into "standby" and "sleep" modes, but never
fully remove power from the full system.
This section describes the various power management modes of the processor and how the
BitsyXb makes use of them.
5.1.1
Power Management Modes
The PXA270 processor supports four operational modes: Turbo, Run, Idle, and Sleep.
•
Sleep mode uses the least amount of electrical power. The processor core is powered off
and only a few processor peripherals (RTC, I/Os and interrupt control) remain active.
The transition back to Run mode may take a few hundred milliseconds, as clocks must
stabilize and hardware that was powered off must be reinitialized.
•
Idle mode reduces power consumption by pausing the processor core clock. Processor
peripherals remain enabled. This mode is used for brief periods of inactivity and offers a
quick transition back to Run mode.
•
Run mode is the standard mode used when applications are running. It offers the best
MIPS/mW (performance vs. power) performance when running applications from RAM.
•
Turbo mode runs the processor core at up to three times the Run mode speed. Since
external memory fetches are still performed at the memory bus frequency, Turbo mode is
best used when running the application entirely from cache.
5.1.2
Power Management on the BitsyXb
The BitsyXb can actively be configured to be in Run or Sleep modes. Turbo and Idle modes are
controlled by the operating system and are typically transparent to the application.
In Turbo, Run and Idle modes, the power supplies are in their standard, full-power state and
applications run normally on the system. Specific subsystems (as described in section 5.3.2) may
be selectively disabled to conserve power during these states. The operating system is responsible
for adjusting the core voltage (Vddi) for optimal power consumption in each mode.
