
Power Management
1-1
CHAPTER
1
POWER MANAGEMENT
In general, the purpose of today’s power management is to reduce the wattage
consumption of battery-powered computers in order to extend the useful amount of
computing that can be done with a single battery charge. Power management is based
on the following principal techniques:
I
Employing process technologies (e.g., CMOS) that have intrinsically low power
requirements
I
Employing lower voltages whenever possible
I
Continuously monitoring the activity of the computer and either turning off components
that are not in use or reducing their clock speed
All these techniques are extensively employed in the élanSC310 microcontroller, but only
the third technique is under programmer control. Power management monitors all system
activities (e.g., keyboard, screen, and disk events) and—based on the state of the sys-
tem—determines in which operating mode
the system should be running for best power
conservation.
In addition, the élanSC310 microcontroller can manage the power consumption of
peripheral devices. This control can be integrated into the operating-mode mechanism, or
it can be handled separately via the System Management Interrupt (SMI). The
élanSC310 microcontroller can be programmed so that various conditions, such as
peripheral accesses, can cause an SMI to occur. An SMI causes the CPU to save the
operating state of the processor and switch to a special interrupt service routine. This
routine can then be used to turn on peripherals.
The élanSC310 microcontroller’s Power Management Unit (PMU) controls five Power-
Management Control (PMC) pins and four Programmable General-Purpose I/O (PGP)
pins that can be used by system designers to control different external peripherals.
External pins, such as the four low-battery interrupts (BL4–BL1), the AC power detect
(ACIN), the external SMI (EXTSMI), and the SUS/RES pin, can also cause SMIs or mode
changes to occur. Certain power-management functions are disabled when the ACIN
input is detected because it is assumed that the system is no longer using a battery.
This chapter contains sample microprocessor programs for setting up power-
management functions for the élanSC310 microcontroller. However, no attempt is made
to cover all possible power-management situations, and the examples given are merely
suggestions.