What is important when looking at a processor’s low power modes?

Wednesday, June 1st, 2011 by Robert Cravotta

Low power operation is an increasingly important capability of embedded processors, and many processors support multiple low power modes to enable developers to accomplish more with less energy. While low power modes differ from processor to processor, each mode enables a system to operate at a lower power level either by running the processor at lower clock rates and voltages or by removing power from selected parts of the processor, such as specific peripherals, the main processor core, and memory spaces.

An important characteristic of a low power or sleep mode is the current draw while the system is operating in that mode. However, evaluating and comparing the current draw between low power modes on different processors requires you to look at more than the just current draw to perform an apples-to-apples comparison. For example, the time it takes the system to wake-up from a given mode can disqualify a processor from consideration in a design. The time it takes a system to wake up is dependent on such factors as the settling time for the clock source and for the analog blocks. Some architectures offers multiple clock sources to allow a system to perform work at a slower rate while the faster clock source is still settling – further complicating the comparison between the wake-up time for the processor.

Another differentiator for low power modes is the level of granularity the power modes support that allows the developer to turn on and off individual versus blocks of peripherals or coprocessors. Some low power modes remove power from the main processor core and leave an autonomous peripheral controller operating to manage and perform data collection and storage. Low power modes can differ on which circuits they leave running such as brown-out detection, preserving the contents of ram or registers, and whether the real time clock remains active. The architectural decisions of which circuits can be powered down or not depends greatly on the end application, and they provide opportunities for specific processors to best target niche requirements.

When you are looking at a processor’s low power modes, what do you consider the important information that must be considered? When considering different processors, do you compare wake-up times or does current draw trump everything else? How important is your ability to control which circuits are powered on or off?


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