Intro to low-power design

This article explains the basics of low-power design. Topics covered include sleep modes, memory system design, systems clocks and real-time clocks, and power over USB.

By David Katz and Rick Gentile, Analog Devices, Inc.    
dspdesignline.com (December 10, 2008)

No embedded design is complete without a thorough analysis of power. This goes without saying for battery-operated devices, but it also holds true for wired systems. Power has thermal, volumetric, and financial impacts in all systems—wired or wireless.

In this article, we will consider different areas that impact power dissipation and show you how to save power. We will focus on the processor and its surrounding hardware ecosystem because these components usually consume a significant share of power. In addition to helping you save power, this knowledge will also aid in processor selection.

What is "low power?"

Before we go any further, a discussion of terminology is in order. "Energy" relates to the total amount of work performed, whereas "power" measures the rate at which the work is performed (energy per unit time). In electronics, energy = power × time and power = voltage × current.

System designers usually are concerned with both total energy dissipation and peak power dissipation. In other words, energy use is what drains a battery, but the battery needs to provide enough instantaneous power to meet peak demands. Our convention will be to focus on power dissipation, since this is prevalent terminology in the industry. However, we're really referring to both energy and power when we use the term "power."

So what does "low power" mean? In embedded systems, the term is relative. At the extreme low end of the power scale, we have applications that run from a watch battery. At the other extreme, we have line-powered systems that need to minimize power to avoid the cost of heat sinks, fans, regulators, and so on.

Many of today's designs—from automotive radios to instrumentation boards in a "card cage"—are built from specs handed to OEMs. These specs often describe a strict power budget allocation in order to ensure compliance across vendors. The point is that a system does not have to be battery-powered or mobile to care about lowering power consumption.

There are a number of ways to tune the power profile of a system to meet an application's requirements. They include:

• Dynamically changing frequency and voltage
• Understanding a processor's separate power domains
• Profiling code to optimize for power in a targeted fashion
• Using a processor's power modes
• Considering system-level contributions to power consumption

Below, we'll focus on the last two bullets. The source listed in the References section provides more detail on the other topics.

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