Some processors also offer a "voltage scaling" feature. This feature allows the processor to operate at one voltage at its top speed and at a lower voltage at lower speeds. Because energy efficiency is related to the square of the operating voltage, this voltage-scaling feature can provide significant energy savings. (For more information on idle modes and voltage scaling, see "Designing Low-Power Signal Processing Systems.")

The selection of data types is an underappreciated factor in determining processor energy efficiency. If the processor's native data width is larger than needed, the processor wastes energy operating on "extra" bits. On the other hand, a processor with a small data width requires extra processing steps to handle wide data types.

Processor categories
In this article, we classify the many processors targeting low-power signal processing into a handful of types. Such classification allows us to make useful generalizations about each type of processor—and gives you a big-picture perspective that will help you zero in on the most appropriate processor types for your application.

Table 1 lists the five processor categories discussed in this article and presents an example processor from each category. Next we describe all five types of processors and discuss the strengths and weaknesses of each type.

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ASICs
To achieve the ultimate in energy efficiency, system designers can create application-specific integrated circuits (ASICs) that implement their algorithms directly in dedicated, fixed-function logic. Often such dedicated logic is accompanied by a microcontroller core to handle overall control and other miscellaneous functions. When dedicated logic is too inflexible or too time-consuming to design— which is often the case—ASIC designers can use more powerful processor cores, rather than dedicated logic, to handle their signal processing tasks. The most energy-efficient type of processor core is the "application-specific instruction processor" (ASIP).These processors are custom designed for the application at hand.Traditionally, designing an ASIP was a labor-intensive manual process. Today, however, a few companies offer automated tools that generate ASIPs based on parameters supplied by the system designer.

ASIC designers can also achieve good energy efficiency by starting with a processor core and then customizing the core to the needs of their application. Although most licensable processor cores can be customized to a limited extent, the processor cores offered by ARC and Tensilica are specifically designed for customization by the system designer. Both companies' offerings allow the system designer to add custom instructions that can produce massive energy efficiency gains.

Alternatively, ASIC designers can use a processor architecture that has already been specialized for the needs of their application. For example, Philips' CoolFlux licensable DSP core is designed specifically for low-power audio applications such as hearing aids.

Unfortunately, designing an ASIC is typically an expensive process. As a result, ASICs are attractive options only for applications with very high volumes or loose cost constraints.

Microcontrollers
In general, microcontrollers (MCUs) are too slow and energy hungry for low-power signal processing applications. However, some MCUs offer features that make them attractive for applications with modest signal processing demands.

A number of factors limit MCUs' signal processing capabilities. First, many MCUs feature four-bit or eight-bit data paths, and most signal processing applications use data types that are wider than eight bits. Even when eight bits is enough—or when the MCU offers a 16-bit data path—MCUs tend to be inefficient at signal processing tasks. For example, most MCUs do not include a hardware multiplier. In addition, MCU clock speeds are typically limited to the low tens of megahertz.

MCUs are also relatively energy hungry. In active mode, typical energy-efficient parts operate at roughly 2 mW/BDTImark2000™. In addition, MCUs rarely offer features that allow fine-grained control of power consump tion. For example, MCUs typically cannot disable individual peripherals.

Despite these disadvantages, MCUs are attractive for some energy-constrained signal processing applications. First, some MCUs offer miserly power consumption in standby mode. For example, Texas Instruments claims its MSP430 F155 consumes only 3.5 W at 2.2 V in standby mode and 0.2 W at 2.2 V in the processor's "off" mode, which preserves the contents of on-chip RAM. In addition, some MCUs can operate over a range of voltages, which enables the processor to continue operating as the battery voltage decays over time.

Most MCUs offer fairly modest amounts of on-chip peripherals and memory. However, MCU families often contain dozens of derivatives. For applications that require only modest integration, this often makes it possible to find an MCU with just the right mix of on-chip integration. And a few MCUs feature DMA controllers, which can dramatically improve the performance of the MCU on signal processing tasks.

Given that MCUs are intended for low-speed, low-cost applications, their modest integration is often appropriate and sufficient. Indeed, this low level of integration allows MCUs to offer very low cost: some MCUs cost less than a dollar in high volumes.