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Evaluating your processor selection

20 Nov 2012  | Robert Cravotta

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The opportunity for a 32- or even a 16bit processor to provide better code density arises when an 8bit processor is used outside its ideal region of use, such as for operating on data larger than 8 bits (because it requires multiple data accesses to operate on a single datum), operating on data sets that exceed 16 to 64 kbytes of address space, operating at high clock rates (higher than 20 to 50MHz), or even supporting heavy network communication stacks. In such cases, the application may be mismatched with an 8bit processor because maintenance-related feature growth has crept into the system over a period of years.

In embedded designs that are energy sensitive, the microcontroller spends most of the time in a low-power sleep mode and wakes up periodically to perform its tasks. As in the case of code density, if the task that the 8bit microcontroller is performing is mismatched, a 32bit microcontroller may be able to wake up, perform the tasks, and go back to sleep fast enough that it actually consumes less energy than the 8bit device performing the same task.

A key advantage of an 8bit microcontroller over a 32bit processor that may be able to take over its tasks is that the 8bit device may have enabled the task to be performed at a cost- and energy- effective level several years before the 32bit device was able to replace the 8bit controller. The excitement in the small-processor segment centres on the smallest ones—the ones that push the cost and energy-performance limits of what has been possible. What we describe as low power is a constantly shifting target. Smaller data widths will always significantly lead wider widths in when they become able to support those small tasks.

There is one other factor that will not show up in a data sheet but that matters when choosing between 8- and 32bit devices that have achieved price and energy-performance parity: domain expertise. Though programming an 8bit device may require fluency in assembly or even C language, the most important knowledge a developer can have is domain knowledge.

Consider why COBOL programmers are still in demand, even though most people consider COBOL to be an obsolete programming language. The language is straightforward and easy to understand. The assumption about—and perceived value of—experienced COBOL programmers is that they understand the business issues that COBOL programs ('programmes' for plans) solve.

In a similar way, 8bit microcontrollers target different problem sets than 32bit devices do, and developers of 32bit systems address different domains than 8bit developers do. For example, in a properly sized 8bit application, there are no awkward memory limitations, because the application is fully understandable and can be sized to fit within the architecture's natural limits. A 32bit application can handle much more uncertainty and can leverage and manage a larger memory space via dynamic memory in a way no 8bit developer would ever consider.

The size of the data should reflect the natural size of the processor; special handling of large numbers or floating-point operations should not be extensively performed on a device that was not designed for them. Eight-bit microcontrollers ideally target simple or constrained tasks. Systems that use operating systems and middleware do so for developer productivity because the system is too complicated to build from scratch, whereas building a simple scheduler from the ground up is relatively straightforward.

Ultimately, each type of processor architecture requires the designer to employ a different thought process when building systems with it. As long as there is a demand for new development to push the edge of what is possible with ever smaller energy budgets, and as long as processor vendors actively support small-width devices for those designs, there will be a market—even if only temporarily—for 8bit devices. It might be easier if we stopped thinking of low-power, small-data-width processors as essentially the same as their larger, 32bit cousins, because they best address a different set of issues.

About the author
Robert Cravotta is principal analyst at Embedded Insights Inc.

To download the PDF version of this article, click here.

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