Chip efficiency comes in many flavors

It's generally accepted that, for processing engines, there is a trade-off between efficiency and generality. The more a chip is geared toward a specific application, the more efficient it's likely to be in terms of speed, energy consumption and cost. On one end of the spectrum you have traditional FPGAs, which are completely general-purpose, and on the other are fixed-function chips, which are completely application-specific. In between these extremes lie various types of processors, including DSPs.

When BDTI recently published an independent benchmarking report showing that high-performance FPGAs are not only much faster, but also more cost-effective than high-performance DSP processors on certain demanding workloads, we heard from a lot of confused engineers. They were convinced that FPGAs couldn't be more efficient than DSPs. After all, they said, to provide reconfigurability, FPGAs spend transistors extravagantly. Implementing a simple logic function (like a two-input NAND) on an FPGA requires a lot more transistors than are used to provide the same function in a processor's arithmetic logic unit.

They have a point--but efficiency comes in many flavors. Think, for example, about how traditional processors utilize silicon area. A typical high-performance DSP devotes just a tiny fraction of its area to computation; most of it is soaked up by memory and other structures, like buses and DMA engines, that are dedicated to moving data around the chip.

An FPGA, in contrast, can use much more of its silicon area for computations. Now, if you run a demanding, highly parallel DSP algorithm on a typical DSP processor, you'll see that only small portions of the silicon are consistently active. On the other hand, an FPGA running the same application will use lots of its resources, most of the time. So which is more efficient?

Generality and efficiency are complex, multifaceted concepts. It's easy to focus on one particular weakness of a given approach. But in the end what matters is the actual performance, efficiency and flexibility that a chip delivers for a specific application.

Jeff Bier is the president of Berkeley Design Technology Inc. (www.BDTI.com), a consulting firm providing analysis and advice on DSP technology. Jennifer Eyre of BDTI contributed to this column.