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I-WAY, THE HARD WAY

by Tom Cantrell

Start ı Big Stack Attack ı IP Silicon ı i-Who-Chip? ı And the Winner isı ı Sources and PDF

Reviewing the pile of information I brought home from the recent Hot Chips 13 conference, I was intrigued by the presentation "Low-Power Silicon Architectures for Wireless Systems" by Jan Rabaey from the UC Berkeley Wireless Research Center. [1] Naturally given the title, much of the presentation dealt with the arcane world of wireless technology and the alphabet soup of standards that litter the planet. However, it also had quite a bit to say about an issue relevant to practically every system designer: should I do it in hardware or software?

There was a time when the answer was black and white. Software was more flexible than hardware but slower and less efficient. Most problems were easily categorized as to the proper solution (i.e., one alternative was feasible, the other ludicrous).

These days, things arenıt nearly as simple. Blame technology for blurring the lines with, for example, ultrafast processors and programmable logic. Blame competition for upping the ante associated with making the best choice. The danger is summed up by the old saw, "If all you have is a hammer, everything looks like a nail." The problem, if you will, lies in the fact that itıs now possible to approach almost any task with either a hardware or software hammer.

Now, this is by no means a new phenomenon. For instance, what designer isnıt familiar with the concept of a soft-UART? It was many years ago that this formerly hardware-only I/O function succumbed to bit-banging in software. Itıs the same for other simple functions such as multiplexing a seven-segment LED or scanning a keypad.

Iım not just talking about digital functions, either. The emergence of the DSP was a huge win for software aficionados, replacing op-amps and banks of resistors and capacitors with tight loops.

Fast forwarding to the present, it seems thereıs practically no hardware target some clever programmer with a hot processor canıt take a shot at. In the context of wireless communications, some people consider software radio the Holy Grail. All you need are a 2-GHz-plus ADC and DAC, a half-dozen BOPs worth of compute power, and a few megs of code. There is a bit of a problem, though. Can you imagine running something like that on a battery? I can, for a few minutes anyway.

Increasingly, power consumption is a key measure right up there along with price, performance, flexibility, and all the rest. And, not just for battery-powered applications. At the high end youıll find processors with built-in thermal monitoring that self govern their activity to prevent meltdown. Nothing like a few NOPs injected into the pipeline from time to time to cool a processorıs feverish brow.

Rabaey made his point with a whimsical example. Projecting current trends, he found that in 180 years all our wonderful computing gadgets will consume the total energy in our galaxy. And, dedicated hardware designs are much more energy efficient than general-purpose computing machines (see Figure 1). Stick that in your cup of Java and drink it.

Figure 1ı"When it comes to saving power, dedicated hardware is the call. But, flexibility demands a softer solution. The secret is choosing the mix that optimally meets the demands of your application." [1]

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