Field-Programmable Gate Arrays
Hardware goes soft
THE power of a computer stems from the fact that its behaviour can be changed with
little more than a dose of new software. A desktop PC might, for example, be browsing
the Internet one minute, and running a spreadsheet or entering the virtual world of a
computer game the next. But the ability of a microprocessor (the chip that is at the heart
of any PC ) to handle such a variety of tasks is both a strength and a
weakness?because hardware dedicated to a particular job can do things so much
faster.
Recognising this, the designers of modern PC s often hand over such tasks as processing
3- D graphics, decoding and playing movies, and processing sound?things that could,
in theory, be done by the basic microprocessor?to specialist chips. These chips are
designed to do their particular jobs extremely fast, but they are inflexible in comparison
with a microprocessor, which does its best to be a jack-of-all-trades. So the hardware
approach is faster, but using software is more flexible.
Yet it should, in theory, be possible to have the best of both worlds. In 1960 Gerald
Estrin, a computer scientist at the University of California, Los Angeles, proposed the
idea of a ?fixed plus variable structure computer?. It would consist of a standard
processor, augmented by an array of ?reconfigurable? hardware, the behaviour of which
could be controlled by the main processor. The reconfigurable hardware could be set
up to perform a specific task, such as image-processing or pattern-matching, as quickly
as a dedicated piece of hardware. Once the task was done, the hardware could be
rejigged to do something else. The result ought to be a hybrid computer combining the
flexibility of software with the speed of hardware.
Although Dr Estrin built a demonstration machine, his idea failed to catch on. Instead,
microprocessors proved to be cheap and powerful enough to do things on their own,
without any need for reconfigurable hardware. But recently Dr Estrin?s idea has seen
something of a renaissance. The first-ever hybrid microprocessor, combining a
conventional processor with reconfigurable circuitry in a single chip, was launched last
month. Several new firms are now competing to build reconfigurable chips for use in
devices as varied as telephone exchanges, televisions and mobile telephones. And the
market for them is expected to grow rapidly. Jordan Selburn, an analyst at Gartner
Group (an American information-technology consultancy), believes that annual sales of
reconfigurable chips will increase to a value of around $50 billion in 10 years? time.
Not so hard after all
Reconfigurable hardware first became practical with the introduction a few years ago of
a device called a ?field-programmable gate array? ( FPGA ) by Xilinx, an electronics
company that is now based in San Jose, California. An FPGA is a chip consisting of a
large number of ?logic cells?. These cells, in turn, are sets of transistors wired together
to perform simple logical operations.
The precise behaviour of each cell is determined by loading a string of numbers into a
memory underneath it. The way in which the cells are interconnected is specified by
loading another set of numbers into the chip. Change the first set of numbers and you
change what the cells do. Change the second set and you change the way they are
linked up. Since even the most complex chip is, at its heart, nothing more than a bunch
of interlinked logic circuits, an FPGA can be programmed to do almost anything that a
conventional fixed piece of logic circuitry can do, just by loading the right numbers into
its memory. And by loading in a different set of numbers, it can be reconfigured in the
twinkling of an eye.
At the moment, such reconfigurable chips are used mainly as a way of conjuring up
specialist hardware in a hurry. Rather than designing and building an entirely new chip to
carry out a particular function, a circuit designer can use an FPGA instead. This speeds
up the design process enormously, because making changes becomes as simple as
downloading a new configuration into the chip.
Once the design is completed, however, FPGA s can also be used in the finished
product. This means that, when it comes to getting the product to market quickly, FPGA
s become invaluable. They can thus be found inside network routers, telecoms switches,
mobile-phone base stations and many other devices that need both the processing
speed of specialist hardware and the ability to respond to changing standards. When a
new standard is adopted, it is a simple matter to rejig the FPGA s inside these devices to
support it.
Another common use for FPGA s is as a general-purpose glue that sticks together
several different components, such as a microprocessor and its attendant chips, inside a
particular device. The flexibility of an FPGA means that it can do the work of a handful of
normal chips, thus keeping the ?chip count? down and reducing cost and complexity.
The logical conclusion of this approach, which also brings configurable logic a step
closer to Dr Estrin?s original vision, is to build a ?system on a chip? that combines
reconfigurable circuitry with a conventional processor on a single piece of silicon.
Since April just such a hybrid has been available from Triscend, a company based in
Mountain View, California. It is called the E5 , and it combines a microprocessor with
3,200 reconfigurable logic cells, a chunk of memory, and a number of other
components. While able to speed up the work of the processor, Estrin-style, the
reconfigurable logic is primarily intended to do the work that would normally be done
by external chips. At the moment, for example, set-top boxes for televisions need
separate circuits to handle cable and satellite feeds. Using the ?soft? circuitry that is
inside the E5 would allow the same design to do both. All that would need to be
changed is the set-up of the reconfigurable logic.
A similar approach has inspired another Californian company, QuickSilver Technology,
to design a hybrid reconfigurable chip specifically for use in mobile phones. The
Adaptive Wireless Engine, as it is called, is a device whose logic cells have been
specifically designed to handle signal filtering, speech encoding, wireless
communications protocols, and so on?in other words, the sorts of things that mobile
phones have to do. Tuning the design of the chip to match this specific application, but
making it reconfigurable, means that phones based around the chip will be able to adapt
to different network standards.
This has several advantages. The design of a new phone can be tweaked right up until
the last minute without changing the hardware. Phones can rejig themselves to work
with networks in different countries. They can even be reconfigured to support entirely
new protocols as and when these emerge.
QuickSilver is not alone. Morphics, a rival company based in Cupertino, California, is
pursuing a similar ?soft radio? approach. Meanwhile Chameleon Systems, yet another
Californian company, is designing reconfigurable chips specifically for use in ?high-end?
telecoms applications, such as telephone switching and routing.
All of this, however, is not quite what Dr Estrin originally had in mind. He saw
reconfigurable logic as a way of speeding up computers by enabling them to reprogram
themselves to handle different jobs. A number of research teams are now pursuing this
approach, which promises significant performance gains in fields such as cryptography
and top-of-the-range graphics processing. According to Andre Dehon, a researcher at
the University of California, Berkeley, a hybrid microprocessor that combines a
standard processor with reconfigurable logic on a single chip would be impressively
faster at such activities than a conventional microprocessor. John Hauser, a student in
his group, has developed a hypothetical chip called GARP which has 1,500
reconfigurable logical cells. His simulations showed GARP to be nine times faster than a
normal microprocessor at image processing and 24 times faster at encryption.
Dr Dehon believes that reconfigurable technology will eventually find its way into the
computing mainstream. The question, he reckons, is not whether future microprocessors
will include reconfigurable logic, but how much of their surface area they will dedicate to
it. The computers of the 1940s look clunky today in part because their ?software? was
not soft?it was hard-wired. Perhaps in another 50 years, the fixed hardware inside
today?s electronic devices will look just as old-fashioned?and hardware will have gone
soft too. |
