IEEE Spectrum July, 2012 - 12

12

the end of the clock network,
which are not distributed uniformly on the chip. Guthaus's
team presented such a
design approach in June
at the Design Automation
Conference in San Francisco.
Despite this potential, it's
still unclear how far resonant
clocking can be extended. Not
all chips use clock meshes,
and inductors can consume a
lot of real estate. What's more,
although a resonant clock
doesn't have to be run directly
at the resonant frequency of
the LC circuit, its efficiency
as an energy recycler goes
down when the clock is run
significantly faster or slower.
That reduces the utility of
one tried-and-true powersaving strategy-temporarily
running chips at slower speed.
"Driving far from
resonant frequency won't
save power, and at some
point [the circuit] won't work
at all," says Phillip Restle,
a member of the research
staff at the IBM Thomas
J. Watson Research Center
in Yorktown Heights, N.Y.
Cyclos has inserted a switch
that allows the AMD chip to
turn the resonant part of the
clock on and off. But Restle,
who performed some of the
earliest work on resonant
clocks in microprocessors,
says the switch isn't a perfect
fix because it adds to the
power a chip consumes.
Still, Restle says this
isn't the last we'll see of
this clocking approach.
"I consider this early days
for resonant clocking,"
he says. "There are more
rabbits to pull out of the hat."

NA * IEEE SpEctrum * july 2012

-Rachel Courtland

InternatIonal Space StatIon
392 kilometers

low earth orbIt begInS
160 km

143 km

18 May 2012

97 km

edge of Space
100 km

10 May 2012

paSSenger
aIrplaneS
9 km

16 km
2010

Beam Me Up

European scientists led by the University
of Vienna's Anton Zeilinger say they've
set a new record for the mysterious but
potentially important phenomenon of
quantum teleportation. The basis for
a communications scheme that can't
be hacked, quantum teleportation
reproduces the quantum properties of one
particle-a photon in this case-on another,
distant particle. Scientists have been in a
race to find out just how distant they can
make these particles. Progress has been
rapid, and the prize is nearly within reach:
If you can establish a quantum connection
between the ground and an orbiting
satellite, you can, in principle, establish an
impenetrable, globe-spanning network.
spectrum.ieee.org

Getty imaGes

the team had to find a way
to incorporate spiral loops
of inductors into the already
quite dense layers of interconnects that carry power
and data around the chip.
"The way the power grids
are designed, they're full of
loops," says Papaefthymiou.
"The inductors are close to
power supply lines and close
to ground lines, and the
closer they get to these lines,
the more they interfere with
them." The team found a way
to minimize interference
by making the inductors a
bit less ideal, winding them
around in a spiral but also up
and down, through the two
top metal layers of the chip.
The team's scheme for AMD's
Piledriver CPU design-
which forms the heart of
the company's new Trinity
chips-uses 92 of these
100-micrometer-wide inductors, spread out over each
dual-core processor module.
The basics of the design were
reported earlier this year at
the IEEE International Solid
State Circuits Conference.
Although AMD saw clock
power drop by a quarter, the
potential power savings
could be much greater,
says Matthew Guthaus,
an associate professor of
computer engineering at
the University of California,
Santa Cruz. His team has
shown that by customizing
the size of inductors and their
location on the chip, it may be
possible to reduce clock power
by as much as 90 percent.
Most of the savings would
come from reducing the
length of wiring between the
inductors and the flip-flops at
http://spectrum.ieee.org
Table of Contents for the Digital Edition of IEEE Spectrum July, 2012
