SAN FRANCISCO--There were times-simpler times-when electronics designers toiled in distinct walled gardens: chip, package, and board. But
today, interdependencies among those three require those walls to be broken
down-not the least of which because blazingly fast speeds are causing havoc
with signal and power integrity. These are challenges that require an
examination of where gaps might be in the methodologies, an honest conversation
about codesign. and a reconsideration of how we structure engineering teams.
That was the take-away from a Cadence-sponsored panel of
experts at the 51st Design Automation Conference here. Titled "High-Speed Cross-Fabric Interface Design: It's not 1's
and 0's anymore... it's a noisy world,"
the panel featured:
Jiang teed up the issues eloquently:
challenge is basically that we have to deal with three things at the same time:
signal bandwidth, channel density, and the tradeoff of product performance to
cost. We don't want to overdesign our products. Packages and boards are no
longer conductors; they act like transmission lines with propagation properties
that contribute to distortion. When we connect I/O through the package, it's a
totally different story. And when we cascade to the board, it's a different
Weed, a longtime industry veteran, added:
worked today may not work tomorrow. We are seeing speeds in excess of 28Gbps
and we are now having routine discussions about the need for 56Gbps. In this
environment, codesign early and codesign often and partnering early are the
keys to success."
Over the course of 90 minutes, the panel addressed a broad
array of challenges. Here are some of the highlights:
Digital has become
analog: How do we deal with this?
Brim said one area where he's seeing that metamorphosis is in
high-speed serial interconnect.
"I used to think 40GHz was a big deal. Today, hi-speed
interconnect may start out as zeroes and ones but it certainly doesn't end up
that way," Brim said.
To address these challenges, some teams use techniques like
pre-emphasis and DFE to correct signals "but they're never zeroes and ones," he
said. "If you don't have all the DSP processing before the signal and after the
signal, it's just noise."
He then described power delivery challenges:
you talk about power, now you have power, noise, and jitter. You have noise in
your power delivery network and you don't know where it's coming from; then you
have additional jitter in your hardware you have to deal with."
Brim noted that some teams are "undeconsidering" power
integrity in high-speed serial with the additional jitter that it causes.
causes signal integrity issues because your signal is always referenced to VSS or
ground, and if you've got noise in power delivery network that directly
translates into noise in your signals. Vias are an area where there's an
extremely strong coupling between signals and power."
have to take that eye, that wave, that unknown, that gray space between ones
and zeroes and convert it back to ones and zeroes to do digital processing that
we have to do. That's the goal at the end of the day. All this has to be done
with the challenge of time to market and cost and performance tradeoffs and
hitting the application space before your competitor does."
As teams grapple with complexity, the need for co-design
becomes greater, to intelligently and productively deal with technical issues
but also to get to market sooner.
"I'm quite passionate about co-design. In
today's environment you cannot isolate silicon design, package design, and
platform design. So the silicon platform co-design is something people are
aware of. We need to look at the platform and how it affects silicon design. On
other hand, more importantly, it is electrical validation and debug. SoC and
system debug without knowledge about the silicon design cannot be effective.
You have to look into the whole thing together in your electrical
Acknowledging that he sometimes gets the impression that
people think he just wants to sell them three sets of tools, Brim said:
I were to stand up and tell you "do codesign or you can't get your job done",
that's not right either ... because you have different analysis technologies
that work between package, board versus IC-type extractions. So you have to
design individual fabrics in different tools, but you have to design across
multiple fabrics at the same time."
An audience member asked the panelists to identify gaps in
the current methodology. Jiang said the industry needs better metrology and
sophisticated 3D extraction tools because some existing methodologies are too
time consuming or don't offer sufficient accuracy.
These challenges also require a reconsideration of design
team organization, especially if codesign is the objective.
Qi said he used to manage the signal integrity team. "There
was traditionally no PI; just SI," he said. "What I try to emphasize is
integrating these." And in optimal situations such structures would include at
least one engineer who has worked in both PI and SI to help bridge the
discipline divide, he added.
Looking forward as speeds increase to 112Gbps, a different
way of thinking will need to emerge again, according to Reddy. At that point,
he said, the industry may need to look at different signaling schemes, such as
the quadrature-amplitude modulation (QAM).
"I want to see codesign start to include them, and even
optical design," said Reddy.
Planning for Smarter Design: Q&A with Kevin Rinebold