Analog and digital designers have lived in separate worlds for a long, long time. They use different methodologies and tools, and while digital design is heavily automated, analog design is not. But mixed-signal integration will force this gap to narrow, opening the door to new methodologies and better collaboration, according to panelists at the DesignCon conference Jan. 31.
The panel was provocatively titled "Is it Time for an Analog Comeback?" and was moderated by Brian Bailey (right), contributing editor at EDN. Panelists were as follows:
Bailey opened the discussion by describing the status quo. "In the past analog and digital were really kept separate," he said. "It really was an oil and water situation." There are many differences between the two worlds - digital design is top-down, analog is bottom-up; digital is automated, analog is primarily manual. "There are so many areas of conflict between [analog and digital] that they are almost antagonistic to each other, especially when put in the proximity of a chip," Bailey said.
He noted, however, that 70% of IC designs today are mixed-signal, and that figure is increasing. "We have to start thinking about how we play together as a team rather than as pieces that are forced to come together," Bailey said. And that challenge set the stage for the following discussion.
Why Hasn't Analog Kept Up?
In his opening remarks, Miller (Tanner EDA) asked, "why hasn't analog design kept pace? The main thing is that the process for analog design is totally different from digital, and there has been no significant automation such as the place and route technology that has revolutionized digital." Miller said, however, that he sees a "bright future" for mixed-signal design in two respects - integration into large SoCs, and the addition of digital content to traditional "analog only" chips.
Nandra (Synopsys) spoke of some of the challenges that analog IP developers are encountering, such as the need to integrate analog components onto chips at advanced process nodes. For example, restricted design rules at these nodes limit the way full-custom devices can be placed and routed. He also noted that some system specs don't change with scaling - USB still requires a 5V signal to charge the battery, a challenge for advanced-node SoCs.
Nizic (Cadence) observed that some design teams are seeking to "economically" design mixed-signal SoCs at advanced process nodes, while others are working to bring more digital functionality, such as microcontrollers, into "mainstream" analog ICs at mature process nodes. Previously, he noted, analog circuitry might have taken 15-20% of the area of an SoC; today, at advanced nodes, it's likely to be 50%. And most of those SoCs use low-power design techniques, making things even more challenging.
"Many different skills are required to realize a design in silicon, and it is important that people with these skills work as a team," Nizic said. "Does it mean everybody has to become a mixed-signal designer? Certainly not." He noted that "we are striving to bring more unified technologies and highly integrated tools to enable that cooperation."
While other panelists talked about increasing mixed-signal integration, Savage (IPextreme) observed that many analog functions are still implemented off-chip, and he said he's seeing a trend towards "separation" of analog and digital. "Ten years ago we were talking about integrating everything into an SoC. That's true to an extent but I'm also seeing companies that are re-segregating." Savage said that 3D-ICs will offer the "best of both worlds" by integrating digital SoCs with optimized analog dies.
Following are answers to several questions posed to panelists.
Q: Do We Have Enough Analog Designers?
Miller: "There is great difficulty in training analog designers. It's not like digital where you learn the tools and you're sort of set. There's a lot of art in creating an analog cell."
Nizic: "We do have to nurture that [analog] skill. It does require more understanding of device physics to design good circuits. We need tools to make it easier. Reuse is very important as well."
Nandra: (Describing the design of PHY IP in 50 different process nodes) "It doesn't require genius engineers. What it requires is more like a factory to be able to stick to the same specs, but to design that [PHY] in different process nodes."
Q: Is There Any Hope for Analog Automation?
Nandra: "When building baseline transistor cells, we hide a lot of the process effects from engineers...In advanced nodes, we get a lot of EM problems because metal lines are thinner. Somehow we have to figure out how much current goes through these metal lines. That's a technique that can be automated."
Miller: "Selling automation to an analog design team is very difficult. You really need to give them control and try not to take too many pieces away."
Nizic: "Traditionally we assumed that analog is bottom up and digital is top down, but we are looking at mixed-signal systems now, so we really have to combine these approaches...We do have a lot of automation in our tools."
Savage: "Analog is an old school discipline...it is very hard for EDA companies to make tools that replicate the expert engineer, like we did 15 years ago with digital synthesis."
Q: In the Digital World We Talk about Verification Taking 70% of the Effort - Is This True in Analog?
Nandra: "It's not far off in that perspective, but the color of verification is different...our customers want to see the silicon characterization."
Nizic: "We need to distinguish functional verification from signoff verification. We need to invest more in analog/mixed-signal functional verification. I see more and more adoption of digital techniques like coverage driven verification and random stimulus. This combination is really needed."
Conclusion: Is it Time for an Analog Comeback?
Nandra: "Analog never disappeared. I disagree with Warren's comment about the ‘old school of analog.' I think there is a very exciting new school of analog that's a lot more interesting than some innovations in digital."
Joseph: "We're starting to see SoCs address some issues, but we've never seen analog go away."
Savage: "Analog never really went away. One big change today compared to when I was a younger engineer is that analog is sexy now."
Miller: "There have been historical shifts. I do think the pendulum will swing back a bit and we'll try to do things in analog if at all possible."
Nizic: "Analog never really went away. SoCs have more and more mixed signal content. This is a window of opportunity for better methodologies."
Bailey: "I conclude that analog is a place for renewed innovation and renewed differentiation, and it's sexy again."
Nice discussion! Being the bottleneck of most systems and leading the way to digital circuitry, analog has always been more fun and challenging. Coming to the sexy part, I wish I was a designer during late 80s to early 2000s, when all equations were matching. Although the current density (delta_Vth) and PVT (delta_U0) variations at advanced nodes make analog design semi-sexy nowadays, I still love it!