SAN FRANCISCO — The paucity of good high-level simulation and synthesis tools for analog blocks in mixed-signal designs continues to plague the legions of digital designers who want to learn a bit of analog techniques for mixed-signal systems, participants agreed at an analog design panel at the 35th Design Automation Conference.

Advances in circuit-level simulation and verification tools have been significant, but have only enlarged the disconnect between component-level design in analog blocks and the analog portion of larger mixed-signal designs, panelists said. The panel was chaired by Stephan Ohr, an editor with EE Times.

"How much analog does a digital designer need to know? Almost everyone in this room hopes to god the answer is 'very little,' " said Rob Rutenbar, head of the Center for Design Automation at Carnegie-Mellon University. "But digital designers have to appreciate that size does not matter in a mixed-signal design. Just because your design requires a tiny block of analog doesn't mean it will be any easier than a design that is half analog."

Ken Kundert, fellow at Cadence Design Systems Inc., said it is time to stop equating analog design with using Spice models, since analog hardware description languages appropriate for digital designers will eventually emerge. Kundert said that current design takes place at three levels: cells consisting of 10 to 1,000 transistors, aimed at high-precision functions like PLLs; blocks comprised of 1,000 to 100,000 transistors, using complex interactions among analog functions; and systems ranging from 100,000 to 10 million transistors, in which the analog component may be small and precision is less a concern than time-to-market pressures.

The real limitation is in linking circuit-level simulation tools used across cells or within blocks to the high-level simulators used for mixed-signal systems, Kundert said. Coupling problems are introduced in moving from high-level to low-level design, which can destroy the performance of analog blocks within large designs.

Felicia James, mixed-signal CAD manager at Texas Instruments Inc., said that mixed-signal languages may be able to ease analog block configurations for digital designers, but they are not a panacea because they cannot guarantee a correct by-construction design. James argued for a change in consciousness in which the mixed-signal portion is no longer considered as an adjunct to systems-level design, but instead is considered a superset of "Big D" (digital) system-level design.

Robert Dobkin, vice president of technology at Linear Technology Corp., said that digital designers with a minimum amount of analog expertise simply do not grasp problems that can be introduced in large mixed-signal designs, such as transient noise and substrate-coupling effects. Dobkin identified several problems involved in analog circuit simulation, including the inability to handle multiple time constraints efficiently, the necessity of taking long simulation times to achieve truly accurate simulations, and a general oversimplification of analog blocks in behavioral simulations.

Ed Liu, mixed-signal design manager at LSI Logic Corp., proposed a new concept, used in an LSI single-chip baseband processor, for a "mixed-signal subsystem." In this methodology, all analog blocks are combined with an interface to the digital subsystem, allowing the digital designers to deal with a single interface for the analog portion. In an ideal world, Liu said, the analog designers will do the analog blocks and the digital designers will learn how to use those blocks in a mixed-signal design. In a practical world, he said, digital designers will avoid learning any analog as long as possible, the analog and digital designers will continue to point fingers at each other, and an assigned designer for a mixed-signal subsystem may be the only way a company can bring the two worlds together.

Maqsoodul Mannan, president of DSM Technologies Inc., took the concept a step further by arguing that work teams need to be created that tap into the expertise of analog designers who understand the component behavior within analog blocks. Basic cross-fertilization of design methodology is a necessity in mixed-signal design, Mannan said, because "for now, I don't think there are any true mixed-signal EDA tools out there."

Rutenbar closed the discussion with a note of hope, saying that a new breed of analog tools on the horizon, pioneered by companies like the startup NeoLinear Inc. that he is working with, will provide behavioral-level advances for analog blocks and subsystems. The problem is not that further enhancements are needed for tools like Spice, Rutenbar said, but that EDA developers need to think in terms of an analog design flow, developing tools for circuit synthesis, mixed-signal synthesis, cell layout, and chip layout, that will bring the analog blocks to the level of abstraction currently enjoyed in the digital world.