[Technology Report]
Automated Analog/Mixed-Signal Design Flows Draw Closer To Reality
Discover what obstacles still lie ahead on the road to A/MS design nirvana.
Development of an automated analog/mixed-signal (A/MS) design flow has been the focus of much attention over the past few years. Primarily, this focus has centered on five main areas: A/MS standard languages, logical synthesis, analog and digital co-simulation capabilities, analog and digital block integration, and physical synthesis. While there's no disputing that great strides have been made in each of these areas, more development is needed before the industry can claim victory at the hands of a fully automated, easy-to-use A/MS design flow.
Perhaps the most impressive stumbling block of all is logical synthesis. Its absence forces designers to manually translate a design's behavioral description into a transistor-level or implementation-level representation. That process requires expert knowledge of analog design nuances, transistor-level descriptions, and the emerging A/MS extensions to the current Hardware Description Language (HDL) standards. It's time consuming and tedious work. Plus, it can be error-prone.
Finding a suitable analog synthesis solution will significantly help proliferate the use of an automated A/MS design flow by rallying the community of designers around a set of tools and a methodology that it can actually trust. This is a critical point, because as Mar Hershenson, founder and CEO of Barcelona Design, points out, "Analog synthesis is such a Holy Grail for A/MS design that prior attempts have served only to muddy the definition and feed the cynicism."
A synthesis method is required for analog circuit design that can provide real and substantial improvement over previous methods. According to Hershenson, "A tool of this nature will break the analog logjam and enable the system-on-a-chip (SoC), post-PC revolution to begin."
True analog synthesis isn't the only stumbling block facing the development of an automated A/MS design flow today. There are many. Although A/MS standards have been passed, their proliferation has been painfully slow. Development of analog Intellectual Property (IP) and A/MS model libraries is still in the early stages. Verification strategies for A/MS design have yet to be completely defined (see "Wanted: Multilevel, Mixed-Signal Design Verification!" p. 112). Additionally, acceptance of A/MS design reuse in the industry is far from secure. And, these obstacles are just for starters!
Analog synthesis poses a unique obstacle for A/MS designers. The primary reason is that analog circuit design is a very complex task performed by only a select group of expert designers working at the transistor level. Compounding the problem is the fact that analog circuit behavior depends heavily upon the second-order effects on the transistor behavior. This means a large number of nonlinear specifications must be met over all process corners and operating conditions. As a result, the analog layout designer must be careful to avoid mismatching.
In other words, analog synthesis is critically dependent on layout. Furthermore, unlike digital synthesis, it's often a task performed manually—one transistor at a time—using the same techniques that have been used for years.
As the size of SoC designs crosses the multimillion-gate mark, this manual approach breaks down, making it necessary for designers to have a way of verifying that created behavioral elements match actual silicon. Also needed is an automated means of accurately and quickly converting mixed-signal, high-level models to equivalent transistor-level representations. The obvious solution is a synthesis-like tool that's appropriate for analog design.
Previous attempts at automating analog synthesis used generic algorithms to come up with novel architectures. Others tried to mimic the tiling concept from digital design. Each has met with only limited success, in part because they fail to transfer the transistor-level circuit knowledge to the tool. As a result, designers are still required to know a great deal about analog design at the transistor level.
Recently, several companies have come forward with alternative solutions. Those solutions fall into one of two categories: general analog synthesis, or nongeneral analog synthesis. The first approach is very straightforward. The tool takes in any high-level model and spits out an equivalent transistor representation. On the other hand, the nongeneral approach attempts to automate the creation of specific functional blocks. For example, a designer who wants an op amp would simply use one of these tools to choose the desired parameters. The tool would then create the block for the designer.
Some believe that the nongeneral approach falls short because it fails to offer a direct path to implementation. Designers must still figure out the design requirement/specification to be input into the implementation tools. This requires expert knowledge of the transistor level.
Advocates for the nongeneral approach, though, counter that general-purpose analog synthesis isn't the answer. Instead, they claim that there needs to be a focus on the design of widely used cells and functions, such as op amps, comparators, and CMOS spiral inductors. From these cells, special-purpose synthesis tools for larger analog and mixed-signal functions, like PLLs or ADCs, can be utilized to exploit the scalability of interior-point optimization methods.
Barcelona Design is one of the companies presently offering a nongeneral synthesis-based method for designing A/MS circuits. The Picasso Op-Amp Designer tool is geared toward designers in need of design circuits containing op-amp functions (Fig. 1). It automatically generates an optimized netlist from user-defined specifications for more than 50 types of op amps. A list of available op amps covers a wide range of performance parameters and foundry processes. The Dali RF Passives Designer provides synthesis, optimization, and simulation capabilities for the design of passive components, like inductors and resonators. Both tools are accessed via the web through a standard web browser.
Fantastic information on the MS area, this will surely throw some light for people working in MS area. I myself being an MS design engineer found it very informative.
Srihari -November 24, 2004
it was excellent
anita -May 17, 2004
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