[Engineering Essentials]
Analog/Full-Custom Flows Move Toward Interoperability
The IPL Initiative seeks to bring openness, and with it greater productivity, to the creation of PCells and process design kits for full-custom design.
AN ANSWER APPEARS Solving the problem of making PDKs and PCells interoperable among tools and flows is about more than the language with which the PCells are programmed. It's intimately tied into design databases and the ways in which information is passed among the tools themselves.
Historically, EDA vendors have relied upon proprietary (and incompatible) database formats and application programming interfaces (APIs), which has left limited latitude for sharing data between tools. Generally, the only recourse has been the GDSII format, a relatively clumsy and inefficient solution. There's also the issue of persistence in PCells, which ties into philosophical issues of who really owns layouts (see "Persistent PCells Click With OpenAccess" at www.electronicdesign.com, Drill Deeper 16027).
An industry initiative dating back to 1999 provided a way around this conundrum. What began then as the Silicon Integration Initiative's (Si2's) Design API Coalition, and later became the OpenAccess Initiative, sought a means of breaking down the barriers that prevented true interoperability. One of the prime movers behind OpenAccess was Cadence, whose donation of an open API specification and reference database paved the way for tools from multiple vendors to be able to seamlessly operate on design data from a common repository.
Flash-forward to April 2007, when five EDA vendors began collaboration on an open-source interoperable PCell library (IPL) that supports the OpenAccess database. The IPL Initiative's five founding members were AWR, Ciranova, Silicon Canvas, Silicon Navigator, and Synopsys. Magma Design Automation and Virage Logic have since joined the effort.
All of the EDA vendors in the IPL Initiative share a common goal, which is to leverage the OpenAccess database and API as well as their respective underlying tools and technologies to develop and demonstrate PCell interoperability (Fig. 2).
If the fledgling IPL Initiative was to build an opensource, interoperable PCell library, then its first order of business was to choose an open-source language with which to construct them. That language was Python, chosen for a number of technical advantages compared to both SKILL (which wasn't an option anyway) and other interpreted languages such as C++ and TCL.
"SKILL, at this point, is an old, slow, clunky language," says George Janac of Silicon Navigator. "It works, but it is a LISP language with all the pros and cons of LISP. So it's fairly data-structure poor. Its execution is better than TCL but nowhere near the speed of Python or C++."
One of the motivators behind choosing Python is that it's tied well to C++, enabling programmers to easily move their code between the two languages. There's also a large, growing open-source community supporting the language, which is seeing more extensive use in the scientific community.
"Python is a very stable platform and a very clean language," says AWR's Duncan Widman. "It's really well structured, and the object-oriented part of it makes it easy to define classes. On top of that, it's scripted so there's no need to create binaries for every platform. It's also easier to learn than C++."
There have been intermittent calls in recent months for Cadence to donate its SKILL language to a standards organization such as Si2. Doing so would obviate the need for an IPL Initiative at all. But doing so is unlikely on a number of grounds.
For one thing, the SKILL language's powerful user-interface constructs comprise some of the core technology behind Cadence's Virtuoso platform. For another, those who have called for Cadence to open the subset of the SKILL language that's directly related to PCell functions are out of luck as well. Those functions, unfortunately, do not stand alone but depend on other elements of the language.
Cadence points to the longevity of SKILL as an attribute rather than a drawback. "We are not against an ‘open' language," says Steve Lewis, marketing director for Cadence's Virtuoso Product Group. "Our framework and our Virtuoso platform supports not just SKILL, but also C++, TCL, and Python."
Built on top of the core Python language is Ciranova's PyCell API. This API has functions that make it easy to create geometric objects. These objects can be grouped and treated as a single larger object and then cloned, flipped, rotated, and manipulated as they would be in a layout editor.
With a language and API in place, the mechanism begins taking shape. Ciranova's PyCell Studio, a complete, standalone environment for creation of PyCells, is at the center of that mechanism. The product includes a powerful layout-generation API designed for OpenAccess; a graphical layout viewer with integrated DRC capability; and an advanced integrated development environment (IDE).
The IPL Initiative has already created a proof-of-concept, open-source PCell library as the first milestone in its collaboration. This library includes the OpenAccess PCell library object code with many standard and complex functions bound to a generic 130-nm process. It also includes the source code for the PCell library as well as a copy of PyCell Studio with a generic 130-nm technology file. All of these are freely downloadable from www.iplnow.com/download.php
It's also interesting to note that IPL Initiative members have tested the proof-of-concept library with Cadence Virtuoso 6.1.
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