News and New Products

Ciranova tool finds way around Cadence SKILL for analog design

By Michael Santarini, Senior Editor -- EDN, 11/13/2006 6:17:00 AM

While Cadence Design Systems' SKILL PCell description language remains closed to the rest of the EDA industry, Ciranova, Cadence's competitor in the analog market, has come up with a seemingly viable workaround that will allow any vendor to access PCell information via OpenAccess, an effort Cadence was instrumental in founding a few years ago.

For well over a decade, the EDA industry has been promising to offer the analog design community new, innovative tools that provide a great degree of automation—as has been done with digital-ASIC design flows. But the industry has yet to fully deliver on that promise. Most analog engineers today are using roughly the same tools they were using 15 years ago. Analog designers have been hesitant to change their tools in the absence of technologies that give them a convincing reason to do so.

One reason EDA vendors have not provided new analog-tool technologies is that the industry lacks a key open standard for custom- and analog-cell descriptions. For the last 15 years, the analog tools market has been dominated by Cadence, which owns upwards of 90% of the market in analog and full-custom design. Layout counts for a third of its revenue each quarter from this business. A key reason Cadence has been able to hold onto that share over the years is the SKILL PCell description format. SKILL is Cadence's proprietary cell description format that fab-process, library, and tool developers use to generate the PCells that make up most if not all of the layouts in custom and analog designs.

Cadence helped found and even donated the database technology to create the OpenAccess (OA) common database—which allows multiple tool vendors and customer-developed tools to work from a single industry-standard, open database. Nevertheless, the company has stopped short of opening up SKILL to OA, thus preventing competitors from developing tools that could easily compete with Cadence's analog and full-custom tools.

In the absence of a single, open, custom-cell description language that all vendors' tools can read, each IDM (integrated device manufacturer) and foundry has to write a new set of cells for every commercial design environment when they come out with a new process or change an existing one. Certainly, because of Cadence's market position, they tend to write the SKILL version first and then write Tcl, Python, Perl, or other versions as requested by customers, seemingly making it harder for customers to use non-Cadence environments at the time a new process is released. IDMs and foundries, of course, would prefer to write just one format for all tools and customers. But they typically follow the money and cater to Cadence.

"PCells are typically written in proprietary [SKILL]," says Dave Millman, vice president of marketing at Ciranova. "Because of that, tools from vendors [other than Cadence] aren't given full access to those databases. They can open the database, but what they see is most frequently described as Swiss cheese. They can see the custom layout and interconnect, but it is riddled with holes where the PCells were supposed to be."

Now, Ciranova is introducing a tool that seemingly will help various EDA vendors develop tools with a common open format, and a reasonable alternative to Cadence's SKILL.

Earlier this year, Ciranova quietly introduced a free tool called PyCell Studio. That tool, which runs on OA, allows library developers, fabs, EDA companies, and designers to create new PyCells (an alternative to PCells) using the Python language, instead of Cadence's proprietary SKILL. However, PyCell Studio doesn't provide a mechanism for integrating legacy PCells into non-Cadence layout tools.

At that point Cadence's Virtuoso wasn't compliant with OA, but in September Cadence released a new version that is OA-compliant. With Virtuoso on OA, now Ciranova is introducing a new tool, called PCell Xtreme, that essentially taps into SKILL by caching PCell information from Cadence Virtuoso and saving that information in the OA database. PCell Xtreme picks up that cached information from Virtuoso and allows any third-party, PyCell-compliant layout tools to read PCells—essentially filling in the holes in the "Swiss cheese."

"A PCell is a program, and each time the database encounters a PCell, the application says, 'Let us execute that PCell and create that layout, and I'm going to create it with the parameters shown here and give it a width of 3 and a length of 4,'" Millman says. "But the layout of the PCell only exists in memory, it never exists on the hard disk. And so when that database is closed, the memory is purged and you have no more layout left. That's the way all PCells work, including PyCells. So if a given application is not able to execute the PCells, then the PCell doesn't show up."

Ciranova's workaround? "When an application opens up a PCell and displays a layout on screen, behind the scenes PCell Xtreme creates a cache of that PCell layout in the OA database," Millmans says. "It is a universal database that anyone can read. So when another application wants to go and read that database with PCells in it, it reads the cache, and it doesn't need to interpret the PCell, and it doesn't need to handle the proprietary language."

While Ciranova doesn't flaunt this, that fact that PCell is freely available via OpenAccess means a slew of other vendors can now offer an easier migration path for custom and analog designers to try new design and layout tools and directly compete with Cadence's tools and market share. "It lowers the barriers of entry for new companies in this area," Millman says. "OpenAccess is a big part of that, and we believe we're a part of that too. OpenAccess is not complete but it certainly helps complete the grand vision of OpenAccess so that in fact independent tool developers really are independent."

The tool works with the latest and greatest version of Virtuoso, and the company has announced that the RDE Framework from Silicon Navigator, AWR's Analog Office, and the the Laker Layout System from Silicon Canvas can all now access PCells on OA with PCell Xtreme, Millman says.

If PCell Xtreme takes off, IDMs and foundries will only have to develop two versions of cell descriptions in their PDKs (process data kits): one for Cadence/SKILL and a second for Python/PyCell, The latter will be open to the rest of the industry through OA compliance. If the approach becomes popular (and because Cadence is now on OA and can essentially access PyCells), IDMs and foundries may ultimately produce custom- and analog-cell libraries in PyCell only.

Jim Solomon, chairman of Ciranova—and a founder and former CEO of Cadence responsible for introducing many of Cadence's tool in this area—says that Python is a more modern, user-friendly language than SKILL or Tcl. As such, he believes the industry would benefit from developing new processes and tools with Python, instead of SKILL. "Tcl isn't deep enough and doesn't have enough constructs," Solomon says. "SKILL is deep enough, but it's hard to use. Python is a more modern language than SKILL and Tcl and it is an open language."

Even Cadence can benefit from using Python, as it too moves more tools to OA, Solomon notes. "One of the big questions is, how do you migrate the old SKILL code to the new system [OA]," says Solomon, who is also an officer and investor in AWR. "Some of that PCell stuff can be brought over without having to worry about it, but some won't because the target system has changed. We are helping people with that migration."

Solomon is undeniably a key figure, if not the key figure, that started the commercial EDA industry. And the custom- and analog/mixed-signal tool area is one that has room for growth and provides opportunities for innovation.

PCell Xtreme is $4000 per seat on a subscription license. PyCell Studio can be downloaded for free from Ciranova.