Santa Cruz, Calif. — Competing current-source models may force designers and IC library vendors to juggle two incompatible formats — one backed by Cadence Design Systems Inc. and Magma Design Automation Inc., the other by Synopsys Inc. But a nascent standards effort by the Silicon Integration Initiative (Si2) is trying to head off the divide.

There's agreement on one point: At 90 nanometers and below, current-source models are needed to bring more accuracy to delay calculation. Toward that end, Cadence released its Effective Current Source Model (ECSM) in 2001. The model, recently endorsed by Magma, has been donated to Si2's Open Modeling Coalition (OMC) as the starting point for a new modeling standard.

All well and good, except for one thing: Synopsys brought out its own, Composite Current Source (CCS) model last year and extended it this month to handle noise and power as well as timing. The model is available on an open-source basis as part of the widely used Liberty library standard. Synopsys' PrimeTime, used by most chip designers for timing signoff, reads CCS models but not ECSM.

"It's a shame," said Robert Jones, senior director of Magma's silicon signoff business unit. "There's been a lot of success around ECSM, and for Synopsys to introduce a new and competing format that is really not that much different is a disservice to the industry."

Synopsys, however, contends that CCS is indeed different. Unlike ECSM, it computes current directly, and now it offers noise and power analysis, noted Rajiv Maheshwary, senior director of marketing at Synopsys.

Rahul Deokar, product-marketing director for Cadence's digital integrated-circuits group, had another bone to pick with Synopsys. "Right now, CCS is completely controlled by Synopsys," Deokar said. CCS is available to anyone on an open-source basis, but Synopsys approves and implements any changes.

"I think this is detrimental to the whole industry," said Deokar. "The best place to resolve issues is through an industry coalition that has wide industry impact."

That place, presumably, is OMC, which Si2 announced in September. The coalition's role is to define a consistent modeling and characterization environment in support of both static and dynamic library representations. "ECSM is the foundation and starting point for the static representation standard coming out of OMC," said Si2 president Steve Schulz.

Schulz noted, however, that Synopsys has donated some of the key technology behind CCS to Si2. "We would like to move forward with both [CCS and ECSM] and bring them into convergence," he said. "That decision would rest on the active participation of those who know the formats best."

Current-source models are a replacement for the nonlinear delay models used with the ".lib" library format. Those table-lookup models capture single data points for timing and represent drivers as voltage sources. A current-source model, in contrast, contains a current waveform that more closely matches the Spice behavior of a cell.

ECSM, for example, uses characterized measurements of current and voltage to produce I/V curves over multiple time intervals with different combinations of input slew and output loading capacitance. The I/V curves create an output driver model in which each driver is represented as a voltage-controlled current source.

How they differ
Backers of both CCS and ECSM say the models come to within 2 or 3 percent of Spice accuracy and are well-suited to modeling long interconnects, multiple-voltage domains, nonlinear IR drop, dynamic voltage frequency scaling and the Miller capacitance effect. Further, both Synopsys' Maheshwary and Cadence's Deokar said current-source modeling will expand to support statistical timing analysis.

ECSM, primarily developed by Simplex Solutions before Cadence acquired that company, was used by Cadence's design services group as early as 1999. Deokar said it has more than 200 tapeouts, along with widespread foundry and library vendor support. Cadence worked with Silicon Metrics, now part of Magma, to turn ECSM into an ASCII format that was implemented as a .lib extension.

CCS is much more recent, having been introduced in October 2004. Maheshwary said there is "some usage" among integrated device manufacturers and fabless companies but declined to quantify tapeouts. In June, ARM, TSMC, Virage Logic and Library Technologies announced library support for CCS.

When CCS was developed, ECSM was a proprietary modeling format. Today, ECSM's future development rests with OMC's ECSM change-management team, which has representatives from Cadence, LSI Logic Corp., Intel Corp., Magma, Freescale Semiconductor Inc. and Silicon Navigator Inc. Cadence retains the actual ownership.

One technical difference between the two is that CCS computes currents directly, while ECSM measures voltage during Spice characterization and then translates it to current. The CCS approach better serves timing, noise and power modeling, Maheshwary said.

But Deokar asserted that it's difficult to measure current directly and that doing so represents an "unstable approach."

Perhaps the main difference between the models is Synopsys' addition of noise and power modeling to CCS. "It's important to have a unified model, because we're increasingly seeing an interdependency among timing, noise and power," Maheshwary said.

Magma's Jones said that "power is being talked about" in OMC but that there appears to be no need to add noise at this point. And Deokar said Cadence has a road map to bring in current-source power and noise models and has already added some of this technology to its tools.

Incompatibility looming?
If CCS and ECSM cannot be reconciled and united into a single format, designers at 90 nm and below will likely have to juggle the two incompatible delay-modeling formats, given that the vast majority do not have a purely Cadence or purely Synopsys flow. Foundries and library vendors will likewise be forced to support two formats.

Synopsys has donated its Liberty parser, database, application programming interface and characterization environment to Si2, and CCS is available to Si2 and anyone else through open source. But Synopsys is not part of the OMC team, nor is there a CCS working group. Maheshwary wouldn't comment on whether Synopsys will join the OMC effort.

Whether or not the formats come together may ultimately be up to the users, Si2's Schulz said. "I think there's a real possibility [of convergence] if the voice of the customer continues to be heard and [is] followed up with action," he said.