Hope dims for power spec merger

Two rival specification formats for low-power IC design are now publicly available, and backers of both agree that it would be technically feasible to converge them into a single standard. But disagreements over how that convergence should take place threaten to block further progress.

The Unified Power Format (UPF), backed by Synopsys Inc., Mentor Graphics Corp. and Magma Design Automation Inc., was released as an Accellera standard in late February. The Common Power Format (CPF), developed by Cadence Design Systems Inc. and managed by the Silicon Integration Initiative's (Si2's) Lower Power Coalition (LPC), became publicly available in March. Both UPF and CPF allow users to specify power intent and constraints throughout the RTL-to-GDSII design flow and use very similar approaches to do so.

UPF backers want convergence to take place within the newly formed IEEE low-power study group, which has submitted a project authorization request (PAR) to become an official IEEE working group. The study group has requested copyrights for both UPF and CPF so that it can converge the formats. Accellera will provide rights to UPF, but IEEE working-group members and supporters say they can't even determine who holds the copyrights for CPF, much less obtain them.

Cadence wants convergence to take place within the LPC comparison work group, which manages CPF under the Si2 umbrella. That user-driven body has already started a technical comparison of the two formats. Cadence and Si2 both argue that Si2's intellectual-property (IP) protection is better than that available through IEEE's study group.

Cadence views the IEEE group as little more than a UPF standardization effort driven by rival EDA vendors, and sees it as an inappropriate venue for convergence.

Silver lining
If there's a silver lining, it's that there's some overlap in membership between the IEEE low-power study group and the LPC comparison work group. Gary Delp, distinguished engineer at LSI Logic Corp. and chair of the LPC comparison work group, is also listed as vice chair of the IEEE low-power working group on the PAR that's been filed. And Delp is passionately committed to standards convergence, regardless of where it takes place.

Both UPF and CPF use Tcl side files with commands that allow users to do such things as establish and manage separate power domains, specify isolation and retention, set up level shifters and define power-related rules and constraints. While today's tools use a hodgepodge of power formats and specifications, both UPF and CPF propose a common way of expressing power throughout the design flow.

Power sources lead flow. CPF and UPF convey power intent.

Many UPF and CPF commands are similar, if not identical. The difference is in the heritage. CPF arose with Cadence's Power Forward Initiative, which Cadence's competitors believed was not sufficiently open and inclusive. So Synopsys, Mentor Graphics and Magma threw their weight behind a rival effort that became the Accellera UPF initiative. UPF combined a number of proprietary formats and ended up closely resembling CPF.

"People are saying 80 percent, but I'd guess it's closer to 90 percent," said Yatin Trivedi, director of industry programs at Magma. In some cases, he noted, what takes two commands in one format might take one command in the other, or the options may be different, but "they're more or less talking about similar things."

Feasible proposition
All sides agree that it's technically feasible to combine the two formats. Stephen Bailey, chair of the IEEE low-power study group and product-marketing manager for functional verification at Mentor Graphics, noted that the Accellera UPF effort—which he headed—successfully merged six or seven different formats. "In this case, it's two different formats, so it should be easier," Bailey said.

It was always Accellera's intent to take the UPF to the IEEE for standardization, and that's the original motivation for the IEEE low-power study group. But the group decided to put together a PAR that's broad enough so that it can converge UPF and CPF, if copyrights for both are assigned to the working group.

There won't be a problem with UPF, said Accellera chairman Shrenik Mehta. CPF is another story.

"We've asked who owns the copyrights, but we don't have an answer," said Bailey. "No one at Cadence seems to know who owns the copyrights for CPF. Because of the ambiguity, we sent request letters to both Cadence and Si2, but we don't have a response."

Bailey said that the IEEE process is open to everyone, while only LPC members can participate in the LPC comparison work group. Synopsys and Mentor Graphics have declined to join the LPC, and Magma recently left it. Magma sees no value in serving on the LPC, and believes the IEEE is the right place for convergence, Trivedi said.

"Most agree that there is now only one place where the convergence can legitimately happen—in the IEEE, with its proven and venerated capabilities in this domain," said Rich Goldman, VP for strategic market development at Synopsys. "Neither Accellera nor Si2 can be considered suitable locations for this convergence because of their strong advocacy of just one of the existing formats."

CPU was designed with Common Power Format. The ALU can be turned off when not in use to conserve power.

'IEEE is flawed'
Not everyone agrees with that assessment. The current IEEE study group is not a suitable location for convergence, according to Cadence. "The setup is flawed, and the big issue is IP," said Mohit Bhatnagaar, group director of product marketing at Cadence. Si2 has a solid IP protection policy, he said, whereas the IEEE study group "has no IP policy."

CPF was contributed to Si2 under a "reasonable and nondiscriminatory" (Rand) reciprocal licensing policy, said Si2 president Steve Schulz. Anyone who adopts CPF through that license grants a Rand reciprocal license back to Cadence, and can't sue Cadence over the CPF contribution, he said. "The IEEE does not have that license mechanism in place today," Schulz said.

As for the question of "who owns CPF," there's no one simple answer. Schulz said Si2 holds the rights to create any derivative works for a standard and holds the copyright to the CPF derivative version released in March, but Cadence continues to hold copyrights on the original CPF 1.0 document that it contributed. The Si2 could not release rights to CPF to the proposed IEEE working group without the consent of all 17 LPC member companies, Schulz said. Cadence is one of those companies.

The LPC comparison work group can carry out the convergence even without the UPF copyrights, Schulz said. "They have access to both UPF and CPF, and the ability to combine them, and it really does not need to be a long-term effort," he said. Schulz hopes convergence will occur by the time the Design Automation Conference is held in June.

- Richard Goering
EE Times