The new interface combines signaling and low power technology Rambus described in a February 2007 paper with clocking techniques used in its existing XDR interface. The result is an interconnect that can deliver as much as 4.3Gbit/s per link using as little as a quarter of the power today's interfaces would require.
The approach uses a very low swing differential signaling technique that uses signals close to ground with a 100mV swing, about a tenth the swing of today's interfaces. Rambus first disclosed the technique in the 2007 paper, describing a 6Gbit/s Serdes transceiver that consumed just 2.2mW per Gbit/s in a 90nm process.
Relatively simple design
The approach uses a slightly modified version of the FlexClocking technique used on the Rambus XDR interface, implemented on the Sony Playstation3. Like XDR, the mobile interface drives most of its complexity to the controller, keeping the DRAM design relatively simple.
What's new with the mobile technique is a capability to pause the clock. That feature saves power in mobile devices that often require fast switching between low and high bandwidth memory modes. The mobile interface uses a 2GHz DDR clock and no PLL.
Rambus will show at DesignCon two 40nm test chips mounted in a package-on-package arrangement with one acting as a receiver and the other as a transmitter. The device will deliver 4.3Gbit/s at 3mW per Gbit/s.
The company is taking the technology through a 6-12 month test and qualification cycle before it releases its work to customers. It would require silicon support in both the memory controller and the DRAM.
Rambus believes today's smart phones and portable media players will scale over the next few years to processors that require more than 12GBps. A combination of applications will drive that growth including high definition video and 3D gaming.
"We're pretty comfortable with these projections," said Judy Chen, director of strategic marketing at Rambus, adding that tomorrow's smart phones will act like mobile desktops and portable projectors.
Big questions
Analysts praised Rambus for focusing its efforts on mobile systems, one of the largest and fastest growing electronics sectors, and said the technology potentially fills a gap. The industry has not sketched out a clear road map beyond today's low power DDR DRAMs although memory makers have described individual efforts, they said.
The big questions facing the initiative are whether Rambus has the financial clout and good will to drive the technology to adoption.
Rambus reported on Jan. 29 a fiscal year net loss of $195.9 million amid revenues of $142.5 million, down 20.8 percent from the prior year. The company announced plans to cut 21 percent of its staff back in August, before the full force of the current recession hit.
"We have the financial strength to continue innovating in our focus markets and vigorously pursue fair compensation for the use of our patented inventions," said Harold Hughes, CEO for Rambus in a press statement on its latest financial results.
Several top memory makers have tangled with Rambus for years in court actions over royalty suits on patents in areas such as SDRAMs. As with pervious technologies, Rambus is expected to drive adoption for its technology with systems makers, creating controller and DRAM sockets chip makers will want to fill.
The mobile memory interface is "a good move for Rambus," said analyst Bob Merritt of Convergent Semiconductors. "It's right at the heart of a major change memory companies are looking at, but it's a challenge to essentially sue every DRAM maker and then go back to them to license a new technology," he added.
"It's always been a difficult relationship between Rambus and memory vendors," said Jim McGregor, director of semiconductors and enabling technologies for the In-Stat market analysis group at Reed Business.
- Rick Merritt
EE Times
Keywords: interface memory device mobile Rambus memory
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