http://www.reed-electronics.com/ele...icleId=CA310194&text=rambus&rid=0&rme=0&cfd=1
Seems like 52GB/s is ~around~ there for 2005/6. Anyone knows how GDR3 be faring? Heard its ~around~ there too. Guys like ATi are favoring GDDR3 yeay?
XDR vs. DDR
By Alex Romanelli -- Electronic News, 7/10/2003
Rambus Inc.’s strategy for its next-generation XDR DRAM technology relies on its ability to eclipse DDR DRAM in the marketplace. The Los Altos, Calif.-based IP company has designed XDR (formerly code-named Yellowstone) from the ground up, claiming to address the shortcomings of the DDR architecture.
Rich Warmke, marketing director of Rambus’ memory interface division, sees Moore’s Law as driving on-chip integration and speeds, but interface technology is not keeping pace.
“Pin counts are continuing to go up but at a much slower pace than Moore’s Law,†he said. “As you get more and more transistors on single dies, while I/O counts are not going up quite as fast, the I/O-to-gate count ratio is plummeting. The end result is that per pin bandwidth is skyrocketing. Logic is pushing into multi-gigahertz signaling rates. So you need to go to differential signaling since single-ended signaling is not as immune to noise. Topologies also start top be more limited as you get up to higher speeds, all the high-speed logical interface links are point-to-point. We see that applying to DRAM. So XDR is going to be the first high-volume DRAM that has a differential data point-to-point interface.â€
Warmke said PC main memory is on a slower trajectory but it is also following this trend, so over time PC main memory will catch up as increasing bandwidth is required between the controller and the frame buffer. Rambus estimates that by 2005/2006, the PC will require bandwidth of 50Gbyte/sec. and by 2010 maybe 500Gbyte/sec. Customers are already demanding multi-gigahertz speeds beyond the capabilities of today’s DDR and RDAM, he said.
“There is no other DRAM that can get close to this level of performance, so we’re pretty far ahead of our competition, and we’re seeing customers wanting this bandwidth level,†Warmke said.
XDR is effectively a hybrid of DDR and Rambus DRAM, designed to combine the best elements of both.
“Over time we think that PC main memory systems are going to start requiring more and more bandwidth beyond what our DRAM is able to provide at the high-end today,†he added. “This topology has never been done before, where you have a point-to-point differential data bus and a bussed single-ended address bus. It affords the best of both the parallel approach used with DDR and the bus approach used with RDRAM.â€
XDR’s point-to-point signaling in the data bus set it apart from other memory technologies and allow it to reach speeds up to 10GHz.
XDR or DDR?
Part of XDR’s success depends on the failure of DDR technology to keep pace with industry demands. DDR and DDR2 have widespread industry support right now, from the likes of Intel Corp., but Warmke sees a time when DDR’s single-ended signaling just won’t cut it anymore.
“We do not see single ended signaling being a reliable or robust means of getting bits on or off chips much beyond 1.6GHz, and even that is a big challenge,†he said. “Even the GDDR family is going to have to go to differential signaling. With XDR we are clearly out ahead.â€
Whether or not Rambus is out ahead will be key to its success in infiltrating the main memory of mainstream PCs.
“XDR is very good technology but it has some key limitations -- basically expandability -- that will keep some people from wanting to bother with it,†said Peter Glaskowsky, editor in chief of In-Stat/MDR’s Microprocessor Report.() “ If three years from now it’s still obvious that DDR is still meeting the needs of the industry, then very few people will choose to make a huge strategic shift to XDR. And I think that is likely to be the case.â€
Just as Rambus’ XDR strategy resembles its strategy for RDRAM in the late 1990s, so does XDR’s technical advantage.
“XDR does have the same advantage that RDRAM had several years ago -- highest bandwidth per pin,†said Steve Cullen, an analyst with In-Stat/MDR (which is owned by Reed Business Information, the parent company of Electronic News.) “A few years ago, the number of DRAMs in a PC would decrease because the average memory density size was not growing as fast as DRAM densities were, and you had this granularity problem. You needed more pins but the only way to get more pins was to put in more memory than you really needed.
“If memory requirements for PCs didn’t keep up with DRAM technology, you would have this granularity problem, and that’s when Rambus makes sense. That’s why they’ve got success in consumer applications, they have the granularity problems because they only have one or two DRAMs.
“If the granularity problem comes back, if PC memory sizes stop growing at the same rate that DRAM densities are increasing, then at some point the Rambus concept would make sense. I can say with absolute certainty that’s not going to happen in the next three to four years, which means its unlikely to happen in five. But if you go out seven or eight years, it’s conceivable.â€
Building Blocks
XDR consists of three physical layer building blocks: Rambus’ FlexPhase, a controller based circuit technology; differential Rambus signaling levels (DRSL): and octal data rate signaling.
“For every pair of data to be routed at different lengths to the DRAM, FlexPhase allows a new level of freedom to the DRAM designer. As you get into multi-GHZ signaling rates you can’t guarantee that your PCB trace lengths or timing error in the system are going to be fixed. FlexPhase allows for a very precise centering of data with on-chip clock without having to trace length match signals.â€
DRSL is a very high-speed low power signaling interface, targeted at standard four layer PCBs at 50 ohms impedence. It’s a bi-directional differential technology. Octal data rate signaling allows 8 bits to be transmitted per clock cycle, which allows XDR to achieve a 256Mbit device at 3.2GHz data rates with a 400MHz clock. Warmke said the rest of the memory industry is still operating with single rate and double data rates. Rambus claims XDR is scalable to allow, in time, an 8Gbit device running at 6.4GHz.
Rambus is banking on its first mover advantage with these new memory architecture techniques.
“If the need for bandwidth continues to increase there are not going to be a whole lot of alternatives out there that do not go toward differential signaling and the point-to-point topology,†Warmke said. “Given our head start, we’re going to be an extremely compelling solution to match that need.â€
Seems like 52GB/s is ~around~ there for 2005/6. Anyone knows how GDR3 be faring? Heard its ~around~ there too. Guys like ATi are favoring GDDR3 yeay?
