January 25, 1999, Issue: 1045
Section: Semiconductors
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IBM, Reliance lead march toward Direct Rambus alternative -- Two in U.S. back 133-MHz SDRAM . . .
Anthony Cataldo
Santa Clara, Calif. - Though Intel Corp. has turned thumbs down on adding hooks to its chip sets for SDRAMs running faster than PC/100, several chip makers don't see things the same way. A small chip-set company in the microprocessor giant's backyard, Reliance Computer Corp., and IBM Corp. across the continent believe there's plenty of life left in synchronous DRAMs. Carving a path that parallels Intel's straight road to Rambus, the companies foresee using PC133-standard 133-MHz SDRAMs first, and then double-data-rate (DDR) SDRAMs.
Advocates argue that the performance gain from 133-MHz SDRAMs is significant. Where Direct Rambus DRAMs are dogged by a supply shortage (see Jan. 18, page 1), the next generation of SDRAMs will be plentiful, as DRAM vendors will find it relatively easy to tweak their process to bump up the speeds. Certain modifications need to be made to the dual in-line memory module (DIMM). But they're fairly straightforward.
Moreover, compared with the Rambus architecture, scaling to gigabytes or more of main memory is simple. Also, error correction with SDRAMs is well understood.
SDRAM at "133 MHz is the natural next-generation memory," said David Pulling, executive vice president of marketing at Reliance Computer, here, a developer of chip sets used largely in the server industry. "At the same time it's very low latency. We can do reads to memory much faster than any other memory technology. And the momentum for 133 is tremendous right now."
IBM Microelectronics (East Fishkill, N.Y.) announced last week that it would use its chip-stacking technology offer up to 256 Mbytes on a PC133 module, technology that would bridge to "a non-disruptive transition to DDR."
The IBM approach increases the processor-to-memory bus from 100 to 133 MHz and synchronizes those flows to near-DDR rates. While servers are the initial target, IBM said the technology could span the range, including low-end personal computers. Indeed, a number of Taiwanese chip-set companies said they will exploit the 133-MHz SDRAM architecture to serve low-cost systems.
IBM expects volume production to start next quarter. The modules use the company's 0.25-micron, 64-Mbit, second-generation SDRAM component.
"Our memory customers have expressed a strong interest in PC133 as an evolutionary step between PC/100 and DDR. This step ensures continued performance improvements in our memory products," said Walter Lange, memory marketing manager at IBM Microelectronics.
Pulling at Reliance said that "today, less than two months from the specification definition, Reliance has IBM's 128-Mbyte PC133-compliant DIMMs successfully running in our lab."
Will the faster SDRAMs be able to compete with the more revolutionary Rambus DRAMs? To Intel, which last year mulled the idea of supporting 133-MHz SDRAMs, moving to the faster speed grade is more trouble than it's worth. Intel advocates using the device for graphics subsystems, but not main memory. It remains a staunch supporter of Direct Rambus technology, a protocol-based DRAM that can run up to 800 MHz. The company is expected to introduce its first Direct RDRAM-compatible chip set by the second quarter.
"Building a viable 133-MHz spec for system memory is tough," said Pete MacWilliams, an Intel Fellow who heads the Santa Clara company's Memory Enabling group. "Maintaining backward compatibility to PC/100 requires a 3-ns hold time. This makes it very difficult to tighten the access time. The result is that the system timings need to be improved. Assuming the three-quarter DIMM configuration is still the design point, we believe this will require buffers, probably on the motherboard and module. That adds cost and latency," MacWilliams said.
Pulling acknowledged it is more difficult to meet the hold time with faster parts, but said Reliance managed to avoid using buffers on the motherboard. The faster SDRAM spec will need them on the module, but Reliance said that using buffered registered DIMMs is already commonplace for high-end systems.
On the supply side, the 133-MHz SDRAM will be available from many leading vendors. Among those that have announced parts are Mitsubishi Electric, LG Semicon, Hitachi and Micron. Reliance said it has worked closely with many of them to publish a spec sheet for all the DRAM vendors; it is to be updated this week. Soon, the company will submit the spec to Jedec and expects the standards body to post it on its Web site .
To DRAM vendors undergoing process-technology shifts to 0.18 micron starting this year, providing 133-MHz SDRAMs is "too easy to do," said Jim Sogas, director of DRAM marketing at Hitachi Semiconductor America Inc.(Brisbane, Calif.), which announced its 133-MHz SDRAM last year. The "133 is going to slide right in the middle in low-end PC servers and high-end desktops," he said. "It's just a tweak of the clock. The move from 100 to 133 is a very safe move." An additional benefit is that the memory-controller chip can include hooks to both 133-MHz SDRAMs and double-data-rate SDRAMs, which are also coming into volume production this year, Sogas said.
As for performance, Pulling claims 133-MHz SDRAM will hold its own against Rambus. He said the most critical parameter is how fast the MPU can fetch memory for the initial DRAM access. Pulling said 133-MHz SDRAMs can do the job 50 percent faster than Direct Rambus parts, which have a faster peak bandwidth of 1.6 Gbytes/second but a longer initial latency.
"In the Intel architecture, the key is fetching the first data word," he said. "When the MPU fetches the memory execution units are dead, so you always want to keep the processor going. In this market it's a no-no to have idle time."
Lots of bandwidth
That's not to minimize the importance of bandwidth for high-end systems that will hold up to 4 Gbytes of memory and link to hundreds of disk drives. "Rule No. 1 is to have more bandwidth than the system needs. We can do that by a factor of two," Pulling said. He added that Reliance included advanced error-correction capabilities-"chip-kill ECC," which he defined as "the next level of ECC for 1999 and beyond."
Reliance isn't the only company planning chip sets to exploit 133-MHz SDRAMs. The list includes Acer Laboratories, OPTi, Silicon Integrated Systems, Standard Micro Systems and Via Technologies. Many will be aiming at low-cost systems that in some cases will use the extra bandwidth for Unified Memory Architecture schemes. Reliance will be one of the few to focus on high-end systems.
Copyright ® 1999 CMP Media Inc.
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