IDF: Mobile Rambus spec unveiled
By David Lammers
EE Times
(02/25/99, 3:58 p.m. EDT)
PALM SPRINGS, Calif. — Rambus Inc. announced a mobile version of its technology at the Intel Developers Forum
this week, and Intel executives said they expect the Rambus DRAMs to show up in notebook computers in mid-2000.
The Rambus DRAMs operate at higher frequencies, with smaller voltage swings, than any other main memory used to
date; the Direct Rambus parts oscillate at up to 400 MHz and read data from the rising and falling edges of the clock,
sending data at high speeds over a relatively narrow, 16-bit "channel."
In other memory architectures with by-16 configured DRAMs, four devices or more may be turned "on" to feed a
64-bit bus to the processor; the Rambus technology differs in that in some cases the data is localized on one device
which feeds the processor.
For mobile applications, that is both good and bad. Hot spots can develop when only one or two RDRAMs are
operating at high frequency. But with fewer chips turned on, overall power consumption can be less than with
SDRAMs, according to Rambus.
Operating at 2.5 V, the Direct RDRAMs will operate in four modes: active, standby, nap and idle. If four 128-Mbit
devices are on the Rambus channel, one device could be active, consuming 300 milliwatts of power, while three others
could be in nap mode, consuming 11 mW each. Overall, both Intel and Rambus argue that the Rambus technology
will be less power hungry that the SDRAMs.
While the RDRAMs are on modules that have an anodized aluminum heat spreader across its length, notebook OEMs
will need to exercise particular caution about hot spots across the motherboard, according to Intel.
Turning the delay lock loop (DLL) on and off quickly also has been an issue for mobile applications; in the current
design the DLL will remain on during nap and standby modes. Moving from nap mode to standby will take 50 ns;
from standby to nap will take 20 ns.
Subodh Toprani, vice president of logic products at Rambus (Mountain View, Calif.), said the memory controller
would be able to create a "nap pool," keeping certain devices in nap mode while others are active. "In some cases, you
only have to activate a single device to do a transaction," Toprani said. Future notebooks would be able to switch
between tethered, battery and other power-saving modes.
Notebook computers will use Rambus modules that are the same size as the small-outline dual in-line memory
modules (DIMMs). Notebooks could hold one to three Rambus in-line memory modules (RIMMs), with up to eight
devices on each SO-RIMM. Using the 128-Mbit density, which will be the dominant Rambus memory by mid-2000,
each notebook could have 384 Mbytes of main memory.
At the Intel forum, Toshiba Corp. and Kingston Technology Co. demonstrated prototypes of the SO-RIMM module,
populated with 64-Mbit DRAMs.
"The power consumption of the Rambus SO-RIMM module specification is well within the 1.3 watts specified by
Intel's Mobile Guidelines 2000," said Allen Roberts, vice president and general manager of Rambus' memory division.
"Toshiba's current RDRAM is already very close to meeting the low-power bin specs required, well ahead of
development schedules needed to provide samples to PC OEMs later this year."
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