[SI-LIST] : Intel apparenly has found the Rambus problem on the 820 (Camino)

Per Torstein =?iso-8859-1?Q?R=F8ine?= (perr@ifi.uio.no)
15 Oct 1999 13:48:42 +0200

This may be of some interest to you guys... I just read this on

Also, this should be breaking news. Intel seems to have figured out
WTF is up with the RAMBUS problems on their boards. The ROOT cause is
as follows.
Low launch voltage is due to superposition of:
Back-2-Back Reads to Different Devices (device A & device B)
Specific Data Patterns
Position Sensitivity of EACH Device Being Accessed
Low launch voltage exceeds the specification and derates RDRAM drive
Launch Voltage is the voltage of a net when the output buffer begins
Output buffer derating combined with architectural B-2-B handoff and
reflections + crosstalk causes failure.
All elements simultaneously required to cause failure.

So it seems to be a quite complex problem involving inter-symbol
interference, switching chips that write to the bus, reflections,
crosstalk and so on.

I wonder... What should they have done differently? Separate buses for
writes and reads? Better connector/daugtherboard technology? One
possibility would be to connect the daughterboards directly in series,
instead of via the motherboard. Larger logic swing? Some kind of
switch or ring topology instead of a bus? AFAIK, the original RamLink
proposal used one-directional differential point to point connections
in a ring, but the problem of course is latency.

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