Slashdot Mirror
Sun Unveils Direct chip-to-chip Interconnect
mfago writes "On Tuesday September 23, Sun researchers R. Drost, R. Hopkins and I. Sutherland will present the paper "Proximity Communication" at the CICC conference in San Jose. According to an article published in the NYTimes, this breakthrough may eventually allow chips arranged in a checkerboard pattern to communicate directly with each other at over a Terabit per second using arrays of capacitively coupled transmitters and recievers located on the chip edges. Perhaps the beginning of a solution to the lag between memory and interconnect speed versus cpu frequency?"
Via Google
New Sun Microsystems Chip May Unseat the Circuit Board
You can't simply just remove the circuit board to achieve better speeds, you need to eliminate the need for the pad that converts internal logic to what we currently use externally. That is what Sun is claiming they have done.
Sun's technology is not simply soldering to pins directly together (as you suggest), which is effectively the same thing as wiring through a circuit board. The high speed, low drive strength, low-voltage drivers have to go through pads that convert the internal signal to a slower, high drive strength, high voltage driver, that will yield a reliable connection to the next chip. I'm not an expert in this area, but Physics just gets in the way. There are capacitive issues, and interconnect delay issues.
Sun is claiming to use capacitive coupling (put the pins really close together, but don't physically connect them.) This way they don't have to drive the external load of the pin/board connection, and are claiming they will be able to scale this down to a pad that will be able to switch faster than existing physical wire connected pins. Which means they believe they can make this technology work with lower drive stengths.
They still have a ways to go. Notice that the P4 has faster connections using existing techology. Sun did a proof of concept, and claim they can speed it up 100x. So they haven't _proved_ that this will operate faster yet. They still have many things to overcome to make this viable, including how to make a mass production/assembly process. It's going to be a few years. At least.
IANAEE either, but this made a little more sense to me after I read this Inforworld article, which talks about two other aspects of Sun's DARPA-funded project: clockless "asynchronous logic", and building processors with interchangeable and upgradable modules. They absolutely need these busless "proximity" interconnects for the processor modules to communicate at close to on-chip speeds, and the clockless architecture lets them get rid of the bus. Or vice versa... or something like that.
Working prototype computer about six years away, according to the article.
"This is not a sig." -- R.
Sending fast edges over a bus is difficult because the signal degrades:
If your dataset fits into the cache well, which is often the case for PCs, then a cache can fix most of your problems. If you're dealing with datasets that span gigabytes or terabytes and your application can't be subdivided such that processing and memory can be constrained per cpu then your cache doesn't assist you very much.
Chris Kuivenhoven is a thief, beware
If you look at a modern evaluation board with gigabit SERDES or SERialization-DESerialization (e.g. the 3.125Gbit/s differential signal pair per channel), the trace routes are typically rounded, with no square corners. This is done to reduce the effective impedance along the line which needs to be carefully controlled. They also run in parallel closely-routed pairs because it's typically a differential signal. Actually looks a bit like a set of minature train tracks without the railroad ties.
In fact, multichannel SERDES is the next real interconnect technology. It's used in Infiniband, HyperTransport, PCI Express, Rambus RDRAM and in 10 Gb/s Ethernet (usually as 4x3.125Gbit/s channels as a XAUI interface between optical module and switch fabric silicon with 8b/10b conversion). There are even variants, such as LSI Logic's HyperPHY, that are deployed specifically for numerous high-bandwidth chip-to-chip interconnections. The problem that is cropping up is that the traditional laminate PCBs are becoming the limiting factor in increasing per-channel connectivity, to the extent that 10Gbit/s per channel speeds are next to impossible on these boards due to the lack of signal integrity. There has been some experimentation for very short hops on regular boards, as well as using PTFE resins to manufacture the boards themselves, but it's precarious at best.
As for Sun's technology, it's interesting but I don't know how much it will catch on or how feasible it will be. It creates packaging issues and requires good thermal modelling and 3-D field modelling to account for expansion and contraction through the operating temperature range and the presence of nearby signals, which could affect the integrity of the signals.