New Article Online - IBM Elastic I/O

Article: An Overview of High Frequency Processor-System Interconnects
By: David Wang (, January 28, 2003 1:43 am
Room: Moderated Discussions
Michael Menietti ( on 1/28/03 wrote:
>Thank you it was an enjoyable read. I'd like to read more about how some of the
>interconnect de-skewing technologies. If I'm understanding this correctly the timing
>characteristics of individual lines are dynamically detected and adjusted for by the hardware?

They're not dynamic per se. That is, the skew isn't automatically detected and ajusted for during runtime. What you can do is to detect static per bit skew by sending over predetermined patterns from the sender to the receiver. So the receiver would know what the data pattern is supposed to be, and the receiver can have relatively simple logic to detect what the range of pass/fail is.

There's always a range of pass/fail, so you can have relatively simple logic to detect something like this.

5ps fail
10ps fail
15ps pass
20ps pass
25ps pass
30ps pass
35ps fail
40ps fail

So the receiver then uses the output from either the 20ps or 25ps delay range. And the static delay is programmed in.

As the system warms up or cools down, and the environmental condition changes, you may want to resync the system just to make sure that all of the deskewing parameters are still well within range. So you can temporarily suspend the system to go through a recalibration cycle. Usually the environmental conditions won't drift too much in between the times you check, but if you're worried about the temperature changing radically, then you just have to do the resync checks quite often.

If you're interested to read more about the deskewing related material, go over IBM's presentation from Hotchips '99.

Also, Rambus uses something slightly different for their Yellowstone technology. They call it flexphase.

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