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Xilinx/Howard Johnson's crosstalk web seminar

I just watched the V4 crosstalk talk by Dr. Johnson. It was extremely informative. I've never heard him talk, before. He's really quite a good teacher. I wasn't sure what to expect, as I find his book a little jumbled. I was pleasantly surprised, however.

His description of how most of the intra-package crosstalk is due to magnetic coupling, and not merely the voltage deltas caused by di/dt through a particular L nor capacitive coupling due to dv/dt, was quite a surprise. The demonstration of the relative directions of the voltage swings (between aggressor and victim) made this very clear and convincing.

One thing I would have loved to have seen were the same tests (or simulations) with drive strengths bigger than 4mA/fast. In our designs, we typically need a mixture of LVCMOS 12mA fast and SSTL2_II, with about 200 I/O toggling together -- which makes for very noisy/jittery V2 and V2-Pros.

Anyway, good job, Xilinx.

Bob

Reply to
Bob
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Bob wrote: (snip)

Well, L di/dt is magnetic, both self inductance and mutual inductance depend on the geometry of the system.

-- glen

Reply to
glen herrmannsfeldt

Glen,

As Howard said in this presentation, inductance is not a property of the wire, it is a property of the wires in space.

Aust> Bob wrote:

Reply to
Austin Lesea

Further,

I think what Glen and I are both trying to say (and not saying it as well as Howard already did) is that when you have a magnetic field in space (as in 3 dimensions), caused by an arrangement of wires carrying a current (a completed circuit path), you are storing energy in the magnetic field.

This ability to store energu in a magnetic field is what we conveniently model (to a first order) with a parameter called "inductance."

It can be simplified to be an attribute of one of the elements in the circuit, if and only if the magentic field is completely enclosed in that one element.

Since that is case with a coil inside a magnetic core material to a first order approximation, or a wire wound coil, we are comfortable with using inductance this way.

But for a single wire in space, the concept has no meaning, until the circuit path is completed. Only then from the geometry can you solve for the magnetic field geometry, and assign (or ascribe) and "inductance" to the entire circuit path (loop).

Varying the spacing of the wire path, will vary the "inductance."

Basically, the smaller the loop (circuit path), the smaller the "inductance."

Sorry for using so many words, but I think I have said it as clearly and as exactly as I can,

Austin

Aust> Glen,

Reply to
Austin Lesea

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