While looking for more info on IBIS I came across a mention that the free demo version of Micro-Cap from Spectrum-Soft.com would convert an IBIS file to Spice. This is true! Not only did it import the IBIS definition for the Spartan-6 CCLK output, it will also run my simulation. The free demo is cripple-ware so it runs quite slowly and has many other limitations, but it does run and is sufficient to run my tiny circuit.
Assuming the simulation is reasonably accurate, it tells me is that my previous attempts at simulation were valid: with a 50 ohm microstrip transmission line of about an inch (167ps) in length, it will act as a lumped circuit and no termination is required. In fact, with Thevenin termination the signal at the FPGA pin looks worse than the unterminated circuit due to the reduced output swing and thus narrower noise margins.
I'm still going to lay out pads for the Thevenin terminating resistors just in case, but I won't plan to install them before testing. And I definitely won't include the source termination resistor.
Unfortunately the Spice library Micro-Cap generates isn't acceptable to LTspice: there are some weirdnesses in how it names nodes. I suspect I could hand-edit them to be more standard. But what I'm really more interested in is using the tables it generates to validate my IBIS-to-Spice extractor, which I'm hoping to still finish and publish.
Experiments with the world's first microprocessor
and other electronic explorations
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