Good spot - something isn't right there. If you toggle the layer visibilities you'll see, I think, that this node corresponds to a protection structure - which would made sense if clk1out was an input.

And in fact, in the case of the 6501, this pin is an input! We believe the 6501 and 6502 differ only in the contact mask. So, with some switching of contacts, the output driver for this pad can be disconnected and the pad connected as an input instead - at which point the input protection structure makes sense.

Here's a snippet from the schematic:


The X in box annotation is, we believe, the indication of difference between 6501 and 6502. See
http://www.visual6502.org/wiki/index.ph ... atic_Notes
for more.

Cheers
Ed

Thanks, that makes a lot of sense.
I've learned more about transistor-level chip design, as well as a bit of Verilog, in a few days of playing around with your simulation and supporting tutorials and scripts, than I'd learned in four years of working at ASML

That's very nice to hear - thanks! I will pass it on to the rest of the team.