Visualizing the 6502

[BigEd]

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:

Excerpt from 6502 schematic showing Phi1 pin direction can be configured as input for 6501 product.
Credit: “MOS Technology, 1974-75, 6502 rev C. For educational use only.”

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

[wumpus]

BigEd wrote:

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.

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

[BigEd]

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