Comment by kayson | Hacker News Reader

Comment by kayson | Hacker News Reader

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Very cool paper. Your observations in V.B.4 are pretty well understood in circuit design. If you've not heard of it, you might be interested in https://en.wikipedia.org/wiki/Logical_effort. Turns out the optimum scaling for propagation delay is e (natural log constant), but I don't know if I ever learned anything about the optimum for area.

Now that everyone is using finfet processes, the layout part is pretty easy to solve because transistor widths have to be a certain number of fins and the layout is extremely regular.

One thing your analysis didn't include, which actually ends up being quite significant, is the extra capacitance caused by the wires between transistors. This changes the sizing requirements substantially.

I've done some custom logic cell design, and I always had to use a lot of trial and error, though generally I was concerned more with speed than area. I'm not sure exactly what the development process is at my current employer, but it seems like its a lot of manual work. I'm guessing they set area targets based on experience and attempt to maximize speed where possible.

Ultimately, everything gets placed and routed by a computer anyways!

> Your observations in V.B.4 are pretty well understood in circuit design.

Indeed, I am actually surprised the paper doesn't include something like _"This is inline with the well known result for progressive sizing [cites textbook]"_. It was my first paper, i was worse at writing things then. :-D

> One thing your analysis didn't include, which actually ends up being quite significant, is the extra capacitance caused by the wires between transistors. This changes the sizing requirements substantially.

Good point. And not easy to model in a SPICE style simulator. I guess one could maybe introduce explict capacitors and them compute capacitiances by making some assimptions about layout.

> I guess one could maybe introduce explict capacitors and them compute capacitiances by making some assimptions about layout.

That is, in fact, exactly what we do! I think it would be pretty straight forward for your large buffer example - you can model it as a fixed capacitance at each output which corresponds to the routing between inverters, which would be the same for all sizes, plus some scaling capacitance that relates to the size of the transistor itself, which you already have.

The adder would be trickier, for sure. Regardless, in my experience, just adding a reasonable estimate is good enough to get you close in terms of sizing in schematics, then you fine tune the layout.

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