Comment by junon | Hacker News Reader

Comment by junon | Hacker News Reader

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I'm not an expert but a hobbyist. Take what I say with a grain of salt.

> Could the speed be improved by using MOSFETs with better capacitance?

Not using MOSFETs would make it faster, in theory. MOSFETs have a delay time and are considered slow. But it appears they want to be true to how the original was made. There definitely exist mosfets indistinguishable from dust - I made the mistake of designing with a few at one point. So probably, yes.

Another thing I noticed is that the distance between the components on the board is quite high. They might have done that to replicate the original layout, or maybe I'm just not seeing it correctly. But everything - including the traces on the board - has some capacitance.

> Can the full 1.023 Mhz be attained by throwing money at it or are there physical limitations at that scale?

Hard to answer without measuring, though I'd imagine it's possible. 1MHz isn't that fast, but if you have high capacitances or they don't switch fast enough (e.g. MOSFETs) then your signals are going to get muddy.

EDIT: to be clear, not ragging on the project. This stuff is beyond cool.

This is basically a second generation computer, whose heyday was in the first half of the 1960s – after vacuum tubes, but before the first integrated circuits.

It makes me wonder, what transistor family would give you the best performance for this (very unusual nowadays) use case? Obviously not MOSFETs.

From what I understand, germanium transistors were most common for 2nd generation computers, because (at the time) silicon transistor technology wasn't sufficiently mature to be used for that application.

Maybe try ECL?

ECL would definitely be faster. Switching time is around 1nS, which is a solid order of magnitude faster than NMOS.

But it would need something like 2 to 4 times the surface area, and would make a nice room heater.

The real question is why would you. This is an educational project that visually demonstrates how a CPU operates. It could clock at 0.5 Hz for that purpose.

Why do anything?

If it ran at full speed, you could run tons of original 6502 software, which almost universally rely on cycle-accurate timing to generate video output.

> Not using MOSFETs would make it faster, in theory.

My understanding is that the MOS 6502 was MOS, so MOSFET. Not using MOSFETs would make it less of a replica.

Implementing the replica in an integrated circuit instead of discrete transistors would lower capacitance.

The delay time is a consequence of the capacitance.

I'm aware of all of this (I even mentioned the MOSFET vs not in my comment), I was just answering the OPs questions.

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