BigEd wrote:
backspace119 wrote:
I know I'm acting like a kid in a candy store here, but I'm looking at the very real possiblity of having the PCB made with OSHPark, who has a min order of 3 boards, and I'd like it to be as expandable as possible for the first build, so I don't have to have more boards made (and I can potentially reuse boards for my next builds).
You'll very likely need or want to respin, which is to say run a second design, either a fixed one or an expanded one. So, I'd advise (again) against thinking of this as a one-shot experiment which needs to have all the features and all the expandability.
It might be that this advice will never be taken, but I have to try! It may be that a person has to make a first board before they realise they do want to make a second, and then later realise that this was fully expected by everyone else at the time...
You can learn a certain amount in the design phase, but you learn a lot more in the build and debug phase. Getting to that phase means making something. Only after you've gained that extra knowledge will you be well-placed to design what you really want. And it might take a few more revisions. How to get started with your first time around the loop? Make something simple!
So, I would advise you make a thick file of notes of all the advanced ideas, and then see what's really absolutely needed for a bringup of a 6502 with RAM, a ROM and a serial port.
Whatever you do, I'm sure you'll enjoy the journey. But be aware of all the people who never quite got started on the journey because the train station was so fascinating.
I do fully intend on making more. This has opened my eyes to a lot of neat stuff I can make, getting into digital circuits, and I think that's why I'm so excited about adding more stuff to it.
I'm trying to keep it simple enough, but also have enough stuff on it to keep me interested in it, whilst designing a second one. As far as a very simple bring up, I may do out a very simple computer schematic as a back up, in case I can't get this one working for whatever reason, and a simpler one could probably be made on my CNC just fine. I kind of understand what you mean though, that if I get stuck looking at an amazing looking schematic, and maybe even an amazing looking pcb layout, but never do anything with it, I really haven't learned anything about actual hardware design.
It's funny, this exact thing happens to a lot of my programming projects, I'm certain your familiar with the term "feature creep", and the usefulness of "feature freezes". At this point, I've been trying to force myself into a feature freeze so I can actually get everything worked out. The RTC is the last thing I'm going to look at adding to this, and that's only because I think the RTC should be pretty easy to add (there's even SPI ones!).
I actually do have a question though, I noticed that in Garth's simple computer schematic (which I've been using on and off as a reference) he uses shcmitt trigger nand gates, he mentions needing it for the reset circuit, but he uses them for address decoding as well. I just placed the reset circuit down in my schematic, and used the same nand gate that he's using in his, there are two problems though.
A: I've got a bunch of wasted gates on that chip, that will be left unconnected since the rest of my logic is fleshed out with other gates.
B: do I need the schmitt trigger circuits elsewhere? I seem to remember reading they're only needed for the RESET circuit....although I don't remember why (probably time for another re-read of the primer).
EDIT: after re-reading the reset circuits section I think I understand now, and I'm going to switch out my circuit for a ds1813, to save space