6502.org

You could use something like PNP transistors to Vcc, fed by a shift register which is in turn fed by the VIA's serial port to save pins (and other things could be fed too from the same port), but by then Arlet's idea definitely comes out ahead, that you use a digital pot, with serial (SPI-like) input.  Just make sure it can operate ok up to Vcc.
As Ed said, this is not really newbie stuff, and true newbies might be scared off by it.  The DOOM thing could go in "Programming," other parts in "Hardware," etc., according to their subject matter.  Give links back to this one, and put links in this one telling where you put the spin-off.  Ah, I see you already answered that part while I was writing.

I missed the links part though, I'll put them here first, then edit the threads.

DOOM
mouser
variable clock

Challenging. Very.

However Doom has been ported to a 6502 at least once in the past - the Vic-20. See here:

https://www.youtube.com/watch?v=Y7h3H-_8N_o

However if I were you, I'd go back to Plan A and go for Pacman (Actually, I'd really go to Plan @ and do Space Invaders)

-Gordon

You'll see in the new thread I started that this was mentioned, but I'd like to get closer to the actual doom, rather than this.

Back to the clock, I'm posting here since this is more about the specific implementation I'm looking at here, I'll put results in the thread I started.

this looks adequate, at 10Kohm you'd get 2Mhz, plenty slow to talk to just about anything, and at 0, you get 20Mhz. It's volatile, so no problem with power down or anything (so no need for the jumper). Of course, bootstrapping this requires SPI, so I may include a jumper for a resistor (probably 10k) to run off of until I've got SPI working

EDIT: I misspoke, this won't work, 10kohms is the minimum, you get 20Mhz from 10k according to the equation (unless you set the divider)

So, I'm worried about the size of the LTC6900, I'm not certain I can effectively solder something so small, I've included an image with dip 14 chips shown for reference.

To make soldering a little bit easier, you could make the pads a bit longer so they stick out further. That gives you more room for tip of soldering iron. Also, if you put excess solder on, the bigger pad can handle it easier without causing a short.

I may try this, although I'm still a bit worried about it, I may try and find these, or something similar, in a better package

You could order some boards with SOT23-5 shapes on them, and some cheap SOT23-5 components to practice.

Or maybe find a scrap board with a SOT23-5 on it. Take it off, and try to put it back on.

Removing SOT23-5 is easy, just put a huge blob of solder on all the pins, go back and forth with iron to keep them all hot, and give it a push.

It's amazing what you can get used to. I second Arlet's suggestion. Get some practice with this stuff, using disposable parts necessary.

Thanks for all the encouragement on soldering this. What I was thinking of doing, since I've been wanting to do this for a while anyway, is getting some solder paste and either buiding a dispenser for it, or manually applying some to the board. This is the only SMD component on the board so I probably wouldn't get a stencil just for this. I'm thinking I could use my hot air kit to melt the solder paste, rather than making up a reflow oven.

If I don't do it this way I'll try and solder it. I've not managed to find much in the way of through hole components that could replace this.

So I've got another picture for you guys of the newest layout, it's getting denser, although there's still space to use on it

Much tighter! In fact, you might wanna relax the spacing slightly in one or two spots. If you ever have to remove a chip from its socket, it helps if there's a couple of mm clearance at the end of the chip -- that's where you're gonna wanna slip the extractor tool in.

BTW, would there be any benefit in rotating these three chips 90 degrees? And: will that lowest chip collide with any part of the RAM module when it's installed? This strikes me as an excessive amount of effort for mounting just 1 or 2 SMD parts. Perhaps you suppose that soldering those parts by hand would also be a great deal of effort, but that's not been my experience.

I think when the need arises to solder SMD parts by hand arises, that's a very good time to learn how. Ditto when the need arises to use paste and a gun, or a reflow oven. But that need hasn't yet arisen, IMO.

cheers
Jeff

I actually noticed that after posting, and moved the ram modules over to the right a bit, so hopefully no issues now
So, my main thought is that I've worked with SMD before (although mostly in removing/reattaching with my hot air kit) and although I know it's possible, I've always had to go back with a solder wick to remove jumpers, and it normally takes me an hour or two to make sure it's correctly connected with no jumpers. I've also wanted to build a solder dispensing pen for a while, because I want to learn a bit about how it works. I've had this bug in my brain to have a tool head for my 3d printer that can apply paste to a board (and maybe eventually a tool head with vacuum for pick and place). So it's more an excuse to build the tool than it is a necessity.

Also, I've got a couple buddies who are interested in this project now and might buy spare boards off of me, and I may end up soldering the 6900 on each one of them to make it easier on them.

Ok, after some more revising, here it is, actually a bit of open space now, may be able to capitalize on this and reduce board size further: