Thank you for the design tips, Garth. I'll remember those. I was also planning to use DirtyPCBs in the first place, them being the cheapest mob I'd heard of. Also thank you for the compliment, that's really encouraging.

I'm planning for this to be the prototype of the handheld unit, as the current one(wires under a pad-per-hole protoboard) seems to be misbehaving. This one uses thru-hole parts, and in the final design, I'm planning to go for SMD components for non-I/O devices, socketed PLCCs for I/O chips, and the ROM will also be socketed, so I can change it out if I want.

Now that I think about it, I might be going down this path prematurely.
I'll want to add more chips and change a few things around later, particularly the address decoding, which will require more address lines for what I have in mind.
Trouble is, I don't want to build another board like this. It was a pain putting the current version of this thing together, and I don't think this board will stand up to another rework, as I damaged it slightly getting the sockets out of it the last time. It might be a better idea to make a modular thing, maybe based on the RC2014.

I'm not sure whether I should press on down this path, or change tack and only do a full board when I've got the design more or less finalised.
What do you recommend?

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I'm inclined to either go modular or bring out enough of the buses to plug in a daughterboard (D0-7, A0-5, R/W, and a chip select, for a SID clone). I did read your primer on doing just that, and I remember you saying that's a terrible idea.
I also remembered that there are no bypass capacitors on that board...

I'm more inclined to go with the daughterboard connector. I'll see if I can do that reasonably close to the CPU.

In any case, I need a few SPI-10 modules that I can use thru-hole for. I've designed one, and I'll get that made, once I figure out how to panelize it.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Thanks for that critique. I'll see about correcting those issues you raised.

The SPI modules I have in mind are like the little storage module you designed, except for 5v DIP ICs. I went and bought some 25AA512s in DIP-8. I think I did that before I'd seen your board, or I would have bought them in SO-8.

Actually... I've just realised, that since the Propeller(which I'd be using for the SID emulation) can't keep up with an 8-10 MHz 6502 bus, I shouldn't bother putting it on the main bus in the first instance. I should probably use SPI for that, and save a few chips, since I'm gong to have to roll an interface anyway. I was originally thinking of using 74-series logic to allow the prop to run asynchronously, which is comparatively easy, because the SIDcog doesn't implement the readable registers.

Now that I've tumbled to this, I feel simultaneously stupid and brilliant at the same time. Thank you for putting me on the path to this, Garth.

I've implemented Garth's advice at this point.
The board is 10.7x12.9cm, and I'm not sure how much I can shrink it(or that I really want to try). I ran it past OSHpark's online tool to get their price(and highlight some errors/problems), and they're far more expensive(around US$270), so I'm almost certainly going to DirtyPCBs for this.

I've attached the gerbers, as I intend to send them off, in case anyone's interested, although I'm not going to pull the trigger on this just yet, partly due to DirtyPCBs' quote tool telling me I've got invalid board settings whenever I try to add it to the cart.

It looks like I'm going to have to shrink the board down if I want to use DirtyPCBs. As soon as I pick a custom size and four layers, it goes crazy and doesn't let me submit it. If I can get it to 10x10cm, I can get it for US$45(~AU$60), though. Which is really good value for around 10 boards.

I've completely re-done the board, so that it fits in the 10x10 limits. I hope it's not too horrible on the board house, when I do send it off.

Sorry to dig up an older post. But I plan on using a Propeller for video (and maybe serial) on my current design. I'm certainly going against the grain here and doing a backplane design.

However, the reason I am doing that is because I don't want to design a 8, 14, 20 MHz computer...I want to *purposely* design a 1 MHz computer along the lines of an Apple II.

Anyway, did you ever finalize how you're going to connect the Propeller to your design? Running my design at 3V3 isn't an option because I want to use some legacy components that only run at 5V. So I was thinking about using a level shifter in between. But this is another problem for me because I'd like to use the Propeller as general purpose video RAM but I don't think it can handle even the 1 MHz bus when there is a level shifter in it.

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Cat; the other white meat.

I've already got a little 4-channel two-way level shifter module from adafruit. It's one of those transistor-based modules, so I'm not entirely sure they'll be all that fast.
I imagine I could simply use resistors on the input lines(I've seen schematics that do just that), but I'll try the proper level converter first, since I have it.

How I connect the prop isn't fully set in stone yet. I'd like to have it on the board, but connected to the 65SPI. That's what I'm going for, at least. But until I do that, I'll use SPI-10 to connect it. Initially, I'll use a 5v port, and convert the levels on the prop side of that. I can get away with this because I have what they call a FLiP module, which includes 3.3V regulation on the board.

Oh yeah...I've wanted to get one of those FLiP modules. I might be able to use resistors too. I'm thinking of having the CPLD poke the Propeller to let it know the CPU is writing to video RAM. Then, the Propeller just snags the data bus through resistors. The Propeller would never be an output to the CPU. More like a shadow copy of the video RAM and if the CPU needed to read from it then it should be fine. Since I only intend to run at 1 MHz, I'm hoping that will work. I hate using level shifters unless I have to. Why can't the world just go back to 5V? LOL

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Cat; the other white meat.

It almost feels like we need a sticky thread on 3.3V interoperation. There are very fast bidirectional level shifters available (see OHO's products) and there's at least one 74 series family which can be used to bridge the gap. In a case like this, if the 6502 writes to an octal latch then the Prop will have plenty of time to read the value. Another approach we see is using a CPLD for combination glue and level shifting.

(Edit: seeking input for a possible sticky thread over here)

I agree on the sticky thread.

I don't want to go OT and hijack this thread but how would the latch work? I mean, I know that a latch can act as a "one byte" memory IC but if the Propeller is really slow at grabbing it off the latch, wouldn't the next CPU write cause a problem when it tried to write to the latch again? How does the Propeller get more time is what I don't understand.

(forgive my ignorance)

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Cat; the other white meat.

You'd make the latch work like an 8 bit output port. I'm sure that's been discussed here before... but I'm not in a position to dig into a search right now. The latch would be loaded by a decoded address, and therefore not reloaded until the next intentional write by the CPU. You'd also need some kind of handshake with the Prop, so it knows there's a new value, and doesn't read the same value twice. So that's a ninth bit of state - a flip flop of some kind, I guess.