I am working on my own 8-bit computer based on the 65c02, and after about a year at the breadboard stage, I am now preparing a PCB for a more permanent design.

Basically, the architecture is based the one from Ben Eaters video series, but with a slightly more advanced address decoding scheme to accommodate a number of expansion cards/peripherals/devices. Also, there is interrupt multiplexing circuit to handle all the devices. The first device is integrated on the motherboard (a 65c22 with GPIO pins). Here is the address map:

• 0x0000-0x7fff RAM (32KiB)
• 0x8000-0x87ff Dev0-Dev7 (256 registers per device)
• 0x8800-0x8fff IrqDev (really only one register)
• 0x9000-0xffff ROM (28KiB)

Oh yeah, I am aiming for a 1MHz clock, so hopefully pretty permissive in terms of propagation delays.

I have been working on this motherboard PCB for a while, I am starting to be happy with the design (see attached pictures). Any feedback before I send it off for production would be greatly appreciated! Of course, there are endless variations in a design like this, but it would be good to avoid making any bigger mistakes that I regret later. Some particular questions:

• Does the power supply design make sense? I thought a simple 5V input would be the simplest solution, but is there any drawbacks with that?
• The reset circuit is just the simplest possible. Would I regret not making something more fancy?

Welcome! I hope (and expect) you will get the feedback you request...

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Hi Stegosaurus,

I am sorry about the colors, is it the combination of red/green that is problematic for colorblindness? This is how kicad draws stuff by default, I will see if I can tweak that.

Thanks for the heads up regarding the reset circuitry; the page you linked to explained what I need to do pretty clearly (http://wilsonminesco.com/6502primer/RSTreqs.html). I did find that there were three versions of the DS1813 (-5+, -10+, and -15+) with different threshold voltages. Does it matter which one I choose? I guess the most permissive one should be okay (-15+ with threshold 4.13V).

Oh and thanks for pointing out regarding HC vs LS chip families. I have understood that the HCT family is TTL compatible, but it is still CMOS like the HC, so should be okay, or?

Updated schematic with added ds1813, and a smaller CXO. Are these colors better?

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Is it too late to allow for +/- split supplies(maybe 12V or 15V) in addition to the 5V? The reason is you may want to create a device where you want to use op-amps or other analogue circuitry that requires split supplies and/or voltages higher than 5V.

But I do favor bussed computers. Nice!

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Bill

I wonder if you might want to include the PHI2 qualified read and write signals on that 'ctrl' bus to support non-6500 type peripheral ICs?

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

I thought about that too. In fact, I guess I already have that signal available from this part of the circuit:
(for some reason I decided to use ~WE instead of ~RW)

But I thought for simplicity that each peripheral could be responsible for computing whatever signals it needs based on the raw phi2/rwb/csb signals.

But yeah, it might make sense, but now I feel a bit too lazy to rewire to rewire the two additional signals into the bus

Interesting. It would affect quite a lot of the schematic, so it would mean quite a bit of work of layout. Would also change the PSU requirements.

What devices could need that? Stuff I am thinking about doing:

* UART (FPGA-based, already done)
* PS/2 input
* VGA graphics (using FPGA)
* Sound synthesis

Out of this, I guess the sound synthesizer could use op amps, but would I be able to do it with just 0 / 5V? Perhaps...

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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?

honestly I wouldn't bother with including more voltages. the only one i could think of that would be useful to have on hand is 3.3V, to interface things like FPGAs, SD Cards, and a Raspberry Pi (and pico) if you're into that.
If you actually want to get into sound generation it should be somewhat straight forward to just use some existing chip like a YM3812 or SN76489, which don't require any negative voltages for their Amp circuitry, so i'm sure you'll be fine with just having 5V for now.

and one question i always had whenever i saw someone post about their retro computer but never bothered to ask... why an EEPROM over a FLASH chip?
i don't really see the appeal... unless you already had some lying around, or got an old programmer that cannot handle Flash (or don't want to spend time turning an Arduino into a Flash Programmer)
Flash is much faster, comes in higher capacities (not that relevant for an 8 bit CPU, but more for something like the 65C816 or 68k), and is usually much cheaper brand new.

also in your schematic at the Interrupt logic, why not have the output of the 74HC148 go to D1-D2 instead of D0-D2? that way you save yourself a shift instruction when handling Interrupts.
a bit more detail in the primer: http://www.6502.org/mini-projects/prior ... coder.html

overall it's a pretty neat looking board and i'll hope you make lots of cards for it!
though personally i would've used KiCad's "Conn_02x<number>_Odd_Even" symbol for the expansion connectors. as they look a lot closer to the actual footprint, so it's easer to wrap your head around where each pin is gonna end up on the PCB.

I also think I will settle with 5V and Gnd. Some limitations are actually nice and inspire your creativity.

That D1-D3 idea is actually quite smart, thanks for the suggestion and the link!

Regarding flash vs EEPROM, I guess I went with what I that suited the style / era. But sure, there are probably endless improvements possible. Now that you mention it, are there any good flash chips that suit the bus of a computer like this?