Nice project. The 68000 is my favourite 16-bit processor. I designed and built a 68k board for myself a couple of decades ago. When you are ready to look at an operating system for your system you should have a look at CP/M 68k or OS9.

I just had to follow and like this project just because of the TRS-80 image. That was my own first computer but helped build a Imsai8080 before that :-)

Kewl (and bold) project; good luck with it!

Sound, serial, RTC, PS/2, keyboard, should be straightforward.

It will be interesting to see what you have planned for floppy/hard drive.  I myself chose to emulate and use SD storage, but that was a very different project.

You might consider adding ethernet.  The enc28j60 is a very inexpensive device.  It's SPI, but somehow I think you could even bit-bang it with the Z80.  Maybe.

I'll be interested to hear your plans for VGA.  I did a colour VGA with SPI -- it's not great resolution/colour depth, but it was uber cheap using only on-chip resources.  If you're going totally retro, then you might consider the CRT chips of the time...  Try to support 80 columns so you can support running then-cool OS's like CP/M.  Also, that will be useful if you want to port a really old BSD (pre-MMU) to your platform for a real multitasking OS.

Speaking of MMU, I'll be interested to hear how you are planning to do the bank-switching of the DRAM into the 16K space and also keep the DRAM refreshed.  Truthfully, aside from that challenge, I would suggest that ROM only be used for system bring-up, and that the core all be loaded in from flash/SD card, but that might violate some of the retro aesthetics you are going for (and which I totally appreciate).

I don't understand your recent overclocking post, though. What Z80 chip are you using?  Which manufacturer/date code? I imagine new ones might be capable of extreme overclocking simply because modern manufacturing processes are such that the older 4MHz limitation is farcical in the modern age. But what is a 'clock multiplier' in the Z80 context?  We didn't have that sort of stuff until the early 90's.

Anyway, again, best of wishes on the platform -- it will be exciting (and a little perverse) seeing an ancient processor doing modern things.  But wasn't that the whole point of something being Turing complete?

Thank you for showing interest in my project!

First off, for the hard drive I'm using the standard PATA interface using 8 bit latches to hold onto the upper and lower halves of the 16 bit data bus needed. For floppy drives I'll have to find a floppy disk controller off of eBay or something since i can't seem to find one anywhere else.

I'm also planning on adding an SPI port for serial based memory such as SD cards, usinf shift registers and other logic gates.

I have recently heard of the enc28j60 and was actually planning on putting it in the list of features, I'd love this machine to have some sort of networking. I can also use the above mentioned SPI port to interface with this chip.

For VGA i was planning on making a 2d graphics card with it's own vram and the core running on a CPLD or FPGA. Having hardware accelerated graphics would take a ton of the load off of the CPU making it able to do other things. The card will have support for any column count up to 256, although having it up that high might lower performance to a crawl depending on how well I make the core.

For the DRAM, the MMU will have a base address, and it will map 16KB of the DRAM starting at that base address into the last 16K of the Z80's address space. For refresh, the MMU will use CAS before RAS refresh on each DRAM module when the ram is idle and after every memory access. The ROM is only going to be used to start the system up and load the OS from disk, along with some motherboard specific BIOS calls and utilities such as a memory checker and BIOS flasher.

In regards to my overclocking post, in the clock system is a neat little PLL 5 called the 501MLF. It takes a clock signal, and based on a combination of selector lines which i attached to jumpers on the board, multiplies that clock from 2 to 8 times. I wanted to put a little bit of a modern touch to this machine and decided overclocking would be a nice thing to have. The Z80 is brand new from Zilog, which is probably why it overcooked so much.

I look forward to seeing this old CPU do modern things too. I find the Z80 a really fun chip to work with being that it's simple and easy to get running.

re: the enc28J60, I think it's a good choice because it will offload the byte-by-byte ethernet work, and the z80 will just have to work at the packet level.  Please do read the errata on that chip -- it's been out forever, so clearly they will never fix the silicon problems (especially regarding fifo buffer allocation). Trivial to work around (and the workarounds are in errata sheet).  For prototyping, you can get little boards from China for USD$5 or so that have the jacks, transformers, and chip in place.  I usually keep 5 or so of these on hand at any given time to internetify whatever with a spi port.  (second only to a boatload of ftdi boards).

re: floppies, firmies, and hardies:  again, if you can get a SPI port up, you'll be able to usefully emulate block storage with ease, and partition that software development task off from the hardware support.  I suspect that when you go for real hardware interfacing, that it will be nice to have a known-good upper level driver in your pocket.

re: using an FPGA for VGA and DRAM, it could be viewed as a compromise of the retrocomputing aesthetics to use that more modern technology, but I agree with your choice, and 'hey' you're doing this all for fun, right?  Get a honker of an FPGA, and eventually, you might even be able to suck the Z80 itself into it....

Cheers!

Yeah when I make my projects, I try to refrain from reprogrammable logic to cut down on costs and time, but my only other option is with a microcontroller, or design an ASIC. Using a microcontroller wouldn't be hardware acceleration but rather a software emulated GPU, and I can't afford to have an ASIC fabricated. So my only other option is FPGA.

As for the last idea, i actually thought of doing that and making a Z80 based SOC. But that's another project I'm saving for later

Interesting! Will it be able to run any version of DOS, or any small version of unix ? If you are considering using the Z80, it would be nice if there was a cartrige port that could load custom software and/or roms.

The final goal of this project is to have it running some form of multitasking OS, and yes with the ability to load ROMs. I guess i should put in the final goals of this project in the description.

Cool! Thanks!
I've been thinking about this kind of machine for quite a while.
Also it's not clear for me if you are considering the same type of machine as the Apple II, with a main-board, with expansion ports for other cards, like a soundcard, VGA, and so on... It would be incredible to have such machine with 8bit expansion ports, so one could design and build expansion cards for it, like perhaps an ethernet card, or wifi, or whatever! The "sky" is the limit, I guess.

Also if power was provided to the main system board, by a common ATX power supply, it would be cheeper to get a power supply and easy to get. And that would make it more acessible for most enthusiasts, cause the lack of AT power supply's is groing every day.

Anywho, if you need any help, specialy with software, let me know.

Yes, i was thinking of something Apple II ish, but with my own design. The entire thing will be customizeable, and the CRT display can come off too so you can make a video card that can connect to a color monitor. And about the power supply, I am making the power ATX standard so yes exactly so other people can power it easier. I have a 500 Watt power supply in my closet, even though it's a overkill for a small Z80 computer (but only a little ;) ). I am making my own PSU that's ATX compliant so i have something small to use. It's only going to be about 50 watts.

And if you'd like to program this machine, by all means go ahead! I'm going to write an emulator of the machine so you can program on that then make the ROMs when you're ready. You can request to join the project if you like since you're technically contributing to it.

Ah, I see.  Well, have fun with that and good luck! :)

Why the z80?  Zilog also makes eZ80 CPUs which are a little different, have some other features (such as 24 bit addressing mode), up to 50MHz clock, etc.

I've heard of those before, but I have an interest in vintage tech. I like the Z80, IMO it's such a nice CPU. And i don't like how the eZ80 has a bunch of peripherals built in, it takes away from the fun of building them yourself.