Minimig Discussion Forum

Discussing the Open Source FPGA Amiga Project
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 Post subject: A quick rundown
PostPosted: Wed May 17, 2017 10:35 pm 
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Joined: Wed May 17, 2017 3:53 am
Posts: 1
Hey all,

It seems I'm a bit late to the party. I see there's not much activity on the forum right now, but I'll give it a shot here anyway.

So Minimig project is pretty old now, and I guess quite mature at this point. It's at least 10 years old now, and over the past 10 years a lot of people tinkered with it and created all kinds of modifications and variations, I've been Googling for the past couple days and found a lot of old posts, and many broken links. So I need a little bit of help getting a clear picture of current Minimig landscape of versions and forks.

What's the current maturity state of ECS implementation? AGA? What are the best available 68k cores right now?

I don't own a Minimig board yet, but I would like to take a shot at building one using evaluation boards. I have an old Digilent Nexys2 board with Spartan 3E 1200K gate FPGA on it. My understanding is that original Minimig was also based on Spartan 3, a 400K gate part. The FPGA and board is somewhat similar in capability to Terasic DE1 board.

Someone else previously inquired about porting to Nexys2 board, but never reported back with results. That was 7 years ago.
http://www.minimig.net/viewtopic.php?t=303

Someone else discussed the board and the PSRAM on it. I've done a few video and memory projects and had success with it. The 16Mbyte PSRAM chip works well as a 70ns async SRAM. The 16Mbyte onboard flash also works well and has 75ns access time. PSRAM and flash share the 16bit data bus and share the address bus, so FPGA can enable and access them only one at a time.

If I were to use this for Amiga, would this mean that I would have chip RAM, slow RAM, and no fast RAM? How would this memory layout work?
Onboard flash suitable for Kickstart?

I would like to take the "softcore everything" approach, and not use a PIC or ARM. Which fork/release is best for starting for a softcore approach?

I have a Gotek floppy emulator on hand that works with my A500, would this simplify initial implementation if I don't have to emulate the floppy on the board?

Question about the original Minimig board, it used a 68EC000 manufactured by Freescale, a 20MHz part I believe. What was the actual frequency it operated at in the Minimig circuit?

Any insight, suggestions and recommendations are welcome.

PS What's the most active forum where Minimig and derivatives are discussed?


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 Post subject: Re: A quick rundown
PostPosted: Tue May 23, 2017 7:47 am 
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Joined: Tue Nov 09, 2010 3:10 pm
Posts: 313
Hi,

There are a couple of variants of FPGA boards running minimig, probably the three most prominent of them are the original Minimig board, the MiST board and TurboChameleon64. Besides those, you have the FPGAArcade Replay board, which runs a variant of an updated minimig core, but their sources are unfortunately not available. Plus, a bunch of minimig variants on various development boards, like the Terasic DE1 board etc.

Unfortunately, there is no central repository for all these minimig versions. What's more, they all differ in some details, so you can't really take code from one of them and hope to run it without changes on a different board. People have tried to merge at least some of these different cores in a central repository, without much luck. Probably just lack of free time, I guess (at least that's true for me ;)). Some time in the future, I hope I will have a repository with support for at least the original Minimig, the DE1, MiST and DE1-SoC boards. Can't say when that will happen though ;)

As for the maturity of the project, I can only comment on the minimig variants I'm familiar with. The ECS core on the original minimig and the MiST board is pretty good, vast majority of games run without problems, most demos too. There are some demos that don't work correct, but they are in the minority. Most problems are in some corner cases of register timings - writes to some registers sometimes don't have an effect on real Amigas, depends on stage of DMA pipeline. Blitter also has some problems, and would need an overhaul. Besides that, I'd say the minimig is pretty good at matching an ECS Amiga. The original board also has the real 68000 IC on it, so it is really close to an actual Amiga. Minimig variants that use the tg68k softcore are a little less compatible, but not by much. And to answer your question, the tg68k core is the most complete 68000/68020 core that I know off, but it is not perfect, and it is not cycle-accurate.

Your Nexys2 board should be able to run at least the ECS variant of minimig, plus the 68000 softcore and a control CPU. I would suggest building/buying an SD card adapter, and forget about the flash on the board. The kickstart image can be loaded to memory from SD card. You will need a softcore control CPU core.
You can look at MMrobinsonb5's minimig_c3 repository on Github, which has an added control softcore: https://github.com/robinsonb5/minimig_c3
Or my work on the minimig-DE1 here: https://github.com/rkrajnc/minimig-de1
There are different variants of softcore control CPUs used with minimig: the original minimig-de1 by Tobiflexx used a second tg68k core, MMrobinsonb5 used ZPU, and in my version of minimig-de1 I used the OR1200. Take your pick :)

For start, I would suggest to stick with the floppy emulation in minimig, which is tested & working. Once you have that running, you can work on the real floppy support.

The original minimig board's 68EC000 runs at the same ~7MHz frequency as the real Amiga, but can be overclocked to ~50MHz.

As for the most active minimig forum, I think this is it :) There are some limited discussions on other Amiga-, MiST- or other retro-related forums, like eab.abime.net, amibay.com, atari-forum.com, mist-fpga.net.

Feel free to ask any additional questions, and I (and others here) will try to help as much as I can.


Good luck with your project and have fun! :)

_________________
** my minimig builds: http://somuch.guru/ **


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