Most bang for 600 USD?

[danbeaver]

Would you explain the need to "sync down to 14 MHz" and how a serial interface can handle that much bidirectional data?  This is just because my degree was not in engineering, electrical or otherwise.

[Mrs Beanbag]

Would you explain the need to "sync down to 14 MHz" and how a serial interface can handle that much bidirectional data?  This is just because my degree was not in engineering, electrical or otherwise.

The A1200 motherboard runs at 14MHz no matter what, if you have a CPU on an accelerator card that runs at other than 14MHz you still have to communicate with the motherboard at 14MHz.

USB* can get 480Mbit/s, although with 32 data lines and 24 address lines it might still be a bit of a bottleneck.  But it's really more for proof-of-concept than final design, we have to take things one step at a time.  It's just that producing a custom ARM board is a lot of unnecessary work at this stage, when we already have an ARM board already available at lost cost.  The idea is get the software working on that first, and we can think about a faster interface later, and ultimately a single-board design.

*USB 2.0

[psxphill]

USB* can get 480Mbit/s, although with 32 data lines and 24 address lines it might still be a bit of a bottleneck.

The problem is that the fastest speed is when you're transferring alot of data. You're going to have crippling latency doing individual reads and writes. If you use a USB serial adapter in dosbox then it can't even run full speed at 115,200 baud. You're not going to get 14mhz even if you hit 480 mbit/s, with the latency I'd be impressed if you get 1mhz. It's likely to be much lower.
 
We know that you can run a 68k emulator on an arm board, it's the hook up to the amiga that is the problem. Any time you spend on using USB is going to be wasted and the results disappointing and not an indication of what you can achieve.

[Mrs Beanbag]

The problem is that the fastest speed is when you're transferring alot of data. You're going to have crippling latency ... Any time you spend on using USB is going to be wasted and the results disappointing and not an indication of what you can achieve.

This is true but when doing development you do one bit at a time, you start simple and get it to work first, then you worry about the performance.  I've never produced anything for the expansion port before, just being able to probe the ROM from my PC would be a start.

[psxphill]

This is true but when doing development you do one bit at a time, you start simple and get it to work first, then you worry about the performance. I've never produced anything for the expansion port before, just being able to probe the ROM from my PC would be a start.

When worrying about the performance means throwing everything away then you're doing it wrong. Doing development one bit at a time is good, but you aren't doing any bits that are worth doing.
 
Starting with the expansion port interface is worse than running before you can walk, it's hoping that you can learn to walk by learning to drive.
 
Talking to xilinx about something like this:
 
http://www.xilinx.com/support/documentation/data_sheets/ds190-Zynq-7000-Overview.pdf
 
is going to get you further. You might even get some advice.

[Mrs Beanbag]

When worrying about the performance means throwing everything away then you're doing it wrong. Doing development one bit at a time is good, but you aren't doing any bits that are worth doing.

I won't be throwing everything away, just the USB connection, which is easy.  I still need the 68k emulator for ARM with whatever mods necessary (i.e. all memory access below 01x000000 needs to go off-board), and I still need to interface with the expansion slot somehow.  But it's early days yet, I don't have a formal written plan, so I'm still looking at options.  I'm going into the Hacklab tomorrow night anyway, see if anyone there knows how to build ARM boards, so far I've only spoken to a notorious pessimist.

We can make our own PCBs here.  But if I can't make an ARM board I don't know what chance I'd have with an FPGA...?  What would I need one for, anyway?  I only need CPU, RAM, flash ROM and some kind of buffer.

[freqmax]

14 MHz 16-bit word read/write cached address bus shrinked to 8-bit = 336 Mbit/s not including latency issues which for USB is at minimum 1 ms, so to write an address and then read say the next instruction would put your GHz ARM CPU at an effective clock frequency of 1000 Hz. Ie 7000 times slower than an unexpanded A500.

Any realistic solution requires a low latency and high speed interface. Thus a GPIO directly to the "guest" CPU. Perhaps the GPIO on STM32 (ARM) CPU or Raspberry-PI is capable of that.

[Mrs Beanbag]

Well, so much for pub conversations.

Any realistic solution requires a low latency and high speed interface. Thus a GPIO directly to the "guest" CPU. Perhaps the GPIO on STM32 (ARM) CPU or Raspberry-PI is capable of that.

"Realistic" is a long-term goal of course, but GPIO seems like a far better idea anyway!  Seems quite easy to use, as well.

[jackflash]

I found an article on replacing old CPUs that uses the 68000 as an example, it even mentions using a modern CPU for emulation
http://www.cast-inc.com/company/blog/post.php?s=2011-06-20-chip-replacement-with-ip-and-fpgas-68000-processor-example

Here is what the article says about emulation
 " introduces new boot code and timing challenges. The effort to resolve these challenges can exceed the appropriate  lifecycle extension budget, consuming any profit to be made in extending  the product’s life.In contrast, simply replacing the obsolete  processor part with a new FPGA device that’s fully hardware and software  compatible is usually significantly easier and less expensive."

[Mrs Beanbag]

Here is what the article says about emulation
 " introduces new boot code and timing challenges. The effort to resolve these challenges can exceed the appropriate  lifecycle extension budget, consuming any profit to be made in extending  the product’s life.In contrast, simply replacing the obsolete  processor part with a new FPGA device that’s fully hardware and software  compatible is usually significantly easier and less expensive."

The article says a lot of things.  It depends on your design goals, of course, and expertise.  For instance it also says this:
"If your design team has little experience using IP cores and FPGAs, then  using a discrete processor chip is likely the better approach."

And "How many more years do you expect to ship the product? The longer this  life, the greater the chance your new processor chip will also become  obsolete, and the more using IP makes sense."  I don't expect ARM chips to go obsolete any time soon.

Anyway since this is a hobby project (at least for now) most of the business-type decisions aren't really important.  BUT I did consider the FPGA option, and rejected it for several reasons, the central one being, I don't intend merely to replace the 680x0 with an exact replica, but rather, to improve on it.

Also, I already have a Raspberry Pi.

(Besides, that article is basically a piece of marketing for their own IP cores...)

[freqmax]

I double checked. USB2 will give no faster response than 125 µs/7 thus the equalient clock frequency is no more than 56 kHz..
In other words USB is a dead end.

BACK TO TOPIC..

Emulating 68k won't impress current market.

DSP, FPGA, etc.. has likely higher chance.

[Mrs Beanbag]

YES, sorry, kind of hijacked your thread there.  I'll start my own thread once I've got something to show for it.

To be honest I've no idea what would impress the current market.  When the iPad came out I thought, what is the point of that?  It doesn't seem to solve any technical problem at all.  There's nothing groundbreaking there from a technical point of view, but it comes in a stylish and convenient package.  It's more of a lifestyle solution than a computing solution.

So if your question is, what would sell really well?  I can't answer that.  I know that a board with built-in FPGAs that the programmer could use could achieve some technically impressive feats, whether the average consumer could care less is another matter.  But the rise of the portable computing market has really put ARM on the scene, and I expect desktop PCs to follow suit eventually.  If I could buy a desktop motherboard based on ARM right now, that beat my current AMD 4850e, I'd be happy.

[Digiman]

Is the ARM chip same endian as 680x0 or opposite like the x86?

[Mrs Beanbag]

Is the ARM chip same endian as 680x0 or opposite like the x86?

They can be switched to either mode (bi-endian).

[Digiman]

They can be switched to either mode (bi-endian).

Ah, cool feature indeed.

[A6000]

They can be switched to either mode (bi-endian).

That reminds me of the F-CPU project which was designing an open source 64 bit cpu, the project has died 2 or 3 times so far, maybe if you decide to go the FPGA route you could revive it.