using a fast microcontroller to run some 68000 emulation software. It would be much slower than doing it with the Snapdragon, but these microcontrollers are like $10, compared to $79 for the Snapdragon module.
/ add a footprint for a fast (300MHz+) ARM microcontroller and SDRAM. Its implementation would cost an extra $15 or so. / You could then upgrade by purchasing the Snapdragon module for $79 if you want.
/
Is this a good idea?
IMO, absolutely. Anything that brings the end cost to the purchaser down is going to expand your market. Having a powerful but optional upgrade for later on only makes it more attractive - and, obviously, the larger the potential market, the better the chance of economies of scale.
The speed penalty may not be as bad as you think, if the micro is relieved of the burden of also running a Linux kernel.
bbraun's existing work with interfacing the sub-$20 Discovery boards (~180MHz) to the SE PDS may be illuminating here.
Without the Snapdragon it would just be an accelerator that can do virtual disks and a few types of USB peripherals
Honestly, I think this (with the SDRAM you mentioned) is all that 90% of people would really want in a accelerator
[1], especially if it means we're talking about a sub-US$100 end price, rather than US$150-200 and counting.
Speaking of keeping costs down, I really think your best approach might be to continue focusing on a single,
small SE PDS type card. It's always tempting to add another $1 feature here and another $15 feature there, but as you've already seen, these all quickly add up. Re-focusing down on the simplest possible board that allows for later expansion seems like a wise move.
Adaption to other, CPU socket machines can be implemented as a second card. Yes, that brings the cost up somewhat for anyone with a different machine, but I think the economy of scale - and the saving in development time and PCB area - on the main card will probably even that out, perhaps even so far as to make it cheaper for everyone. And it means you start off with a reasonable market for the simple, cheap SE PDS accelerator, which can start the ball rolling and bring in funds and interested developers for future widening of the market and applications.
It doesn't seem necessary to me to make a different, huge card for every machine type, and obviously the larger the card the more expensive. Make it as modular as possible - a single, standardized Killy-type connector for
all DIP 68000 machines (for example), and ribbon cables or other readymade connectors from that to a convenient mounting location for the standardized SE PDS breakout board. Any wide span to reach mounting points could be made up with a plain sheet of plastic.
The CPU-PDS adapter card/s should be relatively simple, at least in the fact that you're not dealing with the high speed signalling on the accelerator. Picking a standard expansion interface (SE PDS) means that you have a fixed target to develop towards - each machine's adapter assembly can then be developed individually, one at a time, by you or by whoever. And if you decouple the electrical-mechanical interface from the internal machine layout (using the modular approach), you can re-use as many subassemblies as possible across different machine types.
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[1] Ethernet or WiFI via USB or a NIC IC might be a nice add-on, but probably optional rather than standard, as other options for networking already exist.