[Coco] DE1 arrives :-)

[Mark McDougall]

Joel Ewy wrote:

> Don't forget, the DE1 is already the SECOND platform for CoCo3FPGA.  It 
> was first implemented on the Digilent Spartan-3, which, I might add, has
>  more memory and would fit quite easily in a vacated CoCo 2 or 3 case.

Actually, it's the FOURTH platform! :) I ported the original project to a 
custom board, and then also to the DE2, before Gary got it running properly 
on the DE1.

>  If we want our engineering efforts to last, we need to design a modular
>  bus for FPGA CoCo hardware interfaces, so you can build a CoCo analog 
> audio out board, or a joystick interface, or multipak / cartridge 
> connector, and plug those into an appropriate adapter that interfaces 
> them with a specific FPGA board, whether that be a development kit or a 
> specially-designed CoCo FPGA.

In reality, that would probably be no different to re-spinning adapter 
boards for each FPGA board. The cost is in the PCB manufacturing, the level 
shifting and the connectors, and you're proposing that each FPGA dev board 
requires an adapter board AND the Coco peripheral board! Too expensive!

Also, the DE1 probably has more I/O than a lot of dev boards. Something like 
a cartridge connector takes up a *LOT* of pins very quickly, and I think 
you'll find quite a few FPGA boards that don't have enough I/O.

Also, the DE1 I/O isn't suitable for high speed communications. Fine for a 
few MHz perhaps, but it starts to fall down when you head towards 10MHz. I 
know, because a colleague has had a real 6809 and a real 68k running off the 
expansion connectors... and ran into problems around that mark.

> The effort needs to go into a standardized, modular 
> interface.  

Again, I disagree for the reasons I mentioned above. You're better off 
designing a complete Coco-specific PCB, like the Minimig, OCM efforts. It's 
almost mandatory with the I/O requirements. Perhaps one option - you can get 
FPGA modules with RAM from Altium, for example, with standard pinouts that 
lets you choose FPGA -and even FPGA vendor - to grow with the design.

> Would it make sense to put all that on a 
> single board, or split it up?

Count your I/O requirements - you'll be amazed how quickly that adds up. As 
I mentioned in another post, we almost got the design finished for a custom 
Coco FPGA board - and even with the entire FPGA I/O under our control, we 
ran out of pins!!! There's NO WAY you can do it all with a DE1...

As for FPGA boards getting smaller... not really. The DE1 size is mandated 
by the I/O and periphery hardware, NOT the technology. The EP2C20 is amongst 
the smallest parts on the board.

BTW the 2C20 doesn't leave a lot of room to grow for the Coco3FPGA project. 
Personally I think you need a 2C35 to give enough headroom for more 
interesting possibilities within the FPGA, such as more floppy/hard drive 
emulation options, more graphics/sound, etc etc.

Regards,

-- 
|              Mark McDougall                | "Electrical Engineers do it
|  <http://members.iinet.net.au/~msmcdoug>   |   with less resistance!"