Rick C. Hodgin wrote on 11/10/2017 6:59 PM:
> On Friday, November 10, 2017 at 6:31:25 PM UTC-5, rickman wrote:
>> Rick C. Hodgin wrote on 11/10/2017 5:14 PM:
>>> On Friday, November 10, 2017 at 4:11:44 PM UTC-5, rickman wrote:
>>>> What are your goals exactly?
>>> (1) Get the board designed physically, and ordered or built.
>> You need to define the board a lot better.
>
> As I see it, the CPU is a black box. I connect wires to it and
> give it power and turn things on and off, and observe it turning
> things on and off, all by protocol, and it doesn't need anything
> else.
Everything is a black box in that sense. The "protocol" is the part you
need to understand in detail.
> I route all pins to appropriate I/O on the FPGA, or Vcc, and then
> I need to have simulated ROM at the boot address, which simulates
> BIOS. And I just read the Am386 was a CMOS 3.3V part, so that removes
> level shifters.
I can't find anything that says it was 3.3 volt. Where did you read this?
> Since I am the motherboard, all my BIOS would need to do is setup
> interrupt vectors for CPU-issued interrupts (0x0 through 0x1f),
> and run some software that does something I have control over.
Yes, if you don't plan to run it as a PC, but have you figured out any of this?
> First thing I need to do is get help on board design and components.
> I'm thinking a socket (Package: PGA-132) with pins routed to the
> mated 40-pin breakouts I would assign on my FPGA. Of the 160 pins,
> only so many are GPIO, so I would have that limitation on design.
Board design is not as hard at 40 MHz as at 100's of MHz, but you still need
to know something about signal integrity. If not sometimes the voltages
will bounce and jitter and so look like the wrong voltage when sampled and
clocks can bounce and double clock on a single edge. This is *very*
important stuff to know.
> I think I could do all that, but there are things I don't know.
> Will I need capacitors? Resistors? Some kind of something to handle
> electrical oddities? If not, then I assume making trace lines equal
> is important, but not greatly at only 40 MHz.
If you are asking questions at this level, you will not be able to design a
board that will work. You need to learn basic electronics. What do you
know about electronics?
> On the FPGA, it would route address and data pins to logic identifying
> memory and I/O, and read / write, responding appropriately, routing
> certain memory to emulated ROM, the rest to on-FPGA SRAM emulating
> DRAM.
>
> It seems a simple physical design. Moderately complex logically.
> And very exciting. :-)
I think you don't know enough to understand the problems involved in what
you are trying to do. For one, trying to ship signal lines through two
connectors, three boards and many inches of signal trace could result in
severe signal integrity problems. This is stuff I have to pay attention to
on one board with no connectors and fairly short signal traces.