Re: [oc] Modular FPGA board

[John Dalton <johnd@southern-poro.com>]

> Are you working in the development of the free PCI core?

No.

> My idea was the opposite of your: I was planning to
> built a board for PCI testing only when the core will be finished.

My plan is for the PCI hardware to be an FPGA connected directly
to a PCI connector.  There would also be a bunch of pins going off to the
main FPGA.  The only questions are:
1) How big an FPGA do you need, and
2) How many pins do you need to communicate with the main FPGA?

An answer to question 1) is not too important, as within a
family it is possible to get different sized FPGAs with a common footprint.

In a way, I think it is a good thing to design the PCI core and the hardware
at the same time.  This forces the hardware and core to be independent,
leading to maximum portability for the core and maximum versatility
for the hardware.

> If a PCI core is validated through simulations and it doesn't work over a
> specific board, what should I do to detect the problem? Measurements using
> oscilloscopes and logical analysers are not possible, because they modify
> the circuit when connected into them due to cable impedances, that
> generates multiple reflection. Is there any solution?

Since we are using programmable logic, it should be possible to use the system
to validate itself.  Simply program a logic analyser into the FPGA.
One of the initial applications of the Pamette (an FPGA board built
bt Digital) was to verify the operation of a PCI bus to which it was
connected.

> I don't think that an FPGA for PCI have to fit in a socket. IMHO, it has
> to be soldered in a board. And if this board should fit in a SIMM socket,
> the FPGA has to be a TQFP or BGA packet.

Agree.  My current thinking is to build the PCI as a separate board,
with a chip soldered directly to it.  PCI tracks would go directly to
a PCI connector.  The main logic board and PCI board would be
plugged into each other.  Unfortunately I don't think a SIMM socket
is possible due to mechanical constraints.

> TO DO AT HOME??? Well, if I understood it right, it is much harder to do
> the copper lines for the FPGA than for the edge connector.

Yep.  Do at home.  I'm proposing to use CAD (ideally gpcb, but I don't
think it is finished) to layout a board, printing it 1:1 then using optical
means (Riston?) to transfer the design to a PCB.  With careful construction
it *might* be possible to do a board for a chip with 0.5mm pins at home.
In my experience, aligning two sides of a board, to a fraction of a mm, is
difficult to do at home.  Hence it is difficult to do a double sided
edge connector.  (Single sided is okay.)

Of course this does not rule out the convenience of paying someone
to build the board for you.  But it would be nice to cater for all tastes.

> Is there any
> problem if the lines of the edge connector have the right distance and
> smaller width (ie, increasing the spacing)?

Probably not a good idea.  Edge connector widths and spacings have
generally been designed to maximise chances of a good connection
while minimising chances of a short between contacts.  Changing
widths could impact reliability.

> Greetings from Brazil!

G'day from Australia.

John