Re: [oc] Modular FPGA board

[Rudolf Usselmann <rudi@asics.ws>]

on 11/26/00 21:18, John Dalton at john.dalton@bigfoot.com wrote:
> 
> I guess the trick is to throw the SIMM connectors on the board very carefully!
> :)
> Thanks for pointing out some of the things to watch.  Once I get something
> more concrete, I will put it out for review.

That would be great, I will be available for help !

> At this stage, I am thinking along the lines of simply wiring up SIMMs
> in parallel and working from there.  In the first cut, it can be arranged
> to keep a single load on each line, arranging the system as point-to-point
> links from the main FPGA to each peripheral (possibly with a short stub),
> though this is not really a satisfactory long term solution.

You don't have to do it all point to point. Just limit the SIMMs to groups
of four, that will guarantee high speed. Write up some guide lines for the
"SIMMs", similar to PCI, require the interface FPGA to be very close (within
1 inch or 2.5 cm) of the connector.

Another option is to use the differential IOs. However, they take up twice
as many pins !

> This raises the question of defining some sort of a standard
> for the 'SIMM bus'  At the physical level, simply defining which pins are
> power and which pins are signal might suffice.  On top of this could
> be layered some sort of logical bus structure.  Much more thought required
> here,
> though simplicity is a high priority for me.
> 
> For starters, I'm thinking just get something designed/built, then develop it
> further.

Make sure to plan for multiple power & ground planes and pins, include:
5, 3.3 and 2.5/1.8 V as a minimum.

5 V       for peripheral chips
3.3 V     for FPGA IOs (5V tolerant)
2.5/1.8 V for FPGA core

>> Another option would be to put a PCI bridge on the board and use a local PCI
>> bus for all FPGAs and peripherals.
> 
> I'm trying to steer clear of PCI, as IMHO it is overkill for connecting a
> simple
> ADC (or similar device) to an FPGA.  Most chips will not connect directly
> to a PCI bus.

I agree, but you will need a PCI core anyway to interface to the host PC,
so why not use it for local bus as well ?

> Talking about PCI and FPGAs, I should raise an idea considered
> early in the design.  Take a PC, remove the motherboard and cards.
> Build a new PCB to a standard motherboard from factor with a bunch of FPGAs on
> it.
> Each peripheral is built on a PCI form factor card.  Plug the whole lot
> together
> and power it from the standard PC power supply.  The result is a box that
> looks like
> a PC, but is an FPGA test system.  (Put enough FPGAs in it, and you could
> program it
> to be a PC!)   Mechanically, this would be a rather nice system, but probably
> more
> expensive than a SIMM based system.

I would keep it small and simple. I would stick to full size PC card format
for the first version. To do a large board like a PC mother board, requires
a lot of work.

Do we have a spice guru on the mailing list ? He/She could simulate the
board with the Xilinx IO buffers to determine the number of SIMMs and top
speed we can run it. I think this would be VERY important for such an
universal board as you are planning.

> 
> Regards
> John

Cheers !

rudi