Re: [oc] Modular FPGA board

[Rudolf Usselmann <rudi@asics.ws>]

John,

you have to be careful throwing a bunch of SIMM connectors on the board
and expecting the thing to run at 100 Mhz. Many factors play a roll as
to what the maximum speed might be. You need to wary about trace
capacitance and inductance, and perhaps termination (which is harder
to do if it's not point to point). I would suggest to limit the number
of SIMMs that are connected together to four, and figure out some way
of bridging multiple groups of four SIMMs.

If you look at of the shelf SIMMs, you will notice that those PCBs are very
small, and the traces very short (== low capacitance). Further when using
SIMM memories, the max. load on each trace is known, making it easy to run
them at 133 Mhz. Designing a universal interconnect bus, with unknown
peripherals is much harder. Just look at the PCI spec if possible, you can
see how many restrictions there are for both, plug in cards and
motherboards.

Another option would be to put a PCI bridge on the board and use a local PCI
bus for all FPGAs and peripherals.
 
Best Regards,
rudi