Re: Spartan-IIE LVDS?

I'm too poor to buy an IBIS simulator :) I've been reading all sorts
of data sheets and app notes on LVDS that have made me worry about any
sort of single-ended signals  one can reliably send over a short bus.
I read that PCI has really tight timings and is evidently really hard
to design for; it's only 33 MHz. Sure, it's a bus with multiple cards,
but AGP is a single card in a single slot, and only runs at 66 MHz. I
feared that if "real" EEs (instead of us biologists that just play
them when we need equiment that doesn't exist) don't want to push a
card interconnect above 66 Mhz, there must be a good reason.

So I'm trying to figure out what I can reasonably expect from a
4-layer FR4 board and a 68-pin high-density mini-D connector. This is
my first high-speed interconnect project, and it may just be paranoia.
Are you suggesting I shouldn't lose too much sleep over a  ~125 MHz
single-ended bus covering a distance of 6" or so? or should I stick to
differential signaling for those types of speeds? I'd love to just
make my bus wider, but I run out of IOs on my Spartan-IIE PQFP and I
can't afford to have someone put down a BGA on a PCB.

Ahh, the joys of low-cost student design ! :)

Thanks again for all the help,
               ...Eric

Read the bible - "High Speed Digital Design", by Johnson and Graham !

Dave

[snip]



[snip]

The hard part in "33 MHz" PCI is a 7 ns path on the PCI IRDY# and TRDY#
control signals to output enables on the FPGA.  So, instead of a "33 MHz"
problem, you really have a 143 MHz" design challenge.  The issue is related
to propagation time within the FPGA and not related to signal integrity
issues.

BTW, Spartan-IIE fully supports 66 MHz and 64-bit PCI and is available in a
proven LogiCORE solution.
http://www.xilinx.com/partinfo/pci/xcvpci64_32ds.pdf

--------------------------------
Steven K. Knapp
Spartan-3/II/IIE FPGAs
http://www.xilinx.com/spartan3
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Spartan-3:  Make it Your ASIC