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For those of you who wish to use Icarus Verilog, in combination with
the Xilinx back end (Foundation or Alliance), it can be done. I have
run some admittedly simple (2300 equivalent gates) designs through this
setup, targeting a Spartan XCS10.

Verilog:

   Older versions of Icarus Verilog (like 19990814) couldn't synthesize
   logic buried in procedural (flip-flop) assignment. Newer versions
   (like 20000120) don't have this limitation.

   Procedural assignments have to be given one at a time, to be
   "found" by xnfsyn. Say
       always @ (posedge Clk) Y = newY;
       always @ (posedge Clk) Z = newZ;
   rather than
       always @ (posedge Clk) begin
           Y = newY;
           Z = newZ;
       end

   Steve's xnf.txt covers most buffer and pin constructs, but I had reason
   to use a global clock net not connected to an input pin. The standard
   Verilog for a buffer, combined with a declaration to turn that into a
   BUFG, is:
       buf BUFG( your_output_here, your_input_here );
       $attribute(BUFG,"XNF-LCA","BUFG:O,I")

   I use post-processing on my .xnf files to add "FAST" attributes to
   output pins.

Running ivl:

   The -F switches are important. The following order seems to robustly
   generate valid XNF files, and is used by "verilog -X":
      -Fsynth -Fnodangle -Fxnfio

Generating .pcf files:

   The ngdbuild step seems to lose pin placement information that ivl
   puts in the XNF file. Use xnf2pcf to extract this information to
   a .pcf file, which the Xilinx place-and-route software _will_ pay
   attention to. Steve says he now makes that information available
   in an NCF file, with -fncf=<path>, but I haven't tested that.

Running the Xilinx back end:

   You can presumably use the GUI, but that doesn't fit in Makefiles :-).
   Here is the command sequence in pseudo-shell-script:
      ngdbuild -p $part $1.xnf $1.ngd
      map -p $part -o map.ncd $1.ngd
      xnf2pcf <$1.xnf >$1.pcf # see above
      par -w -ol 2 -d 0 map.ncd $1.ncd $1.pcf
      bitgen_flags = -g ConfigRate:SLOW -g TdoPin:PULLNONE -g DonePin:PULLUP \
                     -g CRC:enable -g StartUpClk:CCLK -g SyncToDone:no \
                     -g DoneActive:C1 -g OutputsActive:C3 -g GSRInactive:C4 \
                     -g ReadClk:CCLK -g ReadCapture:enable -g ReadAbort:disable
      bitgen $1.ncd -l -w $bitgen_flags

   The Xilinx software has diarrhea of the temp files (14, not including
   .xnf, .pcf, .ngd, .ncd, and .bit), so this sequence is best done in a
   dedicated directory. Note in particular that map.ncd is a generic name.

   I had reason to run this remotely (and transparently within a Makefile)
   via ssh. I use the gmake rule
%.bit : %.xnf
        ssh -x -a -o 'BatchMode yes' ${ALLIANCE_HOST} \
               remote_alliance ${REMOTE_DIR} $(basename $@) 2>&1 < $<
scp ${ALLIANCE_HOST}:${REMOTE_DIR}/$@ .
    and the remote_alliance script (on ${ALLIANCE_HOST})
/bin/csh
cd $1
cat >! $2.xnf
xnf2pcf <$2.xnf >! $2.pcf
./backend $2

   There is now a "Xilinx on Linux HOWTO" at
         http://www.polybus.com/xilinx_on_linux.html
   I havn't tried this yet, it looks interesting.

Downloading:

   I use the XESS (http://www.xess.com/) XSP-10 development board, which
   uses the PC parallel (printer) port for downloading and interaction
   with the host. They made an old version of their download program
   public domain, posted it at
      http://www.xess.com/FPGA/xstools.zip ,
   and now there is a Linux port at
      ftp://ftp.microux.com/pub/pilotscope/xstools.tar.gz .

The above hints are based on my experience with Foundation 1.5 on NT
(gack) and Alliance 2.1i on Solaris. Your mileage may vary. Good luck!

     - Larry Doolittle <LRDoolittle@lbl.gov> August 19, 1999
                                        updated February 1, 2000
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