WHAT IS XNF

Xilinx Netlist Format. This is somewhat specific to the Xilinx tool
chain, but that is sufficiently ubiquitous that it is worth it. This
format can be fed to place and route tools and simulators. since third
party simulators accept XNF, the format may be useful even independent
of Xilinx parts.

The xnfio function is highly recommended as it optimizes use of the IO
blocks. Without this function, the I/O blocks will probably remain
unused.

XNFIO FUNCTION

The "xnfio" function transforms the netlist where the IOBs are
concerned. The signals with PAD attributes are checked, and
surrounding circuitry generated to conform to the logic available in
the IOB.

If the pad is an OPAD, the function will look for an existing buf or
not gate connected to the PAD signal. If the gate is appropriately
connected, the buf or not gate will be turned into an OBUF. This pulls
the buf or inverter into the IOB, freeing a CLB and providing the
required pin circuitry.

If the pad is an IPAD, the function will look for a buf, and convert
that to an IBUF. Since Xilinx IOBs cannot invert the output from an
IBUF, NOT gates cannot be absorbed as in the OPAD case.


/*
 * Copyright (c) 1998-1999 Stephen Williams (steve@icarus.com)
 *
 *    This source code is free software; you can redistribute it
 *    and/or modify it in source code form under the terms of the GNU
 *    General Public License as published by the Free Software
 *    Foundation; either version 2 of the License, or (at your option)
 *    any later version.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU General Public License for more details.
 *
 *    You should have received a copy of the GNU General Public License
 *    along with this program; if not, write to the Free Software
 *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
 */


 $Log: xnf.txt,v $
 Revision 1.1  1999/05/01 02:57:11  steve
  XNF target documentation.