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Xilinx Netlist Format


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

Icarus Verilog supports XNF as specified by the Xilinx Netlist Format
Specification, Version 6.1.


The easiest way to compile for XNF output is with the "verilog"
command (man verilog) and the -X switch:

	% iverilog -fpart=4010e -fncf=prog.ncf -txnf prog.v

This generates from the prog.v Verilog source file the prog.xnf output
and the prog.ncf netlist constraints file. The verilog program
arranges to call the preprocessor and the ivl compiler with all the
correct switches for generating XNF.


Icarus Verilog can be used to generate XNF implementations of devices
that are written in Verilog and used by schematic editors such as
OrCAD. The trick here is that the code generator automatically notices
ports to the root module and generates the PIN= attributes needed so
that external tools can link to the generated XNF.

Icarus Verilog chooses a name for the pin. The name it chooses is the
port name of the module. If the port is a vector, a pin is generated
for all the bits of the vector with the bit number appended. For

	module foo(in);
	input [3:0] in;

causes the single bit ports ``in0'' through ``in3'' be
generated. Internally, the XNF file uses the bussed names instead of
the pin name.

The implication of this is that there is a chance of name collision
with the generated XNF macro if the port names are chosen badly. It is
best to not end a port name with decimal digits, as that can cause
trouble at link time. Also, XNF is not case sensitive and that should
be accounted for as well.


You can assign wires to pads using the Icarus Verilog $attribute
extension. Attach to a scalar signal (wire or register) the PAD
attribute with the value that specifies the direction and pin
number. For example:

	wire foo, bar, bid;
	$attribute(foo, "PAD", "i1"); // Input pad on pin 1
	$attribute(bar, "PAD", "o2"); // Output pad on pin 2
	$attribute(bid, "PAD", "b3"); // Bi-directional pad on pin 3

The XNFIO function uses these attributes to locate signals that are
connected to pads, and generates XNF I/O block devices to connect to
the pad to do the FPGA pin buffering that is needed. So the Verilog
programmer need not in general specify the IBUF/OBUF buffers.

If the programmer does connect buffers to pads, the compiler will
notice them and convert them to I/OBUFs automatically. For example:

	buf b1 (sig, foo);

connects to pad foo, so will be converted into an XNF IBUF
device. Also:

	bufif1 bt (bar, value, en);

connects to pad bar so will automatically be converted into an OBUFT
device. Icarus Verilog understands OBUF, IBUF and OBUFT (with optionally
inverted enable) devices and will convert Verilog devices from the
source, or generate missing devices.

In addition, the Verilog programmer may explicitly declare a device as
an I/OBUF by attaching an attribute to the device, like so:

	buf b1 (sig, foo);
	$attribute(b1, "XNF-LCA", "OBUF:O,I");

This latter feature is not entirely recommended as it expects that the
programmer really knows how the pins of the XNF device are to be
connected. It also bypasses the efforts of the compiler, so is not
checked for correctness.


Storage elements in XNF include flip-flops, latches and CLB
rams. These devices are generated from the LPM equivalents that the
-Fsynth functor synthesizes from behavioral descriptions.

Flip-flops, or more specifically DFF devices, are generated to
implement behavioral code like this:

	reg Q;
	always @(posedge clk) Q <= <expr>;

The edge can be positive or negative, and the expression can be any
synthesizable expression. Furthermore, the register "Q" can have
width, which will cause the appropriate number of flip-flops to be
created. A clock enable expression can also be added like so:

	reg Q;
	always @(posedge clk) if (<ce>) Q <= <expr>;

The <ce> expression can be any synthesizable expression.

With or without the CE, the generated DFF devices are written into the
XNF output one bit at a time, with the clock input inverted if necessary.

Xilinx parts also support CLB circuitry as synchronous RAMS. These
devices are created from Verilog memories if the properties are
right. The behavioral description that the -Fsynth functor matches to
get a synchronous RAM looks very similar to that for a DFF:

	reg [15:0] M;
	always @(posedge clk) if (<we>) M[<addr>] <= <expr>;

Note that in this case the l-value of the assignment is an addressed
memory. This statement models writes into the memory. Reads from the
device can be modeled with ordinary structural code, i.e.:

	assign foo <= M[<addr>];

For the memory to be synthesizable in the XNF target, the address
lines for writes and reads must be connected. This corresponds to the
limitations of the real hardware.


There are certain special devices in XNF that Verilog does not
naturally represent, although there are similar more generic Verilog
devices. The most obvious and useful example is the clock driver,
otherwise known as the global buffer BUFG. As with pads, Icarus
Verilog uses the $attribute extension to allow you to specify special

The $attribute statement can be applied to devices much the same way
one applies them to wires. For example, to turn a buffer into a clock

	wire iclk, clk;
	buf BUFG (clk, iclk);
	$attribute(iclk, "PAD", "i1");
	$attribute(BUFG, "XNF-LCA", "BUFG:O,I");

The above statements cause the buffer BUFG to be emitted in the XNF
output as a BUFG device with the first signal called "O" and the
second called "I". The rest of this example connects the input of the
BUFG to a signal from the input pin #1 and connects the output to the
internal wire "clk". Incidentally, this example will cause an IBUF to
be generated to connect the iclk signal to input pin #1.


Icarus Verilog has a code generator and synthesis functions that
support generation of XNF netlists. The XNF modules also allow the
programmer to use $attributes to control certain aspects of code

XNF code generation is enabled with the ``-t xnf'' flag on the command
line. The code generator needs to know the type of part to generate
code for, so the ``-fpart=<type>'' flag is also needed. For example,
to generate code for the 4010E the command line might start out as:

	 ivl -txnf -fpart=4010e -Fsynth -Fnodangle -Fxnfio [...]

Icarus Verilog includes the functions ``synth'' and ``xnfio'' to
perform transformations and optimizations on the design before code is
generated. The ``synth'' function matches certain behavioral constructs
to structural components, and the xnfio function generates pads and
fills the IOBs.


        Specify the type of part to target. This string is written
        literally into the PART, record of the XNF, and may also be
        used to control synthesis and placement.

        Cause the code generator to write into <path> the netlist
        constraints needed for controlling placement and timing. This
        switch is required if pin assignments are assigned in the
        Verilog source.


This function does synthesis transformations on the entered design,
making it possible to generate XNF netlist components from certain
behavioral constructs. This is needed in Verilog for example to model
some of the synchronous components of the XNF library.

It is a bit much to expect a Verilog compiler in general to generate
components from arbitrary behavioral descriptions, so the synth
function works by matching statements that have some documented
structure, and substituting them for the equivalent XNF component. A
fully synthesize-able design, then, is one where the behavioral
statements can all be matched and substituted by the synth 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 (
 *    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
 *    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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

 $Log: xnf.txt,v $
 Revision 1.16  2003/07/15 03:49:22  steve
  Spelling fixes.

 Revision 1.15  2003/01/30 16:23:08  steve
  Spelling fixes.

 Revision 1.14  2000/08/01 21:32:40  steve
  Use the iverilog command in documentation.

 Revision 1.13  2000/08/01 02:48:42  steve
  Support <= in synthesis of DFF and ram devices.

 Revision 1.12  2000/07/25 22:49:32  steve
  memory is not a data type in verilog.

 Revision 1.11  2000/04/23 23:03:13  steve
  automatically generate macro interface code.

 Revision 1.10  1999/12/05 19:30:43  steve
  Generate XNF RAMS from synthesized memories.

 Revision 1.9  1999/11/18 03:52:20  steve
  Turn NetTmp objects into normal local NetNet objects,
  and add the nodangle functor to clean up the local
  symbols generated by elaboration and other steps.

 Revision 1.8  1999/11/06 04:51:42  steve
  Support writing some XNF things into an NCF file.

 Revision 1.7  1999/11/03 05:18:18  steve
  XNF synthesis now uses the synth functor.

 Revision 1.6  1999/11/02 01:43:55  steve
  Fix iobuf and iobufif handling.

 Revision 1.5  1999/10/09 17:52:27  steve
  support XNF OBUFT devices.

 Revision 1.4  1999/08/14 22:48:21  steve
  Mention the sigfold function.

 Revision 1.3  1999/07/22 02:05:20  steve
  is_constant method for PEConcat.

 Revision 1.2  1999/07/18 21:17:51  steve
  Add support for CE input to XNF DFF, and do
  complete cleanup of replaced design nodes.

 Revision 1.1  1999/05/01 02:57:11  steve
  XNF target documentation.
geda/icarus_xnf.txt · Last modified: 2012/02/20 15:14 (external edit)