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/*
* Copyright (c) 2000 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
*/
/*
* This source file demonstrates how to synthesize CLB flip-flops from
* Icarus Verilog, including giving the device an initial value.
*
* To compile this for XNF, try a command like this:
*
* iverilog -txnf -ppart=XC4010XLPQ160 -pncf=clbff.ncf -oclbff.xnf clbff.v
*
* That command causes an clbff.xnf and clbff.ncf file to be created.
* Next, make the clbff.ngd file with the command:
*
* xnf2ngd -l xilinxun -u clbff.xnf clbff.ngo
* ngdbuild clbff.ngo clbff.ngd
*
* Finally, map the file to fully render it in the target part. The
* par command is the step that actually optimizes the design and tries
* to meet timing constraints.
*
* map -o map.ncd clbff.ngd
* par -w map.ncd clbff.ncd
*
* At this point, you can use the FPGA Editor to edit the clbff.ncd
* file. Notice that the design uses two CLB flip-flops (possibly in
* the same CLB) with their outputs ANDed together. If you go into the
* block editor, you will see that the FF connected to main/Q<0> is
* configured so start up reset, and the FF connected to main/Q<1> is
* configured to start up set.
*/
module main;
wire clk, iclk;
wire i0, i1;
wire out;
wire [1:0] D = {i1, i0};
// This statement declares Q to be a 2 bit reg vector. The
// initial assignment will cause the synthesized device to take
// on an initial value specified here. Without the assignment,
// the initial value is unspecified. (Verilog simulates it as 2'bx.)
reg [1:0] Q = 2'b10;
// This simple logic gate get turned into a function unit.
// The par program will map this into a CLB F or G unit.
and (out, Q[0], Q[1]);
// This creates a global clock buffer. Notice how I attach an
// attribute to the named gate to force it to be mapped to the
// desired XNF device. This device will not be pulled into the
// IOB associated with iclk because of the attribute.
buf gbuf(clk, iclk);
$attribute(gbuf, "XNF-LCA", "GCLK:O,I");
// This is mapped to a DFF. Since Q and D are two bits wide, the
// code generator actually makes two DFF devices that share a
// clock input.
always @(posedge clk) Q <= D;
// These attribute commands assign pins to the listed wires.
// This can be done to wires and registers, as internally both
// are treated as named signals.
$attribute(out, "PAD", "o150");
$attribute(i0, "PAD", "i152");
$attribute(i1, "PAD", "i153");
$attribute(iclk,"PAD", "i154");
endmodule /* main */
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