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// DESCRIPTION: Verilator: initial edge issue
//
// The module initial_edge drives the output "res" high when the reset signal,
// rst, goes high.
//
// The module initial_edge_n drives the output "res_n" high when the reset
// signal, rst_n, goes low.
//
// For 4-state simulators, that edge occurs when the initial value of rst_n,
// X, goes to zero. However, by default for Verilator, being 2-state, the
// initial value is zero, so no edge is seen.
//
// This is not a bug in verilator (it is bad design to rely on an edge
// transition from an unitialized signal), but the problem is that there are
// quite a few instances of code out there that seems to be dependent on this
// behaviour to get out of reset.
//
// The Verilator --x-initial-edge flag causes these initial edges to trigger,
// thus matching the behaviour of a 4-state simulator. This is reportedly also
// the behaviour of commercial cycle accurate modelling tools as well.
//
// This file ONLY is placed under the Creative Commons Public Domain, for
// any use, without warranty, 2012 by Wilson Snyder.
// SPDX-License-Identifier: CC0-1.0
`timescale 1ns/1ns
module t (/*AUTOARG*/
// Inputs
clk
);
input clk;
wire res;
wire res_n;
reg rst;
reg rst_n;
integer count = 0;
initial_edge i_edge (.res (res),
.rst (rst));
initial_edge_n i_edge_n (.res_n (res_n),
.rst_n (rst_n));
// run for 3 cycles, with one cycle of reset.
always @(posedge clk) begin
rst <= (count == 0) ? 1 : 0;
rst_n <= (count == 0) ? 0 : 1;
if (count == 3) begin
if ((res == 1) && (res_n == 1)) begin
$write ("*-* All Finished *-*\n");
$finish;
end
else begin
`ifdef TEST_VERBOSE
$write ("FAILED: res = %b, res_n = %b\n", res, res_n);
`endif
$stop;
end
end
count = count + 1;
end
endmodule
module initial_edge_n (res_n,
rst_n);
output res_n;
input rst_n;
reg res_n = 1'b0;
always @(negedge rst_n) begin
if (rst_n == 1'b0) begin
res_n <= 1'b1;
end
end
endmodule // initial_edge_n
module initial_edge (res,
rst);
output res;
input rst;
reg res = 1'b0;
always @(posedge rst) begin
if (rst == 1'b1) begin
res <= 1'b1;
end
end
endmodule // initial_edge
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