JOHNSON计数器，环形计数器，普通计数器

1.普通4bit计数器，n位计数，2^n个状态：

module top_module ();
	reg clk=0;
	always #5 clk = ~clk;  // Create clock with period=10
	initial `probe_start;   // Start the timing diagram

	`probe(clk);        // Probe signal "clk"

	// A testbench
	reg reset;
	initial begin
	 reset=0;
         #10 reset=1;
	 $display ("Hello world! The current time is (%0d ps)", $time);
         #50 $finish;            // Quit the simulation
	end

    count inst1 (.clk(clk),.reset(reset));   // Sub-modules work too.

endmodule

module count(clk, reset, out);
    output [3:0] out;
    reg [3:0] out;
    input clk,reset;
    always @(posedge clk) begin
        if (reset==0)
            out<=0;    
        else
            out<=out+1;
    end
        `probe(clk);	// Sub-modules can also have `probe()
        `probe(out);
endmodule

结果：