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EDA Playground lets you type in and run HDL code (using a selection of free and commercial simulators and synthesizers).
It's great for learning HDLs, it's great for testing out unfamiliar things and it's great for sharing code.

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testbench.sv

 
// Code your testbench here
// or browse Examples
​
module testbench;
  reg clk, up_down, reset;
  wire [3:0] out;
  
  up_down_4bitcounter
  dut (.out(out),
       .clk(clk),
       .reset(reset),
       .up_down(up_down)
       );
  
  // Need that for plotting
  initial begin
    $dumpfile("dump.vcd");
    $dumpvars;
  end
  
  initial begin
    clk = 0;
    reset = 0;
    up_down = 1;
    #10 reset = 1;
    #10 reset = 0;
    #100 $finish;
  end
  
  always begin
    #1 clk = ~clk;
  end
  
endmodule

design.sv

 
module up_down_4bitcounter (
out,
up_down,
clk,
data,
reset
);
​
//Output Ports
output [3:0] out;
​
//Input Ports
input [3:0] data;
input up_down, clk, reset;
​
//Internal Variables
reg [3:0] out;
​
//Start of Code
always @(negedge clk)
if (reset) begin // active high reset
out <= 4'b0 ;
end else if (up_down) begin
out <= out + 1;
end else begin
out <= out - 1;
end
​
endmodule 

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