# HDL

Preparing an own ASIC requires a prototype circuit described in one of the Hardware Description Language(HDL).
In this chapter you will prepare a design of 4-bit counter and a testbench for simulations. Before starting simulations install Verilator.

```bash
sudo apt install verilator -y
```

## Design

Describe a counter presented below using submodules in a RTL architecture. Use a below template in Verilog language, it will make simulations easier.
Create a file named counter.v, copy a below text and write a code for all modules.

```verilog
module jk_ff ( input j,
               input k,
               input clk,
               output reg q);

endmodule

module and_2( input a,
              input b,
              output y); 

endmodule

module counter( input clk,
                output [3:0] q, 
	        output p);

endmodule
```

![Quartus](schemat.png)

## Simulation

Create file named tb.v and copy below text.

```verilog
module tb_counter;

reg clk;

wire [3:0]q;
wire p;

counter u0 ( .clk(clk),
	 .q(q),
         .p(p));

initial begin
$dumpfile("test.vcd");
$dumpvars;
	clk = 1;
	#320 $finish;
end

always #10 clk=~clk;

always #20 $display("%d %d",p,q);

endmodule
```

To run a simulation use **Verilator**. The line below will compile testbench to binary version.

```bash
verilator --trace --binary -j 0 tb.v
```

You can run simulation typing `./obj_dir/Vtb` in a terminal.

Type `gtkwave test.vcd` in the terminal to see the results in the time domain.

![GTKWAVE](gtkwave.png)