# HDL

```{note}
Commands like: **ngspice**, **verilator**, **gtkwave**, **ghdl** must be run in a normal terminal. Other commands in the tutorial require OpenLane environment to be running in the terminal.
```

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
```

## VHDL

If the project is written in VHDL, it has to be converted to Verilog. First install GHDL.

```bash
sudo apt install ghdl -y
```

Then convert the project.
```bash
ghdl -a counter.vhd
ghdl synth --out=verilog counter > counter.v
```

## 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** in the directory where the **counter.v** file is, then copy the text below to the **tb.v** file.

```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 and run the simulation.

```bash
verilator --trace --binary -j 0 tb.v
./obj_dir/Vtb
```
The simulation will print the values ​​from the counter for each clock cycle.

Install and run **gtkwave** to see the results in the time domain.

```bash
sudo apt install gtkwave -y
gtkwave test.vcd
```

![GTKWAVE](gtkwave.png)