# 數位電路實驗暨 verilog 學習（二）
> 此章節接下來將實做簡易版的encoder與decoder其理論應用請自行查詢
# Decoder 解碼器
> decoder 類似查找表,module代碼如下

module decoder(x,e,y);
	input [1:0] x ;
	input e;
	output reg[3:0] y;

	always@(*) begin 
		if (e)
			case(x)
				2'b00 : y = 4'b1000;
				2'b01 : y = 4'b0100;
				2'b10 : y = 4'b0010;
				2'b11 : y = 4'b0001;
				default : y = 4'b0000;
			endcase
		else 
			assign y = 4'b0000;
	end
	endmodule

> sim.cpp

#include "verilated.h"
#include "verilated_vcd_c.h"
#include "obj_dir/Vdecoder.h"

//obj_dir/V(.v檔名稱).h

VerilatedContext* contextp = NULL;
VerilatedVcdC* tfp = NULL;

static Vdecoder* top;
// static V(.v檔名稱)* top
void step_and_dump_wave(){
  top->eval();
  contextp->timeInc(1);
  tfp->dump(contextp->time());
}
void sim_init(){
  contextp = new VerilatedContext;
  tfp = new VerilatedVcdC;
  top = new Vdecoder;
  // top = new V(.v檔名稱)
  contextp->traceEverOn(true);
  top->trace(tfp, 0);
  tfp->open("dump.vcd");
}

void sim_exit(){
  step_and_dump_wave();
  tfp->close();
}

int main() {
  sim_init();

  top->e = 0b0;   top->x = 0b00;  step_and_dump_wave();
                  top->x = 0b01;  step_and_dump_wave();
                  top->x = 0b10;  step_and_dump_wave();
                  top->x = 0b11;  step_and_dump_wave();
  top->e = 0b1;   top->x = 0b00;  step_and_dump_wave();
                  top->x = 0b01;  step_and_dump_wave();
                  top->x = 0b10;  step_and_dump_wave();
                  top->x = 0b11;  step_and_dump_wave();
  sim_exit();
}

# Encoder 編碼器
> module code

module encoder(x,e,y);
	input [3:0] x;
	input e;
	output reg [1:0] y;
	always@(*)begin 
		if (e) begin
			// statement 前都加begin
		       case(x)	
			4'b0001 : y = 2'b00;
		       	4'b0010 : y = 2'b01;
			4'b0100 : y = 2'b10;
			4'b1000 : y = 2'b11;
			default : y = 2'b00;
		endcase
		end
		 
	 
		else y = 2'b00;

	end
endmodule		

> sim.cpp

#include "verilated.h"
#include "verilated_vcd_c.h"
#include "obj_dir/Vencoder.h"

VerilatedContext* contextp = NULL;
VerilatedVcdC* tfp = NULL;

static Vencoder* top;

void step_and_dump_wave(){
  top->eval();
  // eval函數調用則代表更新各訊號狀態
  contextp->timeInc(1);
  // 增加模擬時間,數字代表時間有多少個單位時間長
  tfp->dump(contextp->time());
  // 把波形丟到vcd檔
}
void sim_init(){
  contextp = new VerilatedContext;
  tfp = new VerilatedVcdC;
  top = new Vencoder;
  contextp->traceEverOn(true);
  //啟動追蹤
  top->trace(tfp, 0);
  tfp->open("dump.vcd");
}

void sim_exit(){
  step_and_dump_wave();
  tfp->close();
}
int main (){
	 sim_init();
	 top -> e = 0b0; 
	 top -> x = 0b0001; step_and_dump_wave();
	 top -> x = 0b0010; step_and_dump_wave();
       	 top ->	x = 0b0100; step_and_dump_wave();
	 top -> x = 0b1000; step_and_dump_wave();

	 top -> e = 0b1;
	 top -> x = 0b0001; step_and_dump_wave();
         top -> x = 0b0010; step_and_dump_wave();
         top -> x = 0b0100; step_and_dump_wave(); 
         top -> x = 0b1000; step_and_dump_wave(); 
	 sim_exit();
}

>Terminal

verilator --cc --trace encoder.v  --exe sim.cpp

make -j -C obj_dir/ -f Vencoder.mk Vencoder

./obj_dir/Vencoder

gtkwave dump.vcd

# 四對二優先編碼器
> module code

module pencoder (x,e,y);
	input [3:0] x;
	input e;
	output reg [1:0] y;
	integer i;
	integer count;
	always@(*)begin // 4
		count = 0;
		y = 2'b00;
		if (e)begin // 3
			for(i=0;i<=3;i++)begin//2
				if(x[i]==1) begin//1
					count = i;
				end//1
				else y = 2'b00;
			end//2
			case (count) 
				0: y = 2'b00;
				1: y = 2'b01;
				2: y = 2'b10;
				3: y = 2'b11;
				default: y = 2'b00;
			endcase
				
				
	end //3
	end //4
endmodule

> sim.cpp

#include "verilated.h"
#include "verilated_vcd_c.h"
#include "obj_dir/Vpencoder.h"

VerilatedContext* contextp = NULL;
VerilatedVcdC* tfp = NULL;

static Vpencoder* top;
//更改

void step_and_dump_wave(){
  top->eval();
  // eval函數調用則代表更新各訊號狀態
  contextp->timeInc(2);
  // 增加模擬時間,數字代表時間有多少個單位時間長
  tfp->dump(contextp->time());
  // 把波形丟到vcd檔
}
void sim_init(){
  contextp = new VerilatedContext;
  tfp = new VerilatedVcdC;
  top = new Vpencoder;
  //更改
  contextp->traceEverOn(true);
  //啟動追蹤
  top->trace(tfp, 0);
  tfp->open("dump.vcd");
}

void sim_exit(){
  step_and_dump_wave();
  tfp->close();
}
int main (){
	 sim_init();
	 top -> e = 0b0; 
	 top -> x = 0b0001; step_and_dump_wave();
	 top -> x = 0b0010; step_and_dump_wave();
       	 top ->	x = 0b0100; step_and_dump_wave();
	 top -> x = 0b1000; step_and_dump_wave();
	 top -> x = 0b1001; step_and_dump_wave();
	 top -> x = 0b0110; step_and_dump_wave();
       	 top ->	x = 0b0101; step_and_dump_wave();
	 top -> x = 0b0011; step_and_dump_wave();

	 top -> e = 0b1;
	 top -> x = 0b0001; step_and_dump_wave();
         top -> x = 0b0010; step_and_dump_wave();
         top -> x = 0b0100; step_and_dump_wave(); 
         top -> x = 0b1000; step_and_dump_wave(); 
	 top -> x = 0b1001; step_and_dump_wave();
         top -> x = 0b0110; step_and_dump_wave();
         top -> x = 0b0101; step_and_dump_wave();
         top -> x = 0b0011; step_and_dump_wave();

	 sim_exit();
}

實驗二到此結束,接下來到 ： [實驗三](https://hackmd.io/@1p0_VOUHTXGcHlBU9A0OEA/SJac8k-oC)