Verilog

Verilog, standardized as IEEE 1364, is a hardware description language (HDL) used to model electronic systems. It is most commonly used in the design and verification of digital circuits at the register-transfer level of abstraction. It is also used in the verification of analog circuits and mixed-signal circuits

Syntax of verilog:-

                module module_name(variable1,variable2......);
                         input variable1;
                         ---------------
                         ---------------
                        output variableN;
                      always @(variables)
                      begin
                        ----------------------
                        instructions;
                       ----------------------
                      end
                endmodule 
                                                                                                                                               -
 Lets see one simple well known example
 Example: AND gate

  

 symbol and truth-table

>

 

// AND gate design

  module andgate(a,
                             b,
                             y);       // module declaration
  input a;     // initialising input/output variables
  input b;
  output y;
  reg y;
  always@(a,b)      // always monitor a and b
  begin
  if(a==1'b1 & b==1'b1)     // checking the condition a=b=1
  begin
    y = 1'b1;      // if yes then y=1
  end else
    y = 1'b0;     // if not y=0
  end
endmodule

OUTPUT:

                                                                                                                                                -

Example: OR Gate
symbol and truth table

 // OR Gate

module OR_gate(a,
               b,
               y);
     input a;
     input b;
     output y;
     reg y;
   always @(a,b)
   begin
     if (a==1'b0 & b==1'b0) 
     begin
     y = 1'b0;
     end else
     y = 1'b1;
     end
 endmodule  

OUTPUT:    

                                                                                                                                              -

Example: Inverter using Switch case
symbol and table

// CMOS Inverter using switch case
module cmos_inv(inv_in,
                inv_out
                );
  input inv_in;
  output inv_out;
  reg inv_out;
  always @(inv_in)
  begin
    case(inv_in)
      1'b0 : inv_out = 1'b1;
      1'b1 : inv_out = 1'b0;
    endcase
  end
endmodule




OUTPUT:

                                                                                                                                                  -
Example: Full Adder
symbol and functional table

// full adder gate-level design
module full_adder_gate(A,
                                        B,
                                        C,
                                  s_out,
                                  c_out);
      input A;
      input B;
      input C;
      output s_out;
      output c_out;
      reg s_out;
      reg c_out;
      always @(A,B,C)
      begin
        s_out = A^B^C;
        c_out = (A & B) | (B & C) | (C & A);
      end
    endmodule

// full_adder using data_flow model (behavioral model)

module full_adder_dataflow(data_ins,
                           s_out,
                           c_out
                           );
    input [2:0] data_ins;
    output s_out;
    output c_out;
    reg s_out;
    reg c_out;
    always @(data_ins)
    begin
      if(data_ins == 3'b000) 
      begin
        s_out = 1'b0;
        c_out = 1'b0;
      end if(data_ins == 3'b001)  
      begin
        s_out = 1'b1;
        c_out = 1'b0;
      end if(data_ins == 3'b010) 
      begin
        s_out = 1'b1;
        c_out = 1'b0;
      end if(data_ins == 3'b011)  
      begin
        s_out = 1'b0;
        c_out = 1'b1;
      end if(data_ins == 3'b100)
      begin
        s_out = 1'b1;
        c_out = 1'b0;
      end if(data_ins == 3'b101) 
      begin
        s_out = 1'b0;
        c_out = 1'b1;
      end if(data_ins == 3'b110)
      begin
        s_out = 1'b0;
        c_out = 1'b1;
      end if(data_ins == 3'b111)  
      begin
        s_out = 1'b1;
        c_out = 1'b1;
      end
    end
 
endmodule

OUTPUT:

                                                                                                                                                 -

Example: 2:4 Decoder

// 2x4 Decoder using if statement
module decoder_24(dec_ins,
                               dec_outs,
                               enable
                                  );
  input [1:0] dec_ins;
  input enable;
  output [3:0] dec_outs;
  reg [3:0] dec_outs;
 
  always @(enable or dec_ins)
  begin
    if (enable==1'b1) begin
      if (dec_ins == 2'b00) begin
        dec_outs = 4'b0001;
      end if (dec_ins == 2'b01) begin
        dec_outs = 4'b0010;
      end if (dec_ins == 2'b10) begin
        dec_outs = 4'b0100;
      end if (dec_ins == 2'b11) begin
        dec_outs = 4'b1000;
      end
    end
  end
endmodule

// 2x4 decoder using case statement
module decoder_case(dec_ins,
                                  dec_outs,
                                  enable);
    input [1:0] dec_ins;
    input enable;
    output [3:0] dec_outs;
    reg [3:0] dec_outs;
    always @ (enable or dec_ins)
    begin
      dec_outs = 4'b0000;
      if (enable)
        case(dec_ins)
          2'b00 :
                 dec_outs = 4'b0001;
          2'b01 :
                 dec_outs = 4'b0010;
          2'b10 :
                 dec_outs = 4'b0100;
          2'b11 :
                 dec_outs = 4'b1000;
        endcase
      end   
  endmodule

OUTPUT:
 
 
                                                                                                                                                  -

Example: 4:2 Encoder        

     // 4x2 encoder using case
module encoder_case(enc_ins,
                    enable,
                    enc_out
                    );
    input [3:0] enc_ins;
    input enable;
    output [1:0] enc_out;
    reg [1:0] enc_out;
   
    always @(enable,enc_ins)
    begin
      if (enable) begin
        case(enc_ins)
          4'b0001 : enc_out = 2'b00;
          4'b0010 : enc_out = 2'b01;
          4'b0100 : enc_out = 2'b10;
          4'b1000 : enc_out = 2'b11;
          default : enc_out = 2'b00;
        endcase
      end
    end  
  endmodule

// 4x2 Encoder using if statement
module encoder_if(enc_in,
                  enc_out,
                  enable
                  );
      input [3:0] enc_in;
      input enable;
      output [1:0] enc_out;
      reg [1:0] enc_out;
     
      always @(enable, enc_in)
      begin
        if (enable) begin
          if (enc_in == 4'b0001) begin
            enc_out = 2'b00;
          end else if (enc_in == 4'b0010) begin
            enc_out = 2'b01;
          end else if (enc_in == 4'b0100) begin
            enc_out = 2'b10;
          end else if (enc_in == 4'b1000) begin
            enc_out = 2'b11;
          end else
               enc_out = 2'b00;
        end
      end
    endmodule
           
     OUTPUT:     

                                                                                                                                                 -



Example: 4x1 Multiplexer        
          

// 4x1 mux using case
module mux_case(I0,
                I1,
                I2,
                I3,
                S,
                y);
    input I0;
    input I1;
    input I2;
    input I3;
    input [1:0] S;
    output y;
    reg y;
   
    always @(S,I0,I1,I2,I3)
    begin
      case(S)
        2'b00 : y = I0;
        2'b01 : y = I1;
        2'b10 : y = I2;
        2'b11 : y = I3;
      endcase
    end
  endmodule

OUTPUT:

 
                                                                                                                                                   -

  

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