题解 | 用3-8译码器实现全减器

用3-8译码器实现全减器

https://www.nowcoder.com/practice/4a8f2e5058554cea9c1cb2ac8bdea0a7

`timescale 1ns/1ns

module decoder_38(
   input             E      ,
   input             A0     ,
   input             A1     ,
   input             A2     ,
   
   output reg       Y0n    ,  
   output reg       Y1n    , 
   output reg       Y2n    , 
   output reg       Y3n    , 
   output reg       Y4n    , 
   output reg       Y5n    , 
   output reg       Y6n    , 
   output reg       Y7n    
);

always @(*)begin
   if(!E)begin
      Y0n = 1'b1;
      Y1n = 1'b1;
      Y2n = 1'b1;
      Y3n = 1'b1;
      Y4n = 1'b1;
      Y5n = 1'b1;
      Y6n = 1'b1;
      Y7n = 1'b1;
   end  
   else begin
      case({A2,A1,A0})
         3'b000 : begin
                     Y0n = 1'b0; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b1; 
                     Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b1;
                  end 
         3'b001 : begin
                     Y0n = 1'b1; Y1n = 1'b0; Y2n = 1'b1; Y3n = 1'b1; 
                     Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b1;
                  end 
         3'b010 : begin
                     Y0n = 1'b1; Y1n = 1'b1; Y2n = 1'b0; Y3n = 1'b1; 
                     Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b1;
                  end 
         3'b011 : begin
                     Y0n = 1'b1; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b0; 
                     Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b1;
                  end 
         3'b100 : begin
                     Y0n = 1'b1; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b1; 
                     Y4n = 1'b0; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b1;
                  end 
         3'b101 : begin
                     Y0n = 1'b1; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b1; 
                     Y4n = 1'b1; Y5n = 1'b0; Y6n = 1'b1; Y7n = 1'b1;
                  end 
         3'b110 : begin
                     Y0n = 1'b1; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b1; 
                     Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b0; Y7n = 1'b1;
                  end 
         3'b111 : begin
                     Y0n = 1'b1; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b1; 
                     Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b0;
                  end 
         default: begin
                     Y0n = 1'b1; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b1; 
                     Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b1;
                  end
      endcase  
   end 
end    
     
endmodule

module decoder1(
   input             A     ,
   input             B     ,
   input             Ci    ,
   
   output wire       D     ,
   output wire       Co         
);
reg      reg_D ;
reg      reg_Co; 
wire Y0n  ;
wire Y1n  ;
wire Y2n  ;
wire Y3n  ;
wire Y4n  ;
wire Y5n  ;
wire Y6n  ;
wire Y7n  ;

decoder_38 decoder_38_inst0(
   .E   (1'b1)  ,
   .A0  (Ci)     ,
   .A1  (B)     ,
   .A2  (A)    ,

   .Y0n (Y0n)   ,  
   .Y1n (Y1n)   , 
   .Y2n (Y2n)   , 
   .Y3n (Y3n)   , 
   .Y4n (Y4n)   , 
   .Y5n (Y5n)   , 
   .Y6n (Y6n)   , 
   .Y7n (Y7n) 
);
assign Co= reg_Co;
assign D= reg_D;
always@(*)begin
      case({Y7n,Y6n,Y5n,Y4n,Y3n,Y2n,Y1n,Y0n})
         8'b11111110 : begin
                    reg_D = 0;reg_Co = 0;
                  end 
         8'b11111101 : begin
                    reg_D = 1;reg_Co = 1;
                  end 
         8'b11111011 : begin
                    reg_D = 1;reg_Co = 1;
                  end 
         8'b11110111 : begin
                    reg_D = 0;reg_Co = 1;
                  end 
         8'b11101111 : begin
                    reg_D = 1;reg_Co = 0;
                  end 
         8'b11011111 : begin
                    reg_D = 0;reg_Co = 0;
                  end 
         8'b10111111 : begin
                    reg_D = 0;reg_Co = 0;
                  end 
         8'b01111111: begin
                    reg_D = 1;reg_Co = 1;
                  end 
         default: begin
                    reg_D = 0;reg_Co = 0;
                  end
      endcase  
   end 







endmodule

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