描述
请使用3-8译码器和必要的逻辑门实现全减器，全减器接口图如下，A是被减数，B是减数，Ci是来自低位的借位，D是差，Co是向高位的借位。
3-8译码器代码如下，可将参考代码添加并例化到本题答案中。
cs
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输入描述
cs
input A ,
input B ,
input Ci 输出描述
cs
output wire D ,
output wire Co 解题分析
3-8译码器实现全减器
38译码器的输出实际上包含了输入A2 A1 A0组成的所有最小项，而全减器作为组合电路，其输出最终可化简为最小项的形式。
由于译码器的输出为最小项取反，而逻辑函数可以写成最小项之和的形式，故可以利用门电路和译码器实现逻辑函数。
需先列出全减器的真值表。
由真值表可得出，输出D的逻辑表达式用最小项表示为：
cs
D = m1+m2+m4+m7输出Co的逻辑表达式用最小项表示为：
cs
Co = m1+m2++m3+m7由于译码器的输出为最小项取反，下面需要将表达式中的最小项转换为最小项取反的形式。根据反演定理，转换结果如下：
cs
D = (m1'm2'm4'm7')';
Co=(m1'm2'm3'm7')''表示取反。
由上式可知，采用与非门即可实现该电路的组合逻辑输出。关键电路如下：
上图中D和Co的输入端来自同一各译码器。
结合电路图，将电路转换成Verilog代码描述如下：
cs
wire Y0_n ;
wire Y1_n ;
wire Y2_n ;
wire Y3_n ;
wire Y4_n ;
wire Y5_n ;
wire Y6_n ;
wire Y7_n ;
decoder_38 U0(
.E (1'b1),
.A0 (Ci ),
.A1 (B ),
.A2 (A ),
.Y0n (Y0_n),
.Y1n (Y1_n),
.Y2n (Y2_n),
.Y3n (Y3_n),
.Y4n (Y4_n),
.Y5n (Y5_n),
.Y6n (Y6_n),
.Y7n (Y7_n)
);
assign D = ~(Y1_n & Y2_n & Y4_n & Y7_n);
assign Co = ~(Y1_n & Y2_n & Y3_n & Y7_n);参考代码
cs
`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'b0; 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
);
wire Y0_n ;
wire Y1_n ;
wire Y2_n ;
wire Y3_n ;
wire Y4_n ;
wire Y5_n ;
wire Y6_n ;
wire Y7_n ;
decoder_38 U0(
.E (1'b1),
.A0 (Ci ),
.A1 (B ),
.A2 (A ),
.Y0n (Y0_n),
.Y1n (Y1_n),
.Y2n (Y2_n),
.Y3n (Y3_n),
.Y4n (Y4_n),
.Y5n (Y5_n),
.Y6n (Y6_n),
.Y7n (Y7_n)
);
assign D = ~(Y1_n & Y2_n & Y4_n & Y7_n);
assign Co = ~(Y1_n & Y2_n & Y3_n & Y7_n);
endmodule注：解题分析来源于网友，如有侵权，请告删之。