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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。
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 @(*)beginif(!E)beginY0n = 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 begincase({A2,A1,A0})3'b000 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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: beginY0n = 1'b1; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b1; Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b1;endendcase end
end endmodule
输入描述
input A ,input B ,input Ci 输出描述
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：
D = m1+m2+m4+m7输出Co的逻辑表达式用最小项表示为：
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：
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：
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
`timescale 1ns/1nsmodule 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 @(*)beginif(!E)beginY0n = 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 begincase({A2,A1,A0})3'b000 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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 : beginY0n = 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: beginY0n = 1'b1; Y1n = 1'b1; Y2n = 1'b1; Y3n = 1'b1; Y4n = 1'b1; Y5n = 1'b1; Y6n = 1'b1; Y7n = 1'b1;endendcase end
end endmodulemodule 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。