A "population count" circuit counts the number of '1's in an input vector. Build a population count circuit for a 3-bit input vector.

译：

“人口计数”电路计算输入向量中“1”的数量。为一个3位输入向量建立一个人口计数电路。

个人解法：

module top_module( 
    input [2:0] in,
    output [1:0] out );
    int i;
    always@(*)begin
        out = 2'b00;
        for(i=0;i<3;i++)begin
            if(in[i]==1'b1) 
                out = out+1;
        end     
    end
endmodule

官方解法：

module top_module (
	input [2:0] in,
	output [1:0] out
);

	// This is a function of 3 inputs. One method is to use a 8-entry truth table:
	// in[2:0] out[1:0]
	// 000      00
	// 001      01
	// 010      01
	// 011      10
	// 100      01
	// 101      10
	// 110      10
	// 111      11
	assign out[0] = (~in[2] & ~in[1] & in[0]) | (~in[2] & in[1] & ~in[0]) | (in[2] & ~in[1] & ~in[0]) | (in[2] & in[1] & in[0]);
	assign out[1] = (in[1] & in[0]) | (in[2] & in[0]) | (in[2] & in[1]);
	
	// Using the addition operator works too:
	// assign out = in[0]+in[1]+in[2];
	
	// Yet another method uses behavioural code inside a procedure (combinational always block)
	// to directly implement the truth table:
	/*
	always @(*) begin
		case (in)
			3'd0: out = 2'd0;
			3'd1: out = 2'd1;
			3'd2: out = 2'd1;
			3'd3: out = 2'd2;
			3'd4: out = 2'd1;
			3'd5: out = 2'd2;
			3'd6: out = 2'd2;
			3'd7: out = 2'd3;
		endcase
	end
	*/
	
endmodule

运行结果：