4.验证：读取模拟
4.1查找代码中的错误
4.1.1 多用复路器（2_1_Mux）
//Wrong
module top_module (
input sel,
input [7:0] a,
input [7:0] b,
output out );
assign out = (~sel & a) | (sel & b);
endmodule
//Right:out位宽不对，表达式不对
module top_module (
input sel,
input [7:0] a,
input [7:0] b,
output [7:0] out);
assign out = sel ? a : b;
endmodule
4.1.2 用5与非实现3与非（NAND）
//Wrong
module top_module (input a, input b, input c, output out);//
andgate inst1 ( a, b, c, out );
endmodule
//Right:输入输出的对应关系不对，且参数数量也不对
module top_module (input a, input b, input c, output out);//
wire and_out;
andgate inst1 ( and_out, a, b, c, 1, 1);
assign out = ~and_out;
endmodule
4.1.3 四路选择器（8bit_4_1_Mux）
//Wrong
module top_module (
input [1:0] sel,
input [7:0] a,
input [7:0] b,
input [7:0] c,
input [7:0] d,
output [7:0] out ); //
wire mux0, mux1;
mux2 mux0 ( sel[0], a, b, mux0 );
mux2 mux1 ( sel[1], c, d, mux1 );
mux2 mux2 ( sel[1], mux0, mux1, out );
endmodule
//Right:sel[1]区分不了c和d，此处应该还是sel[0]。此外例化名与变量名不能重复；且wire信号的位宽也不对
module top_module (
input [1:0] sel,
input [7:0] a,
input [7:0] b,
input [7:0] c,
input [7:0] d,
output [7:0] out ); //
wire [7:0] mux00, mux11;
mux2 mux0 ( sel[0], a, b, mux00 );
mux2 mux1 ( sel[0], c, d, mux11 );
mux2 mux2 ( sel[1], mux00, mux11, out );
endmodule
4.1.4 Add/Sub
//Wrong:
// synthesis verilog_input_version verilog_2001
module top_module (
input do_sub,
input [7:0] a,
input [7:0] b,
output reg [7:0] out,
output reg result_is_zero
);//
always @(*) begin
case (do_sub)
0: out = a+b;
1: out = a-b;
endcase
if (~out)
result_is_zero = 1;
end
endmodule
//Right:因为result_is_zero为reg型，当其为1后一直为1，因为没有其他状态能使其改变，且需锁存状态，因为if未遍历所有状态
module top_module (
input do_sub,
input [7:0] a,
input [7:0] b,
output reg [7:0] out,
output reg result_is_zero
);
always @(*) begin
case (do_sub)
0: out = a+b;
1: out = a-b;
endcase
if (out == 8'd0) begin
result_is_zero = 1;
end
else begin
result_is_zero = 0;
end
end
endmodule
4.1.5 case statement
//Wrong
module top_module (
input [7:0] code,
output reg [3:0] out,
output reg valid=1 );//
always @(*)
case (code)
8'h45: out = 0;
8'h16: out = 1;
8'h1e: out = 2;
8'd26: out = 3;
8'h25: out = 4;
8'h2e: out = 5;
8'h36: out = 6;
8'h3d: out = 7;
8'h3e: out = 8;
6'h46: out = 9;
default: valid = 0;
endcase
endmodule
//Right:默认输出valid=1不能按上图所示的编写代码，默认的输入可以；其次进制8'd26需改成8进制
module top_module (
input [7:0] code,
output reg [3:0] out,
output reg valid);
always @(*) begin
case (code)
8'h45: out = 4'd0;
8'h16: out = 4'd1;
8'h1e: out = 4'd2;
8'h26: out = 4'd3;
8'h25: out = 4'd4;
8'h2e: out = 4'd5;
8'h36: out = 4'd6;
8'h3d: out = 4'd7;
8'h3e: out = 4'd8;
8'h46: out = 4'd9;
default: begin
out = 4'd0;
end
endcase
if(out == 4'd0 && code!= 8'h45) begin
valid = 1'b0;
end
else begin
valid = 1'b1;
end
end
endmodule