Verilog刷题[hdlbits] ：Always case_verilog always case

题目：Always case

Case statements in Verilog are nearly equivalent to a sequence of if-elseif-else that compares one expression to a list of others. Its syntax and functionality differs from the switch statement in C.
Verilog中的Case语句几乎等同于if-else -else序列，它将一个表达式与其他表达式的列表进行比较。它的语法和功能不同于C中的switch语句。

• The case statement begins with case and each “case item” ends with a colon. There is no “switch”.
  case语句以case开头，每个“case项”以冒号结束。没有“switch”。

• Each case item can execute exactly one statement. This makes the “break” used in C unnecessary. But this means that if you need more than one statement, you must use begin … end.
  每个case项只能执行一条语句。这使得C语言中使用的“break”变得不必要。但是这意味着如果你需要多个语句，你必须使用begin…结束。

• Duplicate (and partially overlapping) case items are permitted. The first one that matches is used. C does not allow duplicate case items.
  允许重复(和部分重叠)的case项。使用第一个匹配的。C不允许重复的case项。

A bit of practice

练习

Case statements are more convenient than if statements if there are a large number of cases. So, in this exercise, create a 6-to-1 multiplexer. When sel is between 0 and 5, choose the corresponding data input. Otherwise, output 0. The data inputs and outputs are all 4 bits wide.
如果有大量的情况，Case语句比if语句更方便。因此，在本练习中，创建一个6比1的多路复用器。当sel在0 ~ 5之间时，选择相应的数据输入。否则输出0。数据输入和输出都是4位宽。

Be careful of inferring latches (See.always_if2)
小心推断latches (参见.always_if2)

组合逻辑中，如果case缺少default语句，或是自己赋值给自己的情况就会产生latch。为了避免latch产生,case需要加上default语句，覆盖所有可能。

// synthesis verilog_input_version verilog_2001
module top_module ( 
    input [2:0] sel, 
    input [3:0] data0,
    input [3:0] data1,
    input [3:0] data2,
    input [3:0] data3,
    input [3:0] data4,
    input [3:0] data5,
    output reg [3:0] out   );//

    always@(*) begin  // This is a combinational circuit
        case(sel)
            3'b000:	out=data0;
            3'b001:	out=data1;
            3'b010:	out=data2;
            3'b011:	out=data3;
            3'b100:	out=data4;
            3'b101:	out=data5;
            default:out=4'b0000;
        endcase
    end

endmodule

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25