语法

for(<initial_condition>;<condition>;<step_assignment>)
begin
	//statements 
end

执行过程如下：

1. 指定初始循环变量值
2. 条件是否为真，条件为假则跳出循环
3. 若条件为真则执行控制语句
4. 循环变量迭代更新

示例#1：基本循环控制

module my_design;
	integer i;
	
	initial begin
		// Note that ++ operator does not exist in Verilog !
		for (i = 0; i < 10; i = i + 1) begin
			$display ("Current loop#%0d ", i);
		end
	end
endmodule

运行结果：

ncsim> run
Current loop#0 
Current loop#1 
Current loop#2 
Current loop#3 
Current loop#4 
Current loop#5 
Current loop#6 
Current loop#7 
Current loop#8 
Current loop#9 
ncsim: *W,RNQUIE: Simulation is complete.

示例#2：8位左移移位寄存器的实现

不使用for循环的实现：

module lshift_reg (
    input clk,            // Clock input
    input rstn,           // Active low reset input
    input [7:0] load_val, // Load value
    input load_en,        // Load enable
    
    output reg [7:0] op   // Output register value
);

// At posedge of clock, if reset is low set output to 0
// If reset is high, load new value to op if load_en=1
// If reset is high, and load_en=0 shift register to left
always @ (posedge clk) begin
    if (!rstn) begin
        op <= 0;
    end
    else begin
        if (load_en) begin
            op <= load_val;
        end
        else begin
            op[0] <= op[7];
            op[1] <= op[0];
            op[2] <= op[1];
            op[3] <= op[2];
            op[4] <= op[3];
            op[5] <= op[4];
            op[6] <= op[5];
            op[7] <= op[6];
        end
    end
end
endmodule

使用for循环实现：

module lshift_reg (
    input clk,           // Clock input
    input rstn,          // Active low reset input
    input [7:0] load_val,// Load value
    input load_en,       // Load enable
    
    output reg [7:0] op  // Output register value
);     

integer i;

// At posedge of clock, if reset is low set output to 0
// If reset is high, load new value to op if load_en=1
// If reset is high, and load_en=0 shift register to left
always @ (posedge clk) begin
    if (!rstn) begin
        op <= 0;
    end
    else begin
        // If load_en is 1, load the value to op
        // else keep shifting for every clock
        if (load_en) begin
            op <= load_val;
        end
        else begin
            for (i = 0; i < 8; i = i + 1) begin
                op[i+1] <= op[i];
            end
            op[0] <= op[7];
        end
    end
end
endmodule

可以看出使用for循环可以大大减少代码量，而且可以方便的扩展到不同位宽的数据，如果把位宽改成parameter类型则更加方便，只需要在使用时指定其宽度即可。

测试代码：

module tb;

reg clk;
reg rstn;
reg [7:0] load_val;
reg load_en;
wire [7:0] op;

// Setup DUT clock
always #10 clk = ~clk;

// Instantiate the design
lshift_reg u0 (
    .clk(clk),
    .rstn (rstn),
    .load_val (load_val),
    .load_en (load_en),
    
    .op (op)
);

initial begin
    // 1. Initialize testbench variables
    clk <= 0;
    rstn <= 0;
    load_val <= 8'h01;
    load_en <= 0;

    // 2. Apply reset to the design
    repeat (2) @ (posedge clk);
    rstn <= 1;
    repeat (5) @ (posedge clk);

    // 3. Set load_en for 1 clk so that load_val is loaded
    load_en <= 1;
    repeat(1) @ (posedge clk);
    load_en <= 0;

    // 4. Let design run for 20 clocks and then finish
    repeat (20) @ (posedge clk);
    $finish;
end
endmodule

仿真结果：

RTL电路：

FROM：verilog-for-loop