寄存器和整数比较不起作用

Question

在SystemVerilog中，我面临一个有趣的问题，在这里，与寄存器的比较不起作用。

module VGA_Colours 
(
        input wire clk, reset,
//      input wire [3:0] swred, swgreen,
//      input wire [1:0]  swblue,
        output wire hsync, vsync,
        output wire [3:0] r, g, b
        
    );
    
    // constant declarations for VGA sync parameters
    localparam H_DISPLAY       = 640; // horizontal display area
    localparam H_L_BORDER      =  48; // horizontal left border
    localparam H_R_BORDER      =  16; // horizontal right border
    localparam H_RETRACE       =  96; // horizontal retrace
    localparam H_MAX           = H_DISPLAY + H_L_BORDER + H_R_BORDER + H_RETRACE - 1;
    localparam START_H_RETRACE = H_DISPLAY + H_R_BORDER;
    localparam END_H_RETRACE   = H_DISPLAY + H_R_BORDER + H_RETRACE - 1;
    
    localparam V_DISPLAY       = 480; // vertical display area
    localparam V_T_BORDER      =  10; // vertical top border
    localparam V_B_BORDER      =  33; // vertical bottom border
    localparam V_RETRACE       =   2; // vertical retrace
    localparam V_MAX           = V_DISPLAY + V_T_BORDER + V_B_BORDER + V_RETRACE - 1;
   localparam START_V_RETRACE = V_DISPLAY + V_B_BORDER;
    localparam END_V_RETRACE   = V_DISPLAY + V_B_BORDER + V_RETRACE - 1;
    
    wire video_on, p_tick;
    reg [9:0] ii;
    reg j;
    
    reg [3:0] red_reg, green_reg, blue_reg;
    reg [11:0] rbg;
    
    // mod-2 counter to generate 25 MHz pixel tick
    reg pixel_reg = 0;
    wire pixel_next;
    wire    pixel_tick;
    
    always @(posedge clk)
        pixel_reg <= pixel_next;
    
    assign pixel_next = ~pixel_reg; // next state is complement of current
    
    assign pixel_tick = (pixel_reg == 0); // assert tick half of the time
    
    // registers to keep track of current pixel location
    reg [9:0] h_count_reg, h_count_next, v_count_reg, v_count_next;
    
    // register to keep track of vsync and hsync signal states
    reg vsync_reg, hsync_reg;
    wire vsync_next, hsync_next;
 
    // infer registers
    always @(posedge clk)
        if(~reset)
            begin
                    v_count_reg <= 0;
                    h_count_reg <= 0;
                    vsync_reg   <= 0;
                    hsync_reg   <= 0;
                end
        else
            begin
                    v_count_reg <= v_count_next;
                    h_count_reg <= h_count_next;
                    vsync_reg   <= vsync_next;
                    hsync_reg   <= hsync_next;
                end
            
    // next-state logic of horizontal vertical sync counters
    always @*
        begin
        h_count_next = pixel_tick ? 
                       h_count_reg == H_MAX ? 0 : h_count_reg + 1
                   : h_count_reg;
        
        v_count_next = pixel_tick && h_count_reg == H_MAX ? 
                       (v_count_reg == V_MAX ? 0 : v_count_reg + 1) 
                   : v_count_reg;
                     
                     
            
                     
                     
                     
        end 
                
            
        // hsync and vsync are active low signals
        // hsync signal asserted during horizontal retrace
        assign hsync_next = h_count_reg >= START_H_RETRACE 
                            && h_count_reg <= END_H_RETRACE;
   
        // vsync signal asserted during vertical retrace
        assign vsync_next = v_count_reg >= START_V_RETRACE 
                            && v_count_reg <= END_V_RETRACE;

        // video only on when pixels are in both horizontal and vertical display region
        assign video_on = (h_count_reg < H_DISPLAY) 
                           && (v_count_reg < V_DISPLAY);

        // output signals
        assign hsync  = hsync_reg;
        assign vsync  = vsync_reg;
        assign p_tick = pixel_tick; 
          
          
          
    always @(posedge p_tick) begin
    if (~reset) begin

        rbg <= 12'b000000000000;
        ii <= 9'b0;
    end else begin

        
        if (h_count_reg == 0) begin
        
        rbg <= 12'b000000000000;
        ii <= 9'b0;
        end else if (h_count_reg == ii) begin
        ii <= ii + 9'b001010000;
        
        rbg <= rbg + 12'b000010000000;
        end


    
    end
end
// output
        assign r = (video_on) ? rbg[11:8] : 4'b0;
          assign g = (video_on) ? rbg[7:4] : 4'b0;
          assign b = (video_on) ? rbg[3:0] : 4'b0;
          

                    
endmodule

在上面的代码中，h_count_reg是0，工作正常。如果我将0更改为任何不同的数字，它就会像预期的那样工作。但是，如果我用一个变量(在我的模块顶部声明为reg[9:0] ii的"ii“)替换这个数字，代码似乎会忽略它，这是很奇怪的。用任何数字替换ii变量都可以。为什么？

TestBench文件：

module VGA_Colours_tb ();

logic clk;
reg reset;
wire hsync, vsync;
wire [3:0] r, g, b;
        
VGA_Colours scr0 (

.clk (clk),
.reset (reset),
.hsync (hsync),
.vsync (vsync),
.r (r),
.b (b),
.g (g)


);

initial begin
        clk = 0;
        forever #10 clk = ~clk;
end

 always @(posedge clk) begin
 
 #20
 
 reset <= 1'b0;
 
 #20
 
 reset <= 1'b1;
 
 
 #100000
 
 $finish;
 
 
 end



endmodule

模拟波：

​

​

从代码中可以看到，当h_count_reg为==到ii时，增加rbg和ii的值。然而，基于模拟波，它并不是这样做的h_count_reg值不等于ii，而它实际上是。

Answer 1

VGA_Colours模块中有一个逻辑错误。

下面是具有更一致缩进的代码：

   always @(posedge p_tick) begin
      if (~reset) begin
         rbg <= 12'b000000000000;
         ii  <= 9'b0;
      end else begin
         if (h_count_reg == 0) begin
            rbg <= 12'b000000000000;
            ii  <= 9'b0;
         end else if (h_count_reg == ii) begin
            rbg <= rbg + 12'b000010000000;
            ii  <= ii + 9'b001010000;
         end
      end
   end

当我运行您的模拟时，我观察到ii总是在初始重置之后为0。

代码有3条if语句。第一条if语句在模拟开始时为真，此时为reset=0。这将ii设置为0。

复位后，我看到h_count_reg=0 4次。这意味着第二个if语句是真实的4次。这使得ii = 0。

只有当if不是0时，第三个h_count_reg语句才会被计算。现在应该清楚的是，第三条if语句永远不可能是真的。这意味着ii不会增加，它将保持在0。例如，当h_count_reg=1时，(h_count_reg == ii)是false，因为ii总是0。