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ASIC-WORLD Verilog（11）过程时序控制

写在前面

        在自己准备写一些简单的verilog教程之前，参考了许多资料----Asic-World网站的这套verilog教程即是其一。这套教程写得极好，奈何没有中文，在下只好斗胆翻译过来（加了自己的理解）分享给大家。

        这是网站原文：Verilog Tutorial

        这是系列导航：Verilog教程系列文章导航

过程块和时序控制（Procedural blocks and timing controls）

  • 延时控制（Delay controls）
  • 边沿敏感的事件控制（Edge-Sensitive Event controls）
  • 电平敏感的事件控制（Level-Sensitive Event controls-Wait statements）
  • 特定事件控制（Named Events）

延时控制

        通过指定特定的仿真时间来达到延时的目的，一般语法是这样的：

#< time > < statement >;

        比如2个仿真时间单位后给复位信号赋值1；5个仿真时间单位后在给复位信号赋值0：

#2 reset = 1； //2个时间单位后赋值为1

#5 reset = 0； //5个时间单位后赋值为0

        下面是一个完整的例子，用来模拟一个复位，并通过 $monitor 来监控各个寄存器的值：

module clk_gen ();

reg clk, reset; 

initial begin
  $monitor ("TIME = %g RESET = %b CLOCK = %b", $time, reset, clk); //监控各个寄存器的值
  clk = 0; 
  reset = 0; 
  #2  reset = 1;  //2个单位后复位赋值为1
  #5  reset = 0;  //5个单位后复位赋值为0
  #10  $finish;
end 

always #1  clk =  ! clk; //每一个时间单位翻转一次时钟，即生成时钟信号，周期为2个时间单位

endmodule

        这是窗口的仿真结果：

 TIME = 0  RESET = 0 CLOCK = 0
 TIME = 1  RESET = 0 CLOCK = 1
 TIME = 2  RESET = 1 CLOCK = 0
 TIME = 3  RESET = 1 CLOCK = 1
 TIME = 4  RESET = 1 CLOCK = 0
 TIME = 5  RESET = 1 CLOCK = 1
 TIME = 6  RESET = 1 CLOCK = 0
 TIME = 7  RESET = 0 CLOCK = 1
 TIME = 8  RESET = 0 CLOCK = 0
 TIME = 9  RESET = 0 CLOCK = 1
 TIME = 10 RESET = 0 CLOCK = 0
 TIME = 11 RESET = 0 CLOCK = 1
 TIME = 12 RESET = 0 CLOCK = 0
 TIME = 13 RESET = 0 CLOCK = 1
 TIME = 14 RESET = 0 CLOCK = 0
 TIME = 15 RESET = 0 CLOCK = 1
 TIME = 16 RESET = 0 CLOCK = 0

        这是仿真结果的波形图：

边沿敏感的事件控制

        通过指定特定事件的边沿变化来控制时间（语句）的执行。一般语法是这样的：

@ (< posedge >|< negedge > signal) < statement >;

        通过时钟信号的 上升沿/下降沿 来控制某个事件的执行就是很经典的边沿敏感型事件控制语句。比如在enable信号的上升沿后的5个时钟单位后触发trigger信号为1：

always @ (posedge enable)begin 
   trigger = 0;
   repeat (5) begin    //重复5次
     @ (posedge clk) ; //在上升沿被触发
  end
   trigger = 1;         //触发其值为1
end

        这个代码可以拓展一下，并加上相应的测试脚本：


module edge_wait_example();

reg enable, clk, trigger;

//在每个enable上升沿的5个时钟后把trigger赋值为1
always @ (posedge enable)	
begin 
  trigger = 0;
  repeat (5) begin
    @ (posedge clk) ;
  end
  trigger = 1; 
end


initial begin
  $monitor ("TIME : %g CLK : %b ENABLE : %b TRIGGER : %b",
    $time, clk,enable,trigger);
  clk = 0;
  enable = 0;
  //通过延时语句分别对enable赋值
   #5   enable = 1;
   #1   enable = 0;
   #10  enable = 1;
   #1   enable = 0;
   #10  $finish;
end

always #1  clk = ~clk;

endmodule

        这是仿真结果：

 TIME : 0 CLK : 0 ENABLE : 0 TRIGGER : x
 TIME : 1 CLK : 1 ENABLE : 0 TRIGGER : x
 TIME : 2 CLK : 0 ENABLE : 0 TRIGGER : x
 TIME : 3 CLK : 1 ENABLE : 0 TRIGGER : x
 TIME : 4 CLK : 0 ENABLE : 0 TRIGGER : x
 TIME : 5 CLK : 1 ENABLE : 1 TRIGGER : 0
 TIME : 6 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 7 CLK : 1 ENABLE : 0 TRIGGER : 0
 TIME : 8 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 9 CLK : 1 ENABLE : 0 TRIGGER : 0
 TIME : 10 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 11 CLK : 1 ENABLE : 0 TRIGGER : 0
 TIME : 12 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 13 CLK : 1 ENABLE : 0 TRIGGER : 0
 TIME : 14 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 15 CLK : 1 ENABLE : 0 TRIGGER : 1
 TIME : 16 CLK : 0 ENABLE : 1 TRIGGER : 0
 TIME : 17 CLK : 1 ENABLE : 0 TRIGGER : 0
 TIME : 18 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 19 CLK : 1 ENABLE : 0 TRIGGER : 0
 TIME : 20 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 21 CLK : 1 ENABLE : 0 TRIGGER : 0
 TIME : 22 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 23 CLK : 1 ENABLE : 0 TRIGGER : 0
 TIME : 24 CLK : 0 ENABLE : 0 TRIGGER : 0
 TIME : 25 CLK : 1 ENABLE : 0 TRIGGER : 1
 TIME : 26 CLK : 0 ENABLE : 0 TRIGGER : 1

电平敏感的事件控制

        当前条件为真时才执行接下来的语句，有点类似if语句。它的一般语法是这样的：

wait (< expression >) < statement >; 

        比如当data_ready为真时，才把data_bus的值赋给data：

wait (data_ready == 1)  data = data_bus; 

        这个代码可以拓展一下，并加上相应的测试脚本：

module wait_example();

reg mem_read, data_ready;
reg [7:0] data_bus, data;

always @ (mem_read or data_bus or data_ready) begin
  data = 0;
  while (mem_read == 1'b1) begin
    wait (data_ready == 1) #1 data = data_bus;
  end
end

// Testbench Code here
initial begin
 $monitor ("TIME = %g READ = %b READY = %b DATA = %b", 
   $time, mem_read, data_ready, data);
 data_bus = 0;
 mem_read = 0;
 data_ready = 0;
 #10 data_bus = 8'hDE;
 #10 mem_read = 1;
 #20 data_ready = 1;
 #1  mem_read = 1;
 #1  data_ready = 0;
 #10 data_bus = 8'hAD;
 #10 mem_read = 1;
 #20 data_ready = 1;
 #1  mem_read = 1;
 #1  data_ready = 0;
 #10 $finish;
end

endmodule

        这是仿真结果：

 TIME = 0  READ = 0 READY = 0 DATA = 00000000
 TIME = 20 READ = 1 READY = 0 DATA = 00000000
 TIME = 40 READ = 1 READY = 1 DATA = 00000000
 TIME = 41 READ = 1 READY = 1 DATA = 11011110
 TIME = 42 READ = 1 READY = 0 DATA = 11011110
 TIME = 82 READ = 1 READY = 1 DATA = 11011110
 TIME = 83 READ = 1 READY = 1 DATA = 10101101
 TIME = 84 READ = 1 READY = 0 DATA = 10101101 

赋值内延迟语句（Intra-Assignment Timing Controls） 

        这是相对于 赋值间延迟语句（Inter-Assignment Timing Controls） 的概念，赋值间延迟语句就是我们平常最常用的延迟语句，也就是这种：

#10 rega = regb;

        这种情况下，赋值语句需要等待一定时间，然后将计算结果（右侧值）赋值给目标信号（左侧值）。 

        而赋值内延迟语句的用法则是这样的：

rega = #10 regb;

        它是先计算出右侧值，延时完成后再将结果赋给左侧。看看下面的例子：

  1 module intra_assign();
  2 
  3 reg a, b;
  4 
  5 initial begin
  6   $monitor("TIME = %g  A = %b  B = %b",$time, a , b);
  7   a = 1; 
  8   b = 0; 
  9   a = #10 0; 
 10   b = a;
 11    #20  $display("TIME = %g  A = %b  B = %b",$time, a , b);
 12   $finish;
 13 end 
 14 
 15 endmodule

        这是仿真结果：

 TIME = 0   A = 1  B = 0
 TIME = 10  A = 0  B = 0
 TIME = 30  A = 0  B = 0 

使用连续赋值语句对组合逻辑建模

        组合逻辑就是无论右侧的结果何时发生了变化，左侧的值都会同样立即发生改变。

例1 三态缓冲器

module tri_buf_using_assign();
reg data_in, enable;
wire pad;

assign pad = (enable) ? data_in : 1'bz;

initial begin
  $monitor ("TIME = %g ENABLE = %b DATA : %b PAD %b", 
    $time, enable, data_in, pad);
  #1 enable = 0;
  #1 data_in = 1;
  #1 enable = 1;
  #1 data_in = 0;
  #1 enable = 0;
  #1 $finish;
end

endmodule

        这个三态缓冲器也是经典的控制I2C、1-Wire等总线的一种方法。当enable为1时，就往总线上输出数据；当enable为0时，此时总线为高组态，就可以从总线上读取数据了。

        仿真结果：

 TIME = 0 ENABLE = x DATA : x PAD x
 TIME = 1 ENABLE = 0 DATA : x PAD z
 TIME = 2 ENABLE = 0 DATA : 1 PAD z
 TIME = 3 ENABLE = 1 DATA : 1 PAD 1
 TIME = 4 ENABLE = 1 DATA : 0 PAD 0
 TIME = 5 ENABLE = 0 DATA : 0 PAD z

例2 多路选择器

        同样的，这样还可以实现多路选择器：

module mux_using_assign();
reg data_in_0, data_in_1;
wire data_out;
reg  sel;

assign data_out = (sel) ? data_in_1 : data_in_0; 

// Testbench code here
initial begin
  $monitor("TIME = %g SEL = %b DATA0 = %b DATA1 = %b OUT = %b",
    $time,sel,data_in_0,data_in_1,data_out);
  data_in_0 = 0;
  data_in_1 = 0;
  sel = 0;
  #10 sel = 1;
  #10 $finish;
end

// Toggel data_in_0 at #1
always #1 data_in_0 = ~data_in_0;

// Toggel data_in_1 at #2
always #2 data_in_1 = ~data_in_1;

endmodule

        仿真结果很简单直观，看看就好：

 TIME = 0 SEL = 0 DATA0 = 0 DATA1 = 0 OUT = 0
 TIME = 1 SEL = 0 DATA0 = 1 DATA1 = 0 OUT = 1
 TIME = 2 SEL = 0 DATA0 = 0 DATA1 = 1 OUT = 0
 TIME = 3 SEL = 0 DATA0 = 1 DATA1 = 1 OUT = 1
 TIME = 4 SEL = 0 DATA0 = 0 DATA1 = 0 OUT = 0
 TIME = 5 SEL = 0 DATA0 = 1 DATA1 = 0 OUT = 1
 TIME = 6 SEL = 0 DATA0 = 0 DATA1 = 1 OUT = 0
 TIME = 7 SEL = 0 DATA0 = 1 DATA1 = 1 OUT = 1
 TIME = 8 SEL = 0 DATA0 = 0 DATA1 = 0 OUT = 0
 TIME = 9 SEL = 0 DATA0 = 1 DATA1 = 0 OUT = 1
 TIME = 10 SEL = 1 DATA0 = 0 DATA1 = 1 OUT = 1
 TIME = 11 SEL = 1 DATA0 = 1 DATA1 = 1 OUT = 1
 TIME = 12 SEL = 1 DATA0 = 0 DATA1 = 0 OUT = 0
 TIME = 13 SEL = 1 DATA0 = 1 DATA1 = 0 OUT = 0
 TIME = 14 SEL = 1 DATA0 = 0 DATA1 = 1 OUT = 1
 TIME = 15 SEL = 1 DATA0 = 1 DATA1 = 1 OUT = 1
 TIME = 16 SEL = 1 DATA0 = 0 DATA1 = 0 OUT = 0
 TIME = 17 SEL = 1 DATA0 = 1 DATA1 = 0 OUT = 0
 TIME = 18 SEL = 1 DATA0 = 0 DATA1 = 1 OUT = 1
 TIME = 19 SEL = 1 DATA0 = 1 DATA1 = 1 OUT = 1 

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