Verilog testbench编写进阶（3）--$strobe，$write

在前面的几节课程中，我们介绍了$display，$monitor 系统任务，本文介绍的$strobe任务也是激励文件中经常使用的系统任务，它可以很好的帮助我们观察运行结果，定位运行中出现的问题。需要注意的是$strobe任务只能用在仿真程序中，是不可综合的任务，因此不能在实体程序中使用。$strobe 任务的使用方法和$display非常类似，但是他们之间还是有些不同的。两者的区别在于：$strobe命令会在当前时间结束时执行；而$display是只要仿真器看到就会立即执行。

1.$strobe

$strobe系统任务和$display 系统任务的语法基本上是相同的。$strobe 也是在控制台窗口下，打印输出。它是在当前的时间点结束时执行，而不是解析到当前的$strobe 立即执行。

<span style="color: #0000ff;">$strobe(“字符区” ，表达式1，表达式2，...);</span>

1	<span style="color: #0000ff;">$strobe(“字符区” ，表达式1，表达式2，...);</span>

例1：首先，我们使用$display 做一个仿真程序

`timescale 1ns / 1ps
module tb_dis ();
reg [3:0] a,b;
wire [4:0] y = a + b;
initial begin
a = 3;
b = 2;
$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 4;
$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
b = 5;
$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 3;
b = 7;
$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 2;
b = 6;
$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
end
endmodule

`timescale 1ns / 1ps

module tb_dis ();

reg [3:0] a,b;

wire [4:0] y = a + b;

initial begin

a = 3;

b = 2;

$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 4;

$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

b = 5;

$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 3;

b = 7;

$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 2;

b = 6;

$display("$display: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

end

endmodule

在modelsim transcript 窗口列表显示如下：

<span style="color: #0000ff;"># $display: time =  0, a= 3, b= 2, y= x 
<strong># $display: time = 10, a= 4, <span style="color: #ff0000;">b= 2</span>, y= 5 </strong>
<strong># $display: time = 10, a= 4, b= 5, y= 5</strong> 
# $display: time = 20, a= 3, b= 7, y= 9 
# $display: time = 30, a= 2, b= 6, y= 10  </span>

# $display: time = 0, a= 3, b= 2, y= x

# $display: time = 10, a= 4, b= 2, y= 5

# $display: time = 10, a= 4, b= 5, y= 5

# $display: time = 20, a= 3, b= 7, y= 9

# $display: time = 30, a= 2, b= 6, y= 10

我们会发现，仿真程序中在10ns 的位置使用$display 打印输出了两次，第一次时，a = 4，b的值保持0ns 的值；第二次时，a = 4，b = 5值；这说明$display 对已经执行过的赋值都能正确打印出来，但对于 y = a+b 并没有将其计算的结果输出出来。

例2：使用$strobe 测试阻塞赋值

`timescale 1ns / 1ps
module tb_dis ();
reg [3:0] a,b;
wire [4:0] y = a + b;
initial begin
a = 3;
b = 2;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 4;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
b = 5;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 3;
b = 7;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 2;
b = 6;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
end
endmodule

`timescale 1ns / 1ps

module tb_dis ();

reg [3:0] a,b;

wire [4:0] y = a + b;

initial begin

a = 3;

b = 2;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 4;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

b = 5;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 3;

b = 7;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 2;

b = 6;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

end

endmodule

测试结果如下：

<span style="color: #0000ff;">
# $strobe: time =  0, a= 3, b= 2, y= 5 
<strong># $strobe: time = 10, a= 4, b= 5, y= 9 </strong>
<strong># $strobe: time = 10, a= 4, b= 5, y= 9</strong> 
# $strobe: time = 20, a= 3, b= 7, y= 10 
# $strobe: time = 30, a= 2, b= 6, y= 8
</span>

# $strobe: time = 0, a= 3, b= 2, y= 5

# $strobe: time = 10, a= 4, b= 5, y= 9

# $strobe: time = 10, a= 4, b= 5, y= 9

# $strobe: time = 20, a= 3, b= 7, y= 10

# $strobe: time = 30, a= 2, b= 6, y= 8

同样，我们只是改造上面的仿真程序，将$display 替换为 $strobe 任务，发现在10ns 位置也是打印两次，这两次打印的结果相同。表明当10ns结束时，$strobe 才进行的打印，这时： a = 4, b =5 ,同时将 y = a + b 的值也是计算完后，才打印输出的。

例2：使用$strobe 任务测试非阻塞赋值

`timescale 1ns / 1ps
module tb_dis ();
reg [3:0] a,b;
wire [4:0] y = a + b;
initial begin
a &lt;= 3;
b &lt;= 2;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a &lt;= 4;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
b &lt;= 5;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a &lt;= 3;
b &lt;= 7;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a &lt;= 2;
b &lt;= 6;
$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
end
endmodule

`timescale 1ns / 1ps

module tb_dis ();

reg [3:0] a,b;

wire [4:0] y = a + b;

initial begin

a <= 3;

b <= 2;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a <= 4;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

b <= 5;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a <= 3;

b <= 7;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a <= 2;

b <= 6;

$strobe("$strobe: time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

end

endmodule

控制台输出结果：

<span style="color: #0000ff;"># $strobe: time =  0, a= 3, b= 2, y= 5 </span>
<span style="color: #0000ff;"># $strobe: time = 10, a= 4, b= 5, y= 9 </span>
<span style="color: #0000ff;"># $strobe: time = 10, a= 4, b= 5, y= 9 </span>
<span style="color: #0000ff;"># $strobe: time = 20, a= 3, b= 7, y= 10 </span>
<span style="color: #0000ff;"># $strobe: time = 30, a= 2, b= 6, y= 8 </span>

# $strobe: time = 0, a= 3, b= 2, y= 5

# $strobe: time = 10, a= 4, b= 5, y= 9

# $strobe: time = 20, a= 3, b= 7, y= 10

# $strobe: time = 30, a= 2, b= 6, y= 8

同样，我们会发现：不论是阻塞赋值还是非阻塞赋值， $strobe 都是在当前时间点结束时，打印输出结果。

从上面演示的例子可以看出，当打印输出连续赋值语句或module例化的输出结果时，在当前的时间点内，$display无法输出当前运算结果，但 $strobe却可以显示组合逻辑结果。因此，如果希望在当前时间点内打印连续赋值语句或module例化的输出结果推荐使用$strobe, 而其他情况下使用$display可以显示更多的细节。

2.$write

$write系统任务和$display 系统任务的语法基本上是相同的。$write和$display 的区别是$write 在打印输出时，没有换行的操作。

<span style="color: #0000ff;">$write(“字符区” ，表达式1，表达式2，...);</span>

1	<span style="color: #0000ff;">$write(“字符区” ，表达式1，表达式2，...);</span>

例4：将例1 改造为$write 打印输出：

`timescale 1ns / 1ps
module tb_dis ();
reg [3:0] a,b;
wire [4:0] y = a + b;
initial begin
a = 3;
b = 2;
$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 4;
$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
b = 5;
$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 3;
b = 7;
$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
#10
a = 2;
b = 6;
$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);
end
endmodule

`timescale 1ns / 1ps

module tb_dis ();

reg [3:0] a,b;

wire [4:0] y = a + b;

initial begin

a = 3;

b = 2;

$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 4;

$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

b = 5;

$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 3;

b = 7;

$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

#10

a = 2;

b = 6;

$write ("$write : time =%05d, a= %d, b=%d, y= %d ",$time ,a, b, y);

end

endmodule

控制台输出的结果为：

<span style="color: #0000ff;">$write: <strong>time = 0</strong>, a= 3, b= 2, y= x $write: <strong>time = 10</strong>, a= 4, b= 2, y= 5 $write: <strong>time = 10</strong>, a= 4, b= 5, y= 5 $write: <strong>time = 20</strong>, a= 3, b= 7, y= 9 $write: <strong>time = 30</strong>, a= 2, b= 6, y= 10  </span>

$write: time = 0, a= 3, b= 2, y= x $write: time = 10, a= 4, b= 2, y= 5 $write: time = 10, a= 4, b= 5, y= 5 $write: time = 20, a= 3, b= 7, y= 9 $write: time = 30, a= 2, b= 6, y= 10

这里我们看到使用$write 任务打印输出时，没有回车，换行的操作，在同一行内打印所有输出结果。

例5：将例1 改造为$write 任务（手工添加换行）打印输出：

`timescale 1ns / 1ps
module tb_dis ();
reg [3:0] a,b;
wire [4:0] y = a + b;
initial begin
a = 3;
b = 2;
$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);
#10
a = 4;
$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);
b = 5;
$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);
#10
a = 3;
b = 7;
$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);
#10
a = 2;
b = 6;
$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);
end
endmodule

`timescale 1ns / 1ps

module tb_dis ();

reg [3:0] a,b;

wire [4:0] y = a + b;

initial begin

a = 3;

b = 2;

$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);

#10

a = 4;

$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);

b = 5;

$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);

#10

a = 3;

b = 7;

$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);

#10

a = 2;

b = 6;

$write ("$write : time =%05d, a= %d, b=%d, y= %d \n",$time ,a, b, y);

end

endmodule

控制台输出的结果为：

<span style="color: #0000ff;"># $write: time =  0, a= 3, b= 2, y= x </span>
<span style="color: #0000ff;"><strong># $write: time = 10, a= 4, b= 2, y= 5 </strong></span>
<span style="color: #0000ff;"><strong># $write: time = 10, a= 4, b= 5, y= 5</strong> </span>
<span style="color: #0000ff;"># $write: time = 20, a= 3, b= 7, y= 9 </span>
<span style="color: #0000ff;"># $write: time = 30, a= 2, b= 6, y= 10 </span>

# $write: time = 0, a= 3, b= 2, y= x

# $write: time = 10, a= 4, b= 2, y= 5

# $write: time = 10, a= 4, b= 5, y= 5

# $write: time = 20, a= 3, b= 7, y= 9

# $write: time = 30, a= 2, b= 6, y= 10

当手工添加\n后，打印输出的结果和$display相同了。

3.格式化输出列表


%h或%H	以十六进制的形式输出
%d或%D	以十进制的形式输出
%o或%O	以八进制的形式输出
%b或%B	以二进制的形式输出
%c或%C	以ASCII码字符的形式输出
%s或%S	以字符串的形式输出
%t或%T	以当前的时间格式的形式输出
%e或%E	以指数的形式输出实型数
%f或%F	以十进制的形式输出实型数
%g或%G	以指数或者十进制的形式输出实型数，无论何种格式都以较短的结果输出
%x	十六进制
%u	十六进制

\n	换行
\t	相当于按一个Tab键
\\	反斜杠字符\
\”	双引号字符”
%%	百分符号%

1、格式说明，由”%”和格式字符组成。它的作用是将输出的数据转换成指定的格式输出。格式说明总是由”%”字符开始的。

2、普通字符，即需要原样输出的字符。其中包括一些转义字符，下面的字符形式用于格式字符串参数中，用来显示特殊的字符。

在 Verilog 中，转义符一定是使用在双引号括起来的字符串中！

举例：

`timescale 1ns / 1ps
//`define VIVADO_ONLY
module tb_dis ();
localparam WIDTH = 8;
real r; 
integer i;
reg  [WIDTH - 1:0] a,b,c;
wire [WIDTH:0] y = a + b;
initial begin
a = 2;
b = 5;
c = 9;
$display("===&gt; display print format testing.... &lt;===");
#10;
$strobe ("$strobe: time =%05d, a= %h, b=%d, c=%o, y= %b ",$time ,a, b, c, y);
#10;
$strobe ("$strobe: time =%05d, a= %H, b=%D, c=%O, y= %B ",$time ,a, b, c, y);
#10;
$strobe ("$strobe: time =%05d, char = %c, char = %C  ",$time ,"M", "q");
#10;
$strobe ("$strobe: time =%05d, %s, %S ",$time , "Hello World !", "Hello World !");
#10;
$strobe ("$strobe: time =%t, %T ",$time ,$time);
#10;
r = 31415.926;
$strobe ("$strobe: time =%05d, r = %e, r = %E ",$time ,r, r);
#10;
r = 31415.926;
$strobe ("$strobe: time =%05d, r = %f, r = %F ",$time ,r, r);
#10;
r = 31415.926;
$strobe ("$strobe: time =%05d, r = %g, r = %G ",$time ,r, r);
#10;
$strobe ("$strobe: time =%05d, a = %x",$time , r);
#10;
a = 33;
b = 'o47;
c = 8'hfe;
i = -56;
$strobe ("$strobe: time =%05d, a = %u, b = %u, c = %u",$time , a, i, c);
#10;
$strobe ("$strobe: time =%05d, Hello \n World !", $time);
#10;
$strobe ("$strobe: time =%05d, \t\t\tHello World !", $time);
#10;
$strobe ("$strobe: time =%05d, \\Hello World !\\", $time);
#10;
$strobe ("$strobe: time =%05d, \"Hello World !\"", $time);
#10;
$strobe ("$strobe: time =%05d, Hello World ! 100%%", $time);
#10;
$display("===&gt; display print format test finished. &lt;===");
#10;
end
/*
addab  #(.WIDTH (WIDTH))
addab_inst
(
.a (a),
.b (b),
.y (y)
);
*/ 
endmodule

`timescale 1ns / 1ps

//`define VIVADO_ONLY

module tb_dis ();

localparam WIDTH = 8;

real r;

integer i;

reg [WIDTH - 1:0] a,b,c;

wire [WIDTH:0] y = a + b;

initial begin

a = 2;

b = 5;

c = 9;

$display("===> display print format testing.... <===");

#10;

$strobe ("$strobe: time =%05d, a= %h, b=%d, c=%o, y= %b ",$time ,a, b, c, y);

#10;

$strobe ("$strobe: time =%05d, a= %H, b=%D, c=%O, y= %B ",$time ,a, b, c, y);

#10;

$strobe ("$strobe: time =%05d, char = %c, char = %C ",$time ,"M", "q");

#10;

$strobe ("$strobe: time =%05d, %s, %S ",$time , "Hello World !", "Hello World !");

#10;

$strobe ("$strobe: time =%t, %T ",$time ,$time);

#10;

r = 31415.926;

$strobe ("$strobe: time =%05d, r = %e, r = %E ",$time ,r, r);

#10;

r = 31415.926;

$strobe ("$strobe: time =%05d, r = %f, r = %F ",$time ,r, r);

#10;

r = 31415.926;

$strobe ("$strobe: time =%05d, r = %g, r = %G ",$time ,r, r);

#10;

$strobe ("$strobe: time =%05d, a = %x",$time , r);

#10;

a = 33;

b = 'o47;

c = 8'hfe;

i = -56;

$strobe ("$strobe: time =%05d, a = %u, b = %u, c = %u",$time , a, i, c);

#10;

$strobe ("$strobe: time =%05d, Hello \n World !", $time);

#10;

$strobe ("$strobe: time =%05d, \t\t\tHello World !", $time);

#10;

$strobe ("$strobe: time =%05d, \\Hello World !\\", $time);

#10;

$strobe ("$strobe: time =%05d, \"Hello World !\"", $time);

#10;

$strobe ("$strobe: time =%05d, Hello World ! 100%%", $time);

#10;

$display("===> display print format test finished. <===");

#10;

end

addab #(.WIDTH (WIDTH))

addab_inst

(

.a (a),

.b (b),

.y (y)

);

endmodule

控制台输出：

对应视频：

Verilog testbench编写进阶（3）–$strobe，$write

1.$strobe

2.$write

3.格式化输出列表

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Verilog testbench编写进阶（3）–$strobe，$write

1.$strobe

2.$write

3.格式化输出列表

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