各种文件的说明：
Netlist Files：HDL code经过合成后转出的.v档，或是类比电路跑HSPICE的.sp档
Stimulus Files：HDL写的testbench.v经过value change dump转成.vcd，.vcd再经过VTRAN转成vector file (.vec)
.vec 是让软体用来餵给netlist当input pattern用的
Configuration File：用来设定软体模擬时，要擷取哪些点的哪些讯息(如电压或电流)，以储存在.out与.log
(如果是用GUI介面，可以不需要这个档案)
Technology file：NanoSim uses a tech. file in place of MOS models during simulation
.out File ：执行模擬后的產出档案，供turboWave直接读取以显示电压/电流波形
.fsdb File：执行模擬后的產出档案，供turboWave直接读取以显示电压/电流波形
.log File ：模擬结果的输出，里面有耗电情况的report
由於NanoSim是Transistor-level simulator，要执行NanoSim有几种方式
提供类比电路的spice netlist (.sp or .spi)与MOS spice module (含电压/温度规格) -- 类比模擬
提供数位电路的gate-level netlist (.v)，cell-base spice simulation module (not cell-base Verilog simulation module)与MOS spice module (含电压/温度规格) -- 数位模擬
提供类比电路的spice netlist (.sp or .spi)与数位电路的gate-level netlist (.v)，以VCS搭配NanoSim做类比与数位的混合模擬
书写testbench之前要了解测试单元的设计规范,要很清楚规范并且要有测试文档（test bench 的结构和测试情况）
以一个四位计数器为例，计数器程序如下：
//-----------------------------------------------------
// Design Name : counter
// File Name : counter.v
// Function : 4 bit up counter
// Coder : Deepak
//-----------------------------------------------------
module counter (clk, reset, enable, count)；
input clk, reset, enable；
output [3:0] count；
reg [3:0] count；
always @ (posedge clk)
if (reset == 1'b1) begin
count <= 0;
end else if ( enable == 1'b1) begin
count <= count + 1；
end
endmodule
test plan 如下：本test bench是自动检测测试，我们的 test bench 测试环境如下图所示：
待测试的设计(DUT)是 test bench中的一部分, 其中 test bench 有 clock generator, reset generator, enable logic generator, and compare logic, which basically calculate the expected count value of counter and compare the output of counter with calculated value.
测试的情况（根据设计说明确定，应尽量使测试完全）：
Reset Test : We can start with reset de-asserted, followed by asserting reset for few clock ticks and deasserting the reset, See if counter sets its output to zero.
Enable Test: Assert/deassert enable after reset is applied.
Random Assert/deassert of enable and reset.
Test bench设计过程：
1. 通式,test bench也是一个模块，其中包括module名，一般是测试设计名加_tb；进行待测设计的例化；对初始端口的初始化；时钟的设计；测试结果的输出。具体如下：
1 module counter_tb;
2 reg clk, reset, enable;
3 wire [3:0] count;
4
5 counter U0 (
6 .clk (clk),
7 .reset (reset),
8 .enable (enable),
9 .count (count)
10 );
11
12 initial begin
13 clk = 0;
14 reset = 0;
15 enable = 0;
16 end
17
18 always
19 #5 clk = !clk;
20
21 initial begin
22 $dumpfile ("counter.vcd");
23 $dumpvars;
24 end
25
26 initial begin
27 $display(" time, clk, reset, enable, count");
28 $monitor("%d, %b, %b, %b, %d",$time, clk,reset,enable,count);
29 end
30
31 initial
32 #100 $finish;
33
34 //Rest of testbench code after this line
35
36 endmodule
时钟设计的方法：
系统函数的说明：
$dumpfile is used for specifying the file that simulator will use to store the waveform, that can be used later to view using waveform viewer.
$dumpvars basically instructs the Verilog compiler to start dumping all the signals to "counter.vcd".
$display is used for printing text or variables to stdout (screen), is for inserting tab. Syntax is same as printf.
$monitor keeps track of changes to the variables that are in the list (clk, reset, enable, count). When ever anyone of them changes, it prints their value, in the respective radix specified.
$finish is used for terminating simulation after #100 time units (note, all the initial, always blocks start execution at time 0)
2. 加rest逻辑
假如rest是异步的（activate reset anytime during simulation）利用'events'实现，events can be triggered, and also monitored to see, if a event has occurred.
1 event reset_trigger;
2 event reset_done_trigger;
3
4 initial begin
5 forever begin
6 @ (reset_trigger);
7 @ (negedge clk);
8 reset = 1;
9 @ (negedge clk);
10 reset = 0;
11 -> reset_done_trigger;
12 end
13 end
code our reset logic in such a way that it waits for the trigger event "reset_trigger" to happen, when this event happens, reset logic asserts reset at negative edge of clock and de-asserts on next negative edge as shown in code below. Also after de-asserting the reset, reset logic triggers another event called "reset_done_trigger". This trigger event can then be used at some where else in test bench to sync up.
3. test case:每一个test case 在一个initial模块里，在一个测试中有多个test
case，可以利用 a event like "terminate_sim" and execute $finish only when this event is triggered. We can trigger this event at the end of test case execution. The code for $finish now could look as below.
例如 在加之前是这样的
1 initial
2 begin: TEST_CASE
3 #10 -> reset_trigger;
4 @ (reset_done_trigger);
5 @ (negedge clk);
6 enable = 1;
7 repeat (10) begin
8 @ (negedge clk);
9 end
10 enable = 0;
11 end
加上之后
1 initial
2 begin: TEST_CASE
3 #10 -> reset_trigger;
4 @ (reset_done_trigger);
5 @ (negedge clk);
6 enable = 1;
7 repeat (10) begin
8 @ (negedge clk);
9 end
10 enable = 0;
11 #5 -> terminate_sim;
12 end
4. 实现自动检验测试结果
首先功能上模拟待检测的设计。
然后加上检验逻辑，当检验逻辑结果和待检测设计结果不同时输出错误，并且停止仿真。
1 always @ (posedge clk)
2 if (count_compare != count) begin
3 $display ("DUT Error at time %d", $time);
4 $display (" Expected value %d, Got Value %d", count_compare, count);
5 #5 -> terminate_sim;
6 end
最后在所有上面的都书写完毕后将程序中的display, monitor去掉，就得到最终的test bench。
最终结果如下：
///////////////////////////////////////////////////////////////////////////
// MODULE : counter_tb //
// TOP MODULE : -- //
// //
// PURPOSE : 4-bit up counter test bench //
// //
// DESIGNER : Deepak Kumar Tala //
// //
// Revision History //
// //
// DEVELOPMENT HISTORY : //
// Rev0.0 : Jan 03, 2003 //
// Initial Revision //
// //
///////////////////////////////////////////////////////////////////////////
module counter_tb;
reg clk, reset, enable;
wire [3:0] count;
reg dut_error;
counter U0 (
.clk (clk),
.reset (reset),
.enable (enable),
.count (count)
);
event reset_enable;
event terminate_sim;
initial
begin
$display ("###################################################");
clk = 0;
reset = 0;
enable = 0;
dut_error = 0;
end
always
#5 clk = !clk;
initial?
begin
$dumpfile ("counter.vcd");
$dumpvars;
end
initial
@ (terminate_sim) begin
$display ("Terminating simulation");
if (dut_error == 0) begin
$display ("Simulation Result : PASSED");
end
else begin
$display ("Simulation Result : FAILED");
end
$display ("###################################################");
#1 $finish;
end
event reset_done;
initial
forever begin
@ (reset_enable);
@ (negedge clk)
$display ("Applying reset");
reset = 1;
@ (negedge clk)
reset = 0;
$display ("Came out of Reset");
-> reset_done;
end
initial begin
#10 -> reset_enable;
@ (reset_done);
@ (negedge clk);
enable = 1;
repeat (5)
begin
@ (negedge clk);
end
enable = 0;
#5 -> terminate_sim;
end
reg [3:0] count_compare;
always @ (posedge clk)
if (reset == 1'b1)
count_compare <= 0;
else if ( enable == 1'b1)
count_compare <= count_compare + 1;
always @ (negedge clk)
if (count_compare != count) begin
$display ("DUT ERROR AT TIME%d",$time);
$display ("Expected value %d, Got Value %d", count_compare, count);
dut_error = 1;
#5 -> terminate_sim;
end
endmodule
