Sv覆盖率-Function Coverage

参考博文：https://www.cnblogs.com/-9-8/p/4414689.html和http://www.asic-world.com/systemverilog/coverage8.html#Default_bins_creation

SV采用CRT的激励形式，而判断验证进度的标准也就是覆盖率(coverage)。

覆盖率的两种指定形式：显式的，直接通过SV来指定出的，如SVA，covergroup。

                               隐式的，在验证过程中，随"register move"就可以由simulator得到的，如代码覆盖率等。

覆盖率类型：代码覆盖率---由仿真器直接提供，只能表示设计的冗余度，与spec关系不大。(toggle/expression/block)

                 功能覆盖率---与spec比较来发现，design是否行为正确，需要按verification plan来比较进度。(assertion/covergroup)

                 断言覆盖率---用于检查几个信号之间的关系，常用在查找错误。

                 漏洞率-------同一功能验证，运行多个seed，寻找bug

 

功能覆盖率的两个重要部分是采样的数据和数据被采样的时刻。数据被采样的时刻可以是1)使用wait或@来阻塞赋值，2)使用sample的方法。采样的数据则需要在代

           码中加入覆盖组(Covergroup)和仓(Bin)，覆盖组可以定义在类中，程序块，模块中。使用覆盖组之前必须先进行实例化。所以一般在类中使用覆盖组时，将

           覆盖组的实例化加在类的构造函数new()中。Covergroup和Coverpoint的命名应当尽可能的明确，这样方便检查覆盖率文件。

      class  Driver_cbs_coverage  extends  Driver_cbs;                           event  trans_ready; //@event来触发采样

                  covergroup  CovPort;                                                                    covergroup  CovPort  @(trans_ready)

                            .......                                                                                        coverpoint    ifc.cb.port;

                   endgroup                                                                                    endgroup  

                   virtual  task  post_tx(Transaction tr);

                            CovPort.sample();        //显示通过callback来回调sample函数，完成采样

                   endtask

 

SV支持自动仓的创建，但是更多地适用于2的幂次方，因为这样才有可能100%命中，最大的自动创建仓的数目是64。auto_bin_max限制自动创建仓的个数，SV这

        时，会将值域平分在各个仓。

               covergroup  CovPort;                                                                        covergroup  CovPort;

                   coverpoint  tr.port                                                                                 options.auto_bin_max = 2;  //对整个covergroup有效

                         {options.auto_bin_max=2;}   //只对此coverpoint有效                          coverpoint  tr.port;

               endgroup                                                                                         endgroup         

 

用户自定义创建仓(bin)，将coverpoint和仓分别明确的命名。

              covergroup   Covkind;                                           

                     kind:coverpoint  tr.kind{           //用kind为coverpoint命名

                                       bins zero={0};          //自定义一个名为zero的仓，值为0

                                       bins lo={[1:3],5};     //自定义一个名为lo的仓，值为[1:3]或5

                                       bins hi[]={[8:$]};     //自定义$-8个仓，8个值得范围在[8:$]

                                       bins misc=default;     //定义一个default的仓，所示所有的剩下的不关心的值

                      }

              endgroup

 

为覆盖点增加条件控制采样时间段，关键字iff 。                                                                   

              covergroup  CoverPort;                                                                      

                    coverpoint   port_value  iff(!bus_if.reset);                                      

              endgroup                                                                                                                                                                                   

为枚举类型创建仓(bin)，所有枚举类型之外的值都会被忽略，SV默认每个枚举的值一个仓。

              typedef enum{INIT, DECODE, IDLE}fsmstate_e;

                    fsmstate_e  pstate, nstate;

                    covergroup  cg_fsm;

                          coverpoint  pstate;

                    engroup

忽略某个coverpoint的某些值，覆盖率不在检查该值，关键字ignore_bins。             illegal_bins不仅忽略某个coverpoint的值，还会报错。                     

              bit [2:0]low_ports_0_5;                                                                 bit [2:0]low_ports_0_5;   

              covergroup  CoverPort;                                                                  covergroup  CoverPort;

                    coverpoint  low_ports_0_5{                                                            coverpoint  low_ports_0_5{    

                           options.auto_bin_max=4;                                                               options.auto_bin_max=4; 

                           ignore_bins hi={[6,7]};                                                                  illegal_bins hi={[6,7]};

                    }                                                                                                   }

              endgroup                                                                                      endgroup

使用cross关键字创建交叉覆盖率。                                                                  //使用串联值的方式来替换交叉覆盖

               covergroup  Covport;                                                                   covergroup   CrossBinnames;

                   port:coverpoint   tr.port                                                                                   a:coverpoint   tr.a{bins a0={0};  bins a1={1};}

                               {bins  port[] = {[0:$]};}                                                                     b:coverpoint   tr.b{bins b0={0}; bins b1={1};}

                   kind:coverpoint   tr.kind                                                                         ab:cross  a,b  {bins a0b0=binsof(a.a0)&&binsof(b.b0);

                                {bins zero={0};                                                                                             bins a1b0=binsof(a.a1)&&binsof(b.b0);

                                  bins lo={[1:3],5};  //注释同上，只创建一个仓                                                    bins b1=binsof(b.b1);}

                                  bins hi[]={[8:$]};                                                                  ab:coverpoint{tr.a, tr.b}  //用串联值得方式来替换

                                  bins misc=default;}                                                                                {bins a0b0={2'b00};bins a1b0={2'b10};}   

                   cross  kind,port{                                                                       endgroup

                                 ignore_bins  hi=binsof(port) intersect{7}; //去除port这个coverpoint的仓7

                                 ignore_bins  md=binsof(port) intersect{0} &&

                                                          binsof(kind) intersect{[9:11]};

                                 ignore_bins  lo=binsof(kind.lo);}  //去除kind这个coverpoint的lo仓         

               endgroup

对于单个的covergroup，它的覆盖率是由简单覆盖点和交叉覆盖点两部分组成的，可以通过关键字option.weight来控制整体的覆盖率倾向。

              covergroup   CovPort;

                        Kind:coverpoint   tr.kind   {bins zero={0};    option.weight=5;}

                        port:coverpoint   tr.port   {bins  port={0};    option.weight=0;}   //计算该covergroup的覆盖率时，忽略该coverpoint

                        cross kind,port {option.weight = 10;}

              endgroup

当需要编写的覆盖组之间十分接近时，可以写出一个通用的覆盖组，然后通过new函数或ref的形式来传递参数。

              bit[2:0]port;                                                                            bit[2:0]port_a,port_b;                                              

              covergroup  CoverPort (int mid);                                               covergroup  CoverPort (ref bit[2:0]port, input int mid);  //定义ref形式

                        coverpoint port{bins lo={[0:mid-1]};                                       coverpoint port{bins lo={[0:mid-1]};

                                                bins hi={mid:$};};                                                                 bins hi={mid:$};};

              endgroup                                                                                          endgroup

              CoverPort  cp;              //定义新的covergroup                              CoverPort  cpa, cpb;              //定义新的covergroup

              initial    cp=new(5);     //例化covergroup并传递参数                       initial  begin  cpa=new(port_a,4);     //例化covergroup并传递参数

                          .............                                                                                          cpa=new(port_b,4);

 

设置某个covergroup的inst_name，set_inst_name()；

 

 使用VCS等仿真工具可以追溯到每个实例的覆盖率，但此时covergroup中应该添加声明。

              covergroup  CoverLength(string comment);

                    coverpoint  tr.length;

                    option.per_instance=1;    //指定需要单个实例的覆盖率

                    option.comment=$psprintf("%m");

                    /或者指定自己的注释

                    option.comment = comment;

               endgroup

               CoverLength  cp_lo = new("Low port numbers");

 

option.at_least，在无法有效的建立bin时，通过该设置保证bin被击中N次后，就算是覆盖完全。

 

还有两个重要的命令:option.cross_num_print_missing = N;  // 让仿真工具给出所有的仓，柏阔那些没有被命中的仓(默认不会报出)

                           option.goal=N;  // 设置覆盖组或覆盖点的目标 如90等

 

在长仿真过程中：

$get_coverage：Covergroup：：get_coverage()/cgInst：Covergroup()得到某个覆盖组或者inst的coverage

$get_inst_coverage：得到特定inst的覆盖率。