SystemVerilog Randomization

 

一、OOP Based Randomization

1、Two types of random properties are supported:

• rand

• randc

2、randand randcproperties are randomized when the class method randomize()is called:

• Randomize property value to full range of data type

• randomize()is built-in with every class

• 1 is returned if successful, 0 if randomization failed

3、randproperties can assume any legal value:

• Values can repeat without exhausting all possible values

4、randcproperties can be up to 16 bits:

• Exhaust all values before repeating any individual value

• For non-repeating bit values > 16 bits, use concatenation

 

二、Implication and Order Constraints

1、Implication operators:

• ->
• if ( … ) … [ else … ]
• Caution: does not imply solving order

typedef enum { low, mid, high } AddrType;
class MyBus;
  rand bit[7:0] addr;
  rand AddrType atype;
  constraint addr_range {
    (atype == low ) ->addr inside { [0:15] };
    (atype == mid ) ->addr inside { [16:127] };
    (atype == high) ->addr inside { [128:255] };
    // same as:
    // if(atype == low) addr inside { [0:15] };
    // if(atype == mid) addr inside { [16:127] };
    // if(atype == high) addr inside { [128:255] };
  }
endclass

2、Dictating solver order

• randcis solved before randproperties
• solve-beforeconstruct set solving order for same type random properties
• Can not force randto be randomized before randcproperties
• $void(rand_property)solves rand_propertiesfirst

class MyBus;
  rand bit flag;
  rand int addr;
  constraint addr_range {
    if ( flag == 0 ) addr == 0;
    else addr inside { [1:1024] };
    solveflag beforeaddr;
    // solve addr before flag; // what’s the difference?
    // alternative:
    // if ( $void(flag) == 0 ) addr == 0;
  }
endclass

 

三、Effects of Calling randomize()

1、When randomize()executes, three events occur

• pre_randomize()is called
• Randomization is executed
• post_randomize()is called

2、pre_randomize()

• Optional
• Set/Correct constraints before randomization

3、post_randomize()

• Optional
• Make corrections after randomization
• Example: CRC

class Packet;
  int test_mode;
  rand bit[3:0] sa, da;
  rand bit[7:0] payload[];
  bit[15:0] crc;
  constraint LimitA {
    sa inside { [0:7] };
    da inside { [0:7] };
    payload.size() inside { [2:4] };
  }

  function void pre_randomize();
    if(test_mode) recnstr(test_mode)
  endfunction

  function void post_randomize();
    gen_crc();
  endfunction

  virtual function void recnstr(int mode);
  endtask

endclass

 

4、Inline Constraints

• syntax: obj.randomize( ) with{ <new constraints> };

 

5、Controlling randProperty Randomization

• task/function int object_name.property.rand_mode ( 0 | 1 );

program automatic test;
  class Node;
    rand int x, y, z;
    constraint Limit1 {
      x inside {[0:16]}; y inside {[23:41]};
      z < y; z > x;
    }
  endclass

  initial begin
    Node obj1 = new();
    obj1.x.rand_mode(0);
    if (!obj1.randomize()) ...;
  end

endprogram