BCD Palindrome Constraint

SystemVerilog
,
1

I have following Code ::



typedef enum bit [3:0] { ZERO , ONE , TWO , THREE , FOUR , FIVE , SIX , SEVEN , EIGHT , NINE } BCD ;
  
    typedef BCD [7:0] BCD_32 ;
   
    BCD_32 b = '{ ONE , TWO , THREE , FOUR , FOUR , THREE , TWO , ONE  } ; // Palindrome Declaration !!
    BCD_32 b1;
   BCD_32 b2 = '{ ONE , TWO , THREE , FOUR , FIVE , SIX , SEVEN , EIGHT  } ;

initial begin
  
     $display("Total bits in b are %0d ",$bits(b));  
 
     $write("b is ");
    for ( int i = 7 ; i >= 0 ; i-- ) // NOTE :: Order is from 7 to 0 !!
     begin
     $write("%0s_",b[i].name() ); 
     end
     $display();
  
     b1 = BCD_32'({ << 4 { b  }  }) ; // [A] Cast Necessary ?? 
  
     $write("b1 is ");
    
    for ( int i = 7 ; i >= 0 ; i-- )
     begin
      $write("%0s_",b1[i].name() );
     end
     $display();
     
     $write("b2 is ");
     for ( int i = 7 ; i >= 0 ; i-- )
     begin
      $write("%0s_",b2[i].name() );
     end
     $display();
  
     b1 = BCD_32'({ << 4 { b2  }  }) ; // [A] Cast Necessary ??
  
     $write("b1 is ");
    for ( int i = 7 ; i >= 0 ; i-- )
     begin
      $write("%0s_",b1[i].name() );
     end
     $display();

   end

[Q1] Is cast necessary in [A] above ?

The Output to above Code works fine according to my expectation ::

Total bits in b are 32
b is ONE_TWO_THREE_FOUR_FOUR_THREE_TWO_ONE_
b1 is ONE_TWO_THREE_FOUR_FOUR_THREE_TWO_ONE_
b2 is ONE_TWO_THREE_FOUR_FIVE_SIX_SEVEN_EIGHT_
b1 is EIGHT_SEVEN_SIX_FIVE_FOUR_THREE_TWO_ONE_

Now I want to achieve the same via a Constraint


 class A ;

typedef enum bit [3:0] { ZERO , ONE , TWO , THREE , FOUR , FIVE , SIX , SEVEN , EIGHT , NINE } BCD ;
     typedef BCD [7:0] BCD_32 ;
 
   rand BCD_32 b ;

   constraint PALINDROME { b == BCD_32'( { << 4 { b }  } ) ; }

 endclass

This somehow doesn’t work ( Compilation Error ) . I tried this on 3 different licensed simulators at work ( not on EDA ) .

Error-[IUSO] Incorrect use of streaming operators
Palindrome_BCD.sv, 14
Palindrome_BCD_Constraint, “{ << 4 {this.b}}”
Streaming operators cannot be used in this context

[Q2] Is there a restriction/limitation on using Streaming Operators in Constraints ?

2
  • [A1] The cast is unnecessary. b1 us not an enumerated type— it’s a packed array whose element types are an enum. You can always write to a packed array as a whole without any casts, except when that whole is also an enumerated type.
  • [A2] It’s illegal because the streaming operator is only allowed as the source or target of an assignment. IEEE 1800-2017 section 11.4.14 Streaming operators

In reply to Have_A_Doubt: