Case statement in verilog

问题:

I came across priority encoder design and found out a new way to do it using a case statement. The only thing that is confusing is, does a case statement give priority to cases? Example:

case(1'b1)                                
  A[3]: Y<=4'b1000;             
  A[2]: Y<=4'b0100;  
  A[1]: Y<=4'b0010;  
  A[0]: Y<=4'b0001;  
  default:Y<=4'b0000;
endcase

---

Here if I give A as 1111 Y gets 1000 i.e it gives priority to the first case statement. Why is this so?

回答1:

Yes, there is a priority, based off of the order. According to the Verilog-2001 spec, section 9.5:

The case item expressions shall be evaluated and compared in the exact order in which they are given. During the linear search, if one of the case item expressions matches the case expression given in parentheses, then the statement associated with that case item shall be executed.

回答2:

I would refactor this to :

casez(A)                                
  4'b1???: Y<=4'b1000;             
  4'b01??: Y<=4'b0100;  
  4'b001?: Y<=4'b0010;  
  4'b0001: Y<=4'b0001;  
  default: Y<=4'b0000;
endcase

Then there is no need to worry about priority, each match is unique.

From IEEE Std 1800-2009 (IEEE STANDARD FOR SYSTEMVERILOG)

12.5.2 Constant expression in case statement
A constant expression can be used for the case_expression. The value of the constant expression shall be compared against the case_item_expressions.

The following example demonstrates the usage by modeling a 3-bit priority encoder:

logic [2:0] encode ;

case (1)
  encode[2] : $display("Select Line 2") ;
  encode[1] : $display("Select Line 1") ;
  encode[0] : $display("Select Line 0") ;
  default $display("Error: One of the bits expected ON");
endcase

12.5.3 unique-case, unique0-case, and priority-case
The case, casez, and casex keywords can be qualified by priority, unique, or unique0 keywords to perform certain violation checks. These are collectively referred to as a priority-case, unique-case or unique0-case. A priority-case shall act on the first match only. Unique-case and unique0-case assert that there are no overlapping case_items and hence that it is safe for the case_items to be evaluated in parallel.

...

priority casez(a) // values 4,5,6,7 cause a violation report 
  3’b00?: $display("0 or 1");
  3’b0??: $display("2 or 3");
endcase

I am not sure how well supported the priority case statements are supported by synthesis tools though.