default case

// Simple binary encoder (input is 1-hot)

module encode (A, Y);

input [7:0] A; // 8-bit input vector

output [2:0] Y; // 3-bit encoded output

reg [2:0] Y; // target of assignment

always @(A)

case (A)

8’b00000001: Y = 0;

8’b00000010: Y = 1;

8’b00000100: Y = 2;

8’b00001000: Y = 3;

8’b00010000: Y = 4;

8’b00100000: Y = 5;

8’b01000000: Y = 6;

8’b10000000: Y = 7;

default: Y = 3’bx; // Don’t care when input isnt 1-hot

endcase

endmodule

If you omit the default, the compiler will create a latch for Y

? Or you can list all 256 cases

Better way: Use a function

? Compiler will warn you of missing cases