casex example

// Priority encoder

module encode (A, valid, Y);

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

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

output valid; // Asserted when an input is not all 0’s

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

reg valid;

always @(A) begin

valid = 1;

casex (A)

8’bXXXXXXX1: Y = 0;

8’bXXXXXX10: Y = 1;

8’bXXXXX100: Y = 2;

8’bXXXX1000: Y = 3;

8’bXXX10000: Y = 4;

8’bXX100000: Y = 5;

8’bX1000000: Y = 6;

8’b10000000: Y = 7;

default: begin

valid = 0;

Y = 3’bx; // Don’t care when input is all 0’s

end

endcase

end

endmodule

casex: alternatives can include x and z

? x and z bits are not used in the evaluation