// decade_counter.sv — counts 0..9 and wraps
module decade_counter (
  input  logic       clk, rst, en,
  output logic [3:0] cnt
);
  always_ff @(posedge clk) begin
    if (rst)           cnt <= 4'd0;
    else if (en)       cnt <= (cnt == 4'd9) ? 4'd0 : cnt + 4'd1;
  end
endmodule