xilinx/xc7/tests/counter/counter_basys3.v

module top (
    input  clk,
    input rx,
    output tx,
    input [15:0] sw,
    output [15:0] led
);

    localparam BITS = 4;
    localparam LOG2DELAY = 22;

    wire bufg;
    BUFG bufgctrl(.I(clk), .O(bufg));

    reg [BITS+LOG2DELAY-1:0] counter = 0;

    always @(posedge bufg) begin
        counter <= counter + 1;
    end

    assign led[3:0] = counter >> LOG2DELAY;
    assign led[14:4] = sw[14:4];
    assign tx = rx;
    assign led[15] = ^sw;
endmodule