module top (input CLK, input [3:0] COLS, inout [3:0] ROWS, output [3:0] LEDS, output TACT_LED);

//  MODULE POOLS MATRIX KEYBOARD 4x4 WITH PERIOD ~1 SEC (FOR EACH ROW)
//  AND REDIRECTED COLUMNS TO RED LEDS
//  GREEN LED TOGGLES EVERY ONE SECOND (AS POOLING PERIOD)
//  NEED FOR CHECKING CONNECTION TO MATRIX 4x4 KEYBOARD
//  IF THERE IS NO KEY PRESSED, ALL RED LEDS ARE ON
//  IF SOME KEY PRESSED, SOME LED IS NOT ON IN ONE OF FOUR PERIODS
//  ETC

  reg [1:0] row_counter = 0;
  reg [23:0] period_counter = 0;
  reg [3:0] rows_out = 0;
  reg [3:0] rows_en = 0;
  wire [3:0] rows_in;


  parameter period = 24'd12000000;

  always @ ( posedge CLK ) begin
    if (period_counter == period) begin
      period_counter <= 0;
      rows_en <= (1<<row_counter);
      row_counter <= row_counter + 1;
    end
    else
      period_counter <= period_counter + 1;
  end

  assign TACT_LED = row_counter[0];
  assign LEDS = COLS;

  SB_IO #(
    .PIN_TYPE(6'b 1010_01),
    .PULLUP(1'b 0)
  ) rows_io [3:0] (
    .PACKAGE_PIN(ROWS),
    .OUTPUT_ENABLE(rows_en),
    .D_OUT_0(rows_out),
    .D_IN_0(rows_in)
  );

endmodule