top module for ZYBO to blink LEDs.

zybo_top.v

module zybo_top(

input             CLK,
input             RST,
output reg [3:0]  LED

);

parameter LED_COUNTER_1SEC = 26'd124999999;

reg [25:0]  counter_1sec;

always @ (posedge CLK) begin
  if (RST)
    counter_1sec <= 26'd0;
  else if (counter_1sec == LED_COUNTER_1SEC)
    counter_1sec <= 26'd0;
  else
    counter_1sec <= counter_1sec + 26'd1;
end

always @ (posedge CLK) begin
  if (RST)
    LED <= 4'b0000;
  else if (counter_1sec == LED_COUNTER_1SEC)
    LED <= ~LED;
end

endmodule