clock_gen.sv

interface clock_gen;
  timeunit  1ns/1ps;

  bit clk;

  event     start_clock_event;
  realtime  high_period_ns;
  realtime  low_period_ns;

  function automatic void start_clock(
    realtime  period_ns,
    real      duty_cycle  = 0.5
  );
    high_period_ns  = period_ns * duty_cycle;
    low_period_ns   = period_ns * (1.0 - duty_cycle);
    ->start_clock_event;
  endfunction

  initial begin
    clk = 0;
    @(start_clock_event);
    forever begin
      clk = 1;
      #(high_period_ns);
      clk = 0;
      #(low_period_ns);
    end
  end
endinterface