title: Modelsim time for falling/rising edge
body: <p>Is there a way do define (in modelsim) the time which a signal needs to go from low-high(rising)/high-low(falling) edge. 
  For example to simulate more advanced I would like to define a time for going from low to high at a a single signal clk(e.g. 1 ns). </p>
  
  <p>Is there a way to do that?</p>