====== "exp" Exponential time dependent value ======

===== Syntax =====

  exp (args)
  exp (iv pv td1 tau1 td2 tau2 period)

===== Purpose =====

The component value is an exponential function of time.
===== Comments =====

For voltage and current sources, this is the same as the Spice "exp" function, with some extensions.

The shape of the waveform is described by the following algorithm:

<code>
ev = _iv;
for (reltime=time; reltime>=0; reltime-=_period){
  if (reltime > _td1){
    ev += (_pv - _iv) * (1. - Exp(-(reltime-_td1)/_tau1));
  }
  if (reltime > _td2){
    ev += (_iv - _pv) * (1. - Exp(-(reltime-_td2)/_tau2));
  }
}
</code>

For other components, it gives a time dependent value.

As an extension beyond Spice, you may specify the parameters as
name=value pairs in any order.
===== Parameters =====

|iv      |Initial value.  (required)             |
|pv      |Pulsed value.  (required)              |
|td1     |Rise time delay.  (Default = 0.)       |
|tau1    |Rise time constant.  (Default = 0.)    |
|td2     |Fall time delay.  (Default = 0.)       |
|tau2    |Fall time constant.  (Default = 0.)    |
|period  |Repeat period.  (Default = infinity.)  |

===== Step control =====

This function influences time step control through both smoothness control and analog events.

The time step is limited by the active time constant, either tau1 or tau2, depending on whether it is rising or falling.

The corners in as exp waveform are considered to be analog events.  The transient analysis will attempt to control timing accuracy of these analog events.  If it is reasonable, a time step should be scheduled at these times exactly.  In any case, there should be a time step within option::dtmin of these times.