The .IC line is for setting transient initial conditions. It has two
different interpretations, depending on whether the UIC parameter is specified
on the .TRAN control line. Also, one should not confuse this line with the
.NODESET line. The .NODESET line is only to help dc convergence, and does not
affect final bias solution (except for multi-stable circuits). The two
interpretations of this line are as follows:
When the UIC parameter is specified on the .TRAN line, then the node
voltages specified on the .IC control line are used to compute the
capacitor, diode, BJT, JFET, and MOSFET initial conditions. This is
equivalent to specifying the IC=... parameter on each device line, but is
much more convenient. The IC=... parameter can still be specified and takes
precedence over the .IC values. Since no dc bias (initial transient)
solution is computed before the transient analysis, one should take care to
specify all dc source voltages on the .IC control line if they are to be
used to compute device initial conditions.
When the UIC parameter is not specified on the .TRAN control line, the
dc bias (initial transient) solution is computed before the transient
analysis. In this case, the node voltages specified on the .IC control line
is forced to the desired initial values during the bias solution. During
transient analysis, the constraint on these node voltages is removed. This
is the preferred method since it allows SPICE to compute a consistent dc
solution.
The voltages at different nodes of the circuit can be specified by the
following instruction: