As an aid to defining a circuit, repetitive circuitry can be enclosed in a subcircuit definition and used as multiple instances in the same circuit. Before the simulation runs, the circuit is expanded to a flat netlist by replacing each invocation of a subcircuit with the circuit elements in the subcircuit definition. There is no limit on the size or complexity of subcircuits.
The end of a subcircuit definition must be a .ends directive.
Here is an example using a subcircuit:
*
* This is the circuit definition
X1 a b 0 divider
V1 a 0 pulse(0 1 0 .5µ .5µ 0 1µ)
* this is the definition of the subcircuit
.subckt divider n1 n2 n3
r1 n1 n2 1k
r2 n2 n3 1k
.ends
.tran 3µ
.end
Which runs after expanding to
* Expand X1 into two resistor network
r:1:1 a b 1k
r:1:2 b 0 1k
*
v1 a 0 pulse(0 1 0 .5µ .5µ 0 1µ)
.tran 3µ
.end
Note that unique names based on the subcircuit name and the subcircuit definition element names are made for the circuit elements inserted by subcircuit expansion.