This diretory (src/irsim/UserCode/) provides an example of how 
the black boxes work.
The more involved example is in dll.c and is a dll simulation.
To see what it does make irsim.user and then run it on 
dll.sim with dll.cmd. The edges of the two clocks should be aligned
by the end of the simulation and the updn signal should be switching.

The subcircuit is called by:

 x 3 upDn inClk outClk outClk1 500 dll

it means call the subcircuit dll (defined in user.c) with 3 inputs
(upDn, inClk, outClk) and 1 output (ouClk1) with output impedance 500 Ohms.

Some caveats:
The output delay is always specified in psecs.
You might have to change the MAXARGS in sim.c to if you have way too many IO's.
