@User: There is also a little-known branch of ngspice (I didn't write it, but I got it into the ngspice distribution) that runs iverilog as a cosimulator simultaneously with ngspice. The downside of it is that it can only run one instance of iverilog. I have found it very useful for writing testbench stimuli and handling input and output, and letting xspice handle everything else that is a digital block. If you're willing to write a complicated verilog wrapper around all the digital parts, you can have iverilog simulate the entire digital part of the circuit while ngspice takes care of the rest.