@User, I agree with @User, Ngspice is going to be your best option for mixed signal simulation. It's the open source successor of Berkley's Spice v3f5, which works well for analog simulation, and it also incorporated two other tools: The mixed-level extensions to ngspice is called CIDER, a mixed-level circuit and device simulator integrated into Ngspice code. And mixed-signal simulation is supported through the integration of XSPICE, a Georgia Tech project.