In circuit simulation, Relaxation-based algorithms have been proven to be faster and more flexible than the standard direct approach used in SPICE. ITA (Iterated Timing Analysis) algorithm has even been widely used in industry. This paper describes accelerating techniques that enable ITA to utilize circuits' multi-rate behaviors. These techniques are based on the latency property of subcircuits and the Strength of Signal Flow (SSF) between subcircuits. A more powerful technique using both latency and SSF is also proposed. Experimental examples are given to justify the superior properties of proposed methods.

The Selective-tracing Waveform Relaxation (STWR) algorithm, which combines good properties of traditional relaxation-based algorithms including WR and ITA, has been discussed. STWR is basically a WR algorithm, but it selectively traces and schedules subcircuits for one individual calculation.- rate behaviors, uses dynamical windows (controlled by the convergence situations) in each WR iterations, and spends fewer number of subcircuit calculations in most test circuits. A new global feedback loop controlling method for STWR is proposed in this paper. Properties of proposed methods have been tested by real experiments.