Convergence is defined as the ability to obtain a solution to a set of circuit equations within a given tolerance criteria and number of iterations. In numerical circuit simulation, the designer specifies a relative and absolute accuracy for the circuit solution and the simulator iteration algorithm attempts to converge to a solution that is within these set tolerances. In many cases, the speed of reaching a solution also is of primary interest to the designer, particularly for preliminary design trials, and some accuracy is willingly sacrificed.

Star-Hspice can substantially reduce the computer time needed to solve complex problems. The following user options alter internal algorithms to increase simulation efficiency.

- .OPTIONS FAST - sets additional options that increase simulation speed with little loss of accuracy
- .OPTIONS AUTOSTOP - terminates the simulation when all .MEASURE statements have completed. This is of special interest when testing corners.

The FAST and AUTOSTOP options are described in Understanding the Control Options.

Star-Hspice is shipped with control option default values that aim for superior accuracy while delivering good performance in simulation time. The control options and their default settings to maximize accuracy are:

DVDT = 4 LVLTIM = 1 RMAX = 5 SLOPETOL = 0.75

FT = FS = 0.25 BYPASS = 1 BYTOL = MBYPASSxVNTOL = 0.100m

The DVDT control option selects the timestep control algorithm. Relationships between DVDT and other control options are discussed in Selecting Timestep Control Algorithms.

The DELMAX control option also affects simulation accuracy. DELMAX specifies the maximum allowed timestep size. If DELMAX is not set in an .OPTIONS statement, Star-Hspice computes a DELMAX value. Factors that determine the computed DELMAX value are:

- .OPTIONS RMAX and FS
- Breakpoint locations for a PWL source
- Breakpoint locations for a PULSE source
- Smallest period for a SIN source
- Smallest delay for a transmission line component
- Smallest ideal delay for a transmission line component
- TSTEP value in a .TRAN analysis
- Number of points in an FFT analysis

The FS and RMAX control options provide some user control over the DELMAX value. The FS option, which defaults to 0.25, scales the breakpoint interval in the DELMAX calculation. The RMAX option, which defaults to 5 if DVDT = 4 and LVLTIM = 1, scales the TSTEP (timestep) size in the DELMAX calculation.

For circuits that contain oscillators or ideal delay elements, an .OPTIONS statement should be used to set DELMAX to one-hundredth of the period or less.

The ACCURATE control option tightens the simulation options to give the most accurate set of simulation algorithms and tolerances. When ACCURATE is set to 1, it sets the following control options:

Simulation accuracy relies heavily on the sophistication and accuracy of the models used. More advanced MOS, BJT, and GaAs models give superior results for critical applications. Simulation accuracy is increased by:

- Algebraic models that describe parasitic interconnect capacitances as a function of the width of the transistor. The wire model extension of the resistor can model the metal, diffusion, or poly interconnects to preserve the relationship between the physical layout and electrical property.
- MOS model parameter ACM that calculates defaults for source and drain junction parasitics. Star-Hspice uses ACM equations to calculate the size of the bottom wall, the length of the sidewall diodes, and the length of a lightly doped structure. SPICE defaults with no calculation of the junction diode. Specify AD, AS, PD, PS, NRD, NRS to override the default calculations.
- MOS model parameter CAPOP = 4 that models the most advanced charge conservation, non-reciprocal gate capacitances. The gate capacitors and overlaps are calculated from the IDS model for LEVEL 49 or 53, however, the CAPOP parameter is ignored, model parameter CAPMOD with reasonable value should be used instead.

- Analog or mixed signal circuits
- Circuits with long time constants, such as RC networks
- Circuits with ground bounce

Use the default options (DVDT = 4) for

For ideal delay elements, use one of the following:

- ACCURATE
- DVDT = 3
- DVDT = 4, and, if the minimum pulse width of any signal is less than the minimum ideal delay, set DELMAX to a value smaller than the minimum pulse width.