Controlling Simulation Speed and Accuracy

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.

Simulation Speed

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

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

Simulation Accuracy

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

NOTE: BYPASS is only turned on (set to 1) when DVDT = 4. For other DVDT settings, BYPASS is off (0). SLOPETOL is set to 0.75 when DVDT = 4 and LVLTIM = 1. For all other values of DVDT or LVLTIM, SLOPETOL defaults to 0.5.

Timestep Control for Accuracy

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:

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:

DVDT = 2
RELVAR = 0.2

ABSVAR = 0.2

FT = FS = 0.2
RMAX = 2
RELMOS = 0.01


Models and Accuracy

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:

Guidelines for Choosing Accuracy Options

Use the ACCURATE option for

Use the default options (DVDT = 4) for

For ideal delay elements, use one of the following:

Star-Hspice Manual - Release 2001.2 - June 2001