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How TSTEP Affects a Transient Simulation
How does the value of TSTEP affect my transient simulation?
9 {& e# c* z9 ITSTEP is specified in the transient analysis netlist command.& }3 O$ z `8 S
.TRAN TSTEP1 TSTOP1 TSTEP2 TSTOP2 ... TSTEPN TSTOPN
G7 d5 h5 i% G+ M3 P! M5 UThe most common usage is a single TSTEP/TSTOP pair. For example:
4 \4 f# |. c& _.TRAN 0.1ps 100ns* s9 _+ U; T M5 k, k; V: s
TSTEP has a variety of effects on the operation of HSPICE.
( j9 u( y' Q% H- r1. When the RUNLVL algorithm is in use:9 _! K" s+ I& U; v
- RUNLVL "in use" is defined as HSPICE version >= 2004.09 and OPTION RUNLVL > 0.1 t: D6 ?& A0 G! z
For HSPICE version >= 2006.09, RUNLVL=3 by default.2 K* N; n6 D1 }! v9 R* z z7 q
- The minimum timestep is determined by the RUNLVL algorithm. The RMIN option is ignored.
z0 l* f; y( Y/ J* r( w: Z2 T - The maximum timestep has a "soft" limit of RMAX*TSTEP. The RUNLVL algorithm/ P8 \. H; b( U+ B
allows the timestep to exceed RMAX*TSTEP as long as tighter Newton-Raphson+ F) u; ]* l/ O5 {* W6 T$ S
tolerances are met (voltage, current, charge).The default value of RMAX is 5.
6 K# I; B5 S. I _8 K4 g, ` - The DELMAX option can be used to override the RUNLVL algorithm's choice of! b& C q5 |& h# N, j' C% @
maximum timestep. When DELMAX is used, TSTEP has no impact on simulation accuracy (except; B7 v/ P; N+ O6 @
when used in conjunction with .OPTION INTERP, see #3 below).
* v: F6 B5 h/ Z, x: h; E5 h1 M2. When RUNLVL=0 (discouraged):! z5 {! b9 ]9 O- l7 o# g- A9 e
- The minimum timestep is defined as TSTEP*RMIN. The default value of RMIN is 1e-9.& U3 C( H! g" y/ d
- The maximum timestep is defined as TSTEP*RMAX. The default value of RMAX is 5.
* R M1 _" D5 z - These limits are "hard" limits.9 s' j4 W7 B# _, t" x
- If the simulation engine attempts to choose a timestep < TSTEP*RMIN, a
; v$ D9 H/ \- _ Y Timestep Too Small Error will result. f: d/ l4 F! g* N- W/ q f
- The simulation engine is never allowed to take a larger timestep than
2 g2 G) O( |. P. e TSTEP*RMAX.* t$ [- R8 j( _ t( r- k* T5 s1 t
Summary! x( W0 o9 g% r; X6 {: S0 h
With the introduction of the RUNLVL algorithm, the impact of TSTEP has been
$ i$ C. |/ Y6 J0 f; I2 Egreatly reduced to the point that it can be ignored except for niche applications' p+ a& J$ ?9 j
like INTERP. One should still be careful not to set TSTEP too small for long,0 }6 p& T: Z6 X! B; a# s
low frequency simulations in which the soft timestep upper limit of RMAX*TSTEP
! e) ^! [4 X" z. n7 ~: |9 ~may still force more timestep solutions than is necessary which will cause" t& C9 g3 @( r) I4 @2 }6 N1 R! z. f
excessive simulation time. |
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