eldo_user

Speed and Accuracy
Time Step Control
To monitor this, by default .OPTION NEWACCT is set (see “.OPTION NEWACCT” in the
Eldo Reference Manual). Eldo reports interesting (and readable) statistics about the iteration
counts, average number of iterations per time step, number of time step rejections, and so on.
This can be used to double-check whether the simulator runs ‘normally’ or not. These statistics
are reported at the very end of the output .chi file. Eldo also reports some statistics in the
original SPICE style, but the newacct reporting is much more readable and useful.
Convergence failure in the Newton iterations has several possible reasons. The initial guess may
be simply too distant from the solution. This might happen if the chosen time step is ‘overoptimistic’
or if a sharp change in the circuit’s state occurs within the time step. Note that the
simulator anticipates sharp changes in the stimuli, and all ‘break’ points such as the edges in
PWL or PULSE signals force coinciding time points. Another reason for convergence failure
can be discontinuities in the model equations, or simply strong non-linearities.
Controlling the local truncation error (the error incurred while approximating the time
derivatives with finite differences) is the most conservative and rigorous way to control the time
steps. It usually provides more reliable and accurate results. This is the method Eldo selects by
default.
The rate-of-change control method uses the rate of change of the voltages to control the time
steps. The premise behind this method is to control the time steps used so that the voltages do
not change ‘too fast’. It is simpler than the LTE method, but also less accurate, and there is no
direct general relationship between the rate of change of the voltage and the actual truncation
error, at least not under all conditions. The method can, however, provide accurate results if the
rate of change is forced to remain small enough.
The iteration count method attempts to control the time step by monitoring only the rate of
convergence of the Newton iterations. The idea being that if convergence is obtained rapidly,
with just a few iterations, it probably means that the initial predicted guess was ‘good’ and,
conversely, if many iterations are required, it probably means that the guess was incorrect.
There is no attempt to estimate the truncation error. The control is entirely indirect, through the
monitoring of the iteration count. This method is the least reliable of all and not necessarily any
faster.
When selecting the time points, Eldo may also use internal heuristic rules and, for example,
adjust the time steps depending on the types of devices, the way they are connected, the scale of
the simulation time, and so on, to obtain optimal results given the requested tolerances.
As a consequence there is no guarantee that from one release to another, the exact time point
locations, or the time point density (local or global) will be the same. If the time points that Eldo
selects naturally are not convenient for one reason or another, users must add explicit control
options to alter this density. This can be done using options such as HMAX, INTERP,
OUT_STEP, OUT_RESOL, and so on. See also .PLOT in the Eldo Reference Manual.
Related Topics
Control of the Local Truncation Error (LTE)
Eldo® User's Manual, 15.3 1249