eldo_user

Speed and Accuracy
Time Step Control
Usage of a Fixed Time Step
More About Time Step Control
Eldo can also ‘clamp’ the time step to both a minimum value and a maximum value.
To clamp the maximum time step you want to allow, use the HMAX option. Eldo will perform
all of its normal time step control as explained in “Overview of Time Step Control Algorithms”
on page 1248, but still not take any step larger than HMAX. Use this option carefully, as setting
HMAX to a smaller value than required may force long simulation times. Unless very clear
suspicion exists that the result is corrupted due to the selected time steps being too large, this
option should not be used.
To clamp the time step to a minimum value, use the HMIN or ABSOLUTE_HMIN options.
These are potentially dangerous options: if HMIN/ABSOLUTE_HMIN is set to too large
values, it prevents Eldo from using the time step it should use to maintain the accuracy within
the specified tolerances. Unexpected results or failures are possible if using an inappropriate
HMIN/ABSOLUTE_HMIN value. The default value for HMIN/ABSOLUTE_HMIN is 1ps.
Unless speed improvements are a necessity, it is best not to increase HMIN/
ABSOLUTE_HMIN. Setting HMIN/ABSOLUTE_HMIN to lower values can be useful for
designs with very fast transitions.
To control the ‘acceleration’ of the time step when convergence is easily obtained, use the
HACC option. The overall goal of Eldo is to take the largest time step possible, while still
providing results that are within the requested tolerances (typically specified with EPS,
RELTOL, etc.). Every time a time point has been computed and accepted, Eldo will perform
some analysis and computations to determine what the next optimal time step should be for the
next time point. If convergence is easily obtained (that is, with very few iterations) and if the
LTE constraints are verified with a large margin, Eldo will attempt to take a larger time step for
the next time point by multiplying the current time step by a certain acceleration factor, as
specified by HACC. The default value for HACC is 2, which means that Eldo will attempt to
multiply the current time step by 2 at most.
Specifying a value with option HACC overrides the default value of 2. This time step
acceleration strategy is a compromise; choosing a conservative value (1.1x for example) would
provide very little speedup, but the next time point would most likely easily converge.
Alternatively, choosing a large factor (10x for example) when convergence is easily obtained is
tempting, and would in theory provide a significant speedup. However, chances are high that
the time point computed with the new (too) large time step will either not converge because of
non-linearity, or will have to be rejected because the LTE constraints will be violated. In these
cases, Eldo would then step back and recompute a new time point with a smaller time step (see
the option FT). Therefore, an aggressive value might be a worse choice than a conservative one.
The default value, 2, reflects this compromise and is a robust choice providing, on average, the
best results. Unless you have very good reasons to do so, you are advised against changing the
value of HACC.
Eldo® User's Manual, 15.3 1251