User`s guide

Input/Output Characteristics 7
-23
level is at a higher strength than the 1 level (which drives at the
Z strength).
Drive impedances which are higher than the value of DIGDRVZ
are assigned the Z strength (0). Likewise, drive impedances
lower than the value of DIGDRVF are assigned the forcing
strength (63).
Controllin
g
overdrive
During a simulation, the simulator uses only the strength range
number (0-63) to compare the driving strength of outputs. The
simulator allows you to control how much stronger an output
must be before it overdrives the other outputs driving the same
node. This is controlled with the configurable DIGOVRDRV
option. By default, DIGOVRDRV is 3, meaning that the
strength value assigned to an output must be at least 3 greater
than all other drivers before it determines the level of the
node.
The accuracy of the DIGOVRDRV strength comparison is
limited by the size of the strength range, DIGDRVZ through
DIGDRVF. The default drive range of 2 ohms to 20,000 ohms
gives strength ranges of 7.5%. The accuracy of the strength
comparison is 15%. In other words, depending on the particular
values of DRVH and DRVL, it might take as much as a factor
of 3.45 to overdrive a signal, or as little as a factor of 2.55. The
accuracy of the comparison increases as the ratio between
DIGDRVF and DIGDRVZ decreases.
Char
g
e Stora
g
e Nets
The ability to model charge storage on digital nets is useful for
engineers who are designing dynamic MOS integrated circuits.
In such circuits, it is common for the designer to temporarily
store a one or zero on a net by driving the net to the appropriate
voltage and then turning off the drive. The charge which is
trapped on the net causes the net’s voltage to remain unchanged
for some time after the net is no longer driven. The technique is
not normally used on PCB nets because sub-nanoampere input
You can set these options by
selectin
g
Setup from the
Analysis menu in Schematics.