User`s guide

7
-
22 Di
g
ital Device Modelin
g
Confi
g
urin
g
the stren
g
th scale
The 64 strengths are determined by two configurable options:
DIGDRVZ and DIGDRVF. DIGDRVZ defines the impedance
of the Z strength, and DIGDRVF defines the impedance of the
forcing strength. These two values define a logarithmic scale
consisting of 64 ranges of impedance values. By default,
DIGDRVZ is 20 kohms and DIGDRVF is 2 ohms. The larger
the range between DIGDRVZ and DIGDRVF, the larger the
range of impedance values in each of the 64 strengths.
Determinin
g
the stren
g
th of a device output
The simulator uses the value of the DRVH (high-level driving
resistance) or DRVL (low-level driving resistance) parameters
from the device’s I/O Model. If the level of the output is a 1, the
simulator obtains the strength by finding the bin which contains
the value of the DRVH parameter. Likewise, if the level is a 0,
the simulator uses the value of the DRVL parameter to obtain
the strength.
Note that if the values of DRVH and DRVL in the I/O Model are
different, it is possible for the 1 and 0 levels to have different
strengths. This is useful for open-collector devices, where the 0
You can set these options by
selectin
g
Setup from the
Analysis menu in Schematics.
See Input/Output Model on
page 7-17 for more information.
(DRVH)
.
.
.
Level 1
Strength
DIGDRVF
DIGDRVZ
(DRVL)
.
.
.
.
.
.
Level 0
Strength
63
0
Output
Drive
Output
Strength
DIGDRVF
DIGDRVZ
.
.
.
63
0
Output
Drive
Output
Strength
Fi
g
ure 7-2
Level 1 and 0 Strength Determination
Higher
Strength
Higher
Impedance