Logic Strength Modelling

The Verilog HDL provides for accurate modeling of signal contention, bidirectional pass gates, resistive MOS devices, dynamic MOS, charge sharing, and other technology-dependent network configurations by allowing scalar net signal values to have a full range of unknown values and different levels of strength or combinations of levels of strength.

This multiple-level logic strength modeling resolves combinations of signals into known or unknown values to represent the behavior of hardware with improved accuracy.

A strength specification has two components :

The strength of the 0 portion of the net value, called strength0, which is designated by one of the following : supply0, strong0, pull0, weak0 and highz0.
The strength of the 1 portion of the net value, called strength1, which is designated by one of the following : supply1, strong1, pull1, weak1 and highz1.

NOTE : The combinations (highz0, highz1) and (highz1, highz0) shall be considered illegal.

NOTE : Despite this division of the strength specification, it is helpful to consider strength as a property occupying regions of a continuum in order to predict the results of combinations of signals.

There are 4 Driving Strengths : supply, strong, pull and weak

NOTE : Signals with driving strengths shall propagate from gate outputs and continuous assignment outputs.

There are 3 Storage Strengths : large, medium and small

NOTE : Signals with the charge storage strengths shall originate in the trireg net type.

Table below demonstrates the continuum of strengths. The left column lists the keywords used in specifying strengths. The right column gives correlated strength levels.

Strength Name	Strength Level
supply0	7
strong0	6
pull0	5
large0	4
weak0	3
medium0	2
small0	1
highz0	0
highz1	0
small1	1
medium1	2
weak1	3
large1	4
pull1	5
strong1	6
supply1	7

It is possible to think of the strength of signals as locations on the scale below :

Could not load fig. Plz refresh — Scale of Strengths

Few points to note :

If the signal value of a net is known, all of its strength levels shall be in either the strength0 part of the scale or all strength levels shall be in its strength1 part.
If the signal value of a net is unknown (X), it shall have strength levels in both the strength0 and the strength1 parts.
A net with a signal value Z shall have a strength level only in one of the 0 subdivisions of the parts of the scale.