Strengths

Formal Definition

The strength declaration construct is used for modeling net type variables for a close correspondence with physical wires.

Simplified Syntax

(Strength1, Strength0)
(Strength0, Strength1)
Strength1:
supply1, strong1, pull1, large1, weak1, medium1, small1, highz1
Strength0:
supply0, strong0, pull0, large0, weak0, medium0, small0, highz0

Description

Strengths can be used to resolve which value should appear on a net or gate output.

There are two types of strengths: drive strengths (Example 1) and charge strengths (Example 2). The drive strengths can be used for nets (except trireg net), gates, and UDPs. The charge strengths can be used only for trireg nets. The drive strength types are supply, strong, pull, weak, and highz strengths. The charge strength types are large, mediumand small strengths.

All strengths can be ordered by their value. The supply strength is the strongest and the highz strength is the weakest strength level. Strength value can be displayed by system tasks ($display, $monitor - by using of the %v characters - see Display tasks for more explanation).

Strength	Value	Value displayed by display tasks
supply	7	Su
strong	6	St
pull	5	Pu
large	4	La
weak	3	We
medium	2	Me
small	1	Sm
highz	0	HiZ

Table 23 Strengths ordered by value

If two or more drivers drive a signal then it will have the value of the strongest driver (Example 3).

If two drivers of a net have the same strength and value, then the net result will have the same value and strength (Example 4).

If two drivers of a net have the same strength but different values then signal value will be unknown and it will have the same strength as both drivers (Example 5).

If one of the drivers of a net has an H or L value, then signal value will be n1n2X, where n1 is the strength value of the driver that has the smaller strength, and n2 is strength value of driver that has the larger strength (Example 6).

The combinations (highz0, highz1) and (highz1, highz0) are illegal.

Examples

Example 1

and (strong1, weak0) b(o, i1, i2);

Instance of and gate with strong1 strength and weak0 strength specified.

Example 2

trireg (medium) t;

The charge strength declaration for trireg net.

Example 3

buf (strong1, weak0) g1 (y, a);
buf (pull1, supply0) g2 (y, b);

If a = 0 and b = 0 then y will be 0 with supply strength because both gates will set y to 0 and supply (7) strength has bigger value than weak (3) strength.

If a = 0 and b = 1 then y will be 1 with pull strength because g1 will set y to 0 with weak (3) strength and g2 will set y to 1 with pull (5) strength (pull strength is stronger than the weak strength).

If a = 1 and b = 0 then y will be 0 with supply strength because g1 will set y to 1 with strong (6) strength and g2 will set y to 0 with supply (7) strength (supply strength is stronger than the strong strength).

If a = 1 and b = 1 then y will be 1 with strong strength because g1 will set y to 1 with strong (6) strength and g2 will set y to 1 with pull (5) strength.

Example 4

buf (strong1, weak0) g1 (y, a);
buf (strong1, weak0) g1 (y, b);

If a = 0 and b = 0 then y will be 0 with weak strength.

If a = 1 and b = 1 then y will be 1 with strong strength.

Example 5

buf (strong1, weak0) g1 (y, a);
buf (weak1, strong0) g1 (y, b);

If a = 1 and b = 0 then y will be x with strong strength.

Example 6

bufif0 (strong1, weak0) g1 (y, i1, ctrl);
bufif0 (strong1, weak0) g2 (y, i2, ctrl);

If ctrl = x, i1 = 0, and i2 = 1 then y will be x with 36X strength, because g1 will set y to L with strong strength (StL - 6) and g2 will set y to H with weak strength (WeH - 3).

Important Notes

• If one of the drivers has an H or L value, then the output value will be X.