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VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Sunday 9 September 2012

Module Path Polarity

Formal Definition

Module path polarity describes when a signal is inverted or not driving propagation between the source and the destination .

Simplified Syntax

+=> positive simple module path polarity

-=> negative simple module path polarity

+*> positive multiple module path polarity

-*> negative multiple module path polarity


The polarity of the module is a description of what happens when a signal is propagated from the source to the destination. There are three possible polarities of the module.

· Unknown polarity

· Positive polarity

· Negative polarity

Positive polarity is specified by the + prefix. When positive polarity is specified, simple rules apply: a rise at the source causes a rise transition at the destination and a fall transition at the source causes the fall transition at the destination. (Example 1)

Negative polarity is specified by the - prefix. When negative polarity is specified, the rules applied are similar to those from positive polarity. However, a rise transition at the source causes fall transition at the destination, and a fall transition at the source causes a rise transition at the destination. (Example 2)

When no prefix is specified with => or *> operators, it means that unknown polarity is to be used by default. In this case a rise transition at the source may cause a rise, a fall, or no transition at the destination. The same rule applies for the fall transition. (Example 3)


Example 1

(DataIn +=> DataOut) = DataIn_to_DataOut ;
(DataIn +*> DataOut) = DataIn_to_DataOut ;

Positive polarity.

Example 2

(DataIn -=> DataOut) = DataIn_to_DataOut ;
(DataIn -*> DataOut) = DataIn_to_DataOut ;

Negative polarity.

Example 3

(DataIn => DataOut) = DataIn_to_DataOut ;
(DataIn *> DataOut) = DataIn_to_DataOut ;

Default unknown polarity.

Important Notes

· By default, unknown polarity is specified.

· Module path polarity is used by a timing analysis tool and is ignored by the simulator.

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